5-PRINCIPLES OF INHERITANCE AND VERIATION
CHAPTER NO.5 PRINCIPLES OF INHERITANCE AND VERIATION
A36
Students do you ever think! How the characters of
your parents come to
you? And why your brothers or sisters don’t resemble
to you. Even the twins are not alike, why?
The process by which characters are transferred from
one generation to
another is called GENETICS or The transfer of
characters from parents to off springs is called Pe The difference in traits of
individuals of a progeny from each other and from their parents are called
Sir Gregor John Mendel, the Great Botanist, worked
on the PEA plants to discover the hidden secrets of HEREDITY and VARIATIONS.
He is known as the FATHER of GENETICS.
Let us discuss various definitions and the work of
the FATHER OF
GENETICS: MENDELISM, that
we will come across to understand the concept of genetics thoroughly.
IMPORTANT DEFINITIONS:
VARIATIONS:
The differences in traits of individuals of a progeny from each other and from
their parents are called variations.
Types of variations-
1. Somatic variations
2. Germinal variations
1. Somatic variations:-
These are the variations which affect the body cells of
the organisms. These are caused by environment, use
and disuse of organs,
efforts by individuals. These are non- inheritable.
2. Germinal variations:-
They are produced in the germ cells of an organisms and are inheritable.
GENETICS -
The branch of biology which deals with inheritance and variations is called
genetics.
TRAIT-
It is an inherited character such as purple or white color of a flower,shape of
a pod etc.
CHARACTER-
Feature of an individual such as flower color, skin color etc.
UNIT FACTOR- Unit of
inheritance which controls a trait. This term was used by Mendel.
GENE-
It
is the unit of inheritance which is carried from parent by gamete.Unit factor
of Mendel was called gene by Johansson 1909. Gene occurs in a
chromosome and controls the expression of a
character and cooperation with other genes and environment.
ALLELES-
Alleles are slightly different forms of the same gene which are found at a
particular place on homologous chromosomes and control the same character.
CONTRASTING CHARACTERS-The
traits which always appear in two opposing conditions.
HOMOZYGOUS- Both the alleles
of the character are similar e.g. TT, tt. It
is pure for a trait and breeds true.
HETEROZYGOUS-
It carries dissimilar alleles. e.g.Tt. It is seldom pure
and produces offspring with different genotype.
GENOTYPE- It is the
genetic constitution of an individual.
PHENOTYPE-
It is the external manifestation of gene product brought to expression.
MONOHYBRID CROSS-
It is a cross between two organisms of a species which is made to study the
inheritance of a single pair of alleles.
DIHYBRID CROSS- It is a cross
between two organisms of a species which is made to study the inheritance of a
two pair of alleles.
F1 GENERATION-
It is 1* filial generation produced from a cross between the genetically
different individuals called parents.
F2 GENERATION-
It is the 2" filial generation which arises as a result of
inbreeding or interbreeding amongst individuals of
F1 generations.
GREGOR JOHN MENDEL
Gregor John Mendel is known as the father of
genetics because he was the first to
demonstrate the mechanism of transmission of
characters from one generation to
another. Mendel performed his experiments with
garden pea plant Pisum sativum.
He conducted artificial/cross pollination
experiments using several true breeding
varieties. He selected 7 pairs of contrasting characters
in his experiments.
It is an annual plant with a short life cycle so
several generations can be
studied within a short period.
Ithas perfect bisexual flowers.
Flowers are self-pollinating.
It is easy to get pure lined plants for several generations.
Pea plants could easily be raised maintained and
handled.
Anumber of easily detectable contrasting characters
were available.
Mendel’s observations:
F-1 progenies always resembled one of the parents;
trait of the other parent
was not seen.
F-2 progenies expressed both the parental traits in
the proportion of 3:1.
The contrasting traits did not show any blending.
In dihybrid cross, he got identical results as in
monohybrid cross.
Phenotypes of F-2 generations appeared in the ratio
of 9:3:3:1.
Based on his hybridization experiments, Mendel,
proposed the three laws of
inheritance as:
Law of dominance.
Law of segregation.
Law of independent assortment.
Law of dominance: This law states that when two
alternate forms of a trait are
present in an organism, only one factor expresses
itself in F-1 progeny, and is
called dominant, while the other which does not show
its expression is called
recessive.
Law of segregation: This law states that the factors
or alleles of a pair segregate
from each other during gamete formation such that
gamete receives only one of the
two factors. They do not show any blending.
The following cross between a pure breeding plant with yellow, round seeds {RRVY)\ with nraen wrinkled ceade (rrvv\ ran he takan an avampnle of thic law
Law of independent assortment: According to this law
the two factors of each character separate out independent of the factors of
other characters during gamete formation and get randomly rearranged in the
offspring.
Reasons for Mendel’s success:-
He selected only pure breeding varieties of pea
plant.
He studied the inheritance of a character for three
or more generations.
He took one or two characters at one time for his
experiments.
He kept a record of every observation.
He took care to avoid contamination from pollen
grains brought by insects.
Rediscovery of Mendel’s work:-Scientists Hugo de
Vries, Carl Correns, Eric Von rediscovered Mendel’s work.
PART: A SHORT ANSWER TYPE QUESTIONS:
A) MCQs:
1. Who is the “father of genetics”?
(a) Hugo de Vries
(b) Eric Von
(c) Gregor John Mendel
(d) Carl Correns
2. Who coined the term gene?
(a) Gregor John Mendel
(b) Johansson
(c) Hugo de Vries
(d) Carl Correns
3. Mendel’s experimental organism was:
(a) homo sapiens
(b) drosophila melanogaster
(c) oryza sativa
(d)Pisum sativum
B) FILL Ups:
1. The phenotypic ratio in the f2 generation of a
dihybrid cross is .....
2. Unit factor of Mendel was called ‘gene’ by .....
C) TRUE/FALSE:
1. A gamete receives both the factors of a gene.
2. Alleles are the same forms of the same gene that
controls same character.
ANSWER KEY: PART-A
A)MCQs:
1. Ans. (c) Gregor John Mendel because other
scientists rediscovered Mendel’s work
2. Ans. (b) Johansson because Mendel used the word
unit factor and other scientists rediscovered Mendel’s work.
3. Ans. (d) Pisum sativum because it is the
scientific name of pea plant.
B) FILL UPs.:
Ans 1. ratio 9:3:3:1.
Ans 2. Johansson
C) TRUE/FALSE:
Ans 1. FALSE: A gamete receives only single factor
of a gene.
Ans 2. FALSE: Alleles are the DIFFERENT forms of the
same gene that controls
same character.
PART: B SHORT ANSWER TYPE QUESTIONS:
1. Differentiate between genotype and phenotype.
2. Why Mendel is called father of Genetics?
PART: C LONG ANSWER TYPE QUESTIONS:
1. Why Mendel selected pea plant for his
experiments?
2. Explain the law of dominance using a monohybrid
crosses?
A37
INTRODUCTION
MONO means ONE. (One character)
HYBRID means product made by crossing two
individuals
CROSS means breeding of two parents with different
genes to produce offsprings (youngones).
MONOHYBRID CROSS means cross made between
individuals having contrasting traits( expression of a character), in order to
study the inheritance of a pair of alleles or factors of a single character.
Some Basic Terms Used
1. Gene or Factor- Gene is a functional unit of
hereditary material. Chemically
gene is a segment of DNA and equivalent to factor of
Mendel.
2. Allelomorphs or Alleles- Alleles are alternative
forms of same genes.
3. Homozygous-_The organisms in which both the genes
of a character are
identical is said to be homozygous.T and T ort andt.
4. Heterozygous-_The organisms in which both the
genes of a character are
unlike is said to be heterozygous.T and t.
5. Phenotype-_|t expresses the observable external
characters of Individuals
like form, colour etc. It is expression of a
character.
6. Genotype- It is the genetic constitution of an
organism.
7. Pure Line- Generations of homozygous individuals
which produce offsprings
of only one type.
MONOHYBRID CROSS
When only one allelic pair of a single character is
considered in cross
breeding, it is called monohybrid cross.
For Example, Mendel studied inheritance of tall and
dwarf traits, of single
character of HEIGHT; in Pea plant he crossed variety
of tall plants with
another variety with dwarf plants. Mendel’s crosses
between the two different varieties always produce F: generation of tall
plants. When F;:plants were self-pollinated, F: generation produced tall and
dwarf plants in the ratio of 3:1.
On the basis of his observations on monohybrid
cross, Mendel proposed a set of
generalisations which resulted into the formation of
following three laws of
inheritance:-
I. Principle of paired factors:
A character is represented by at least two factors
which lie on two homologous
chromosomes at the same locus. They may represent
same or alternate
expressions of the same character, these are called
alleles.
ll. Principle of Dominance:
In a heterozygous individual, a character is represented
by two contrasting
factors called alleles out of which only one is able
to express its effect in the
individual. It is called dominant factor. The other
allele which does not show its
effect in a heterozygous individual is called
recessive factor.
This law explains why individuals of F, generation
express trait of only one
parent, it is also able to explain 3:1 ratio in F2
generation.
ll. Principle of Segregation:
The two factors of a character present in an
individual separate at the time of
gamete formation get randomly distributed to
different gametes and then again
get paired in different offsprings.
In monohybrid cross shown above, in cross between
pure tall pea plant and dwarf plant the hybrids in F, generation are all tall
though they have also received the factor for dwarfness. This is because factor
for tallness is dominant over factor for dwarfness. In F2 generation we get
tall and dwarf plants in the ratio of 3:1.
Because only one of the two factors passes into the
gamete, fifty percent of male and female gametes formed by F; plants have the
factor for tallness and fifty percent carry the factor for dwarfness.
LET US TEST WHAT WE HAVE LEARNT!
PART: A VERY SHORT ANSWER TYPE
QUESTIONS:-
A) MCQ
1.A cross between two plants resulted in
offspring having few dwarf plants.
What would be the genotypes of both the
parents?
a) TT and Tt
(b) Tt and Tt
(c) TT and TT
(d) None of the above
2.Which condition is heterozygous?
(a)TT
(b) Tt
(c)tt
(d) None of the above
3. Which law cannot be deduced from
monohybrid cross?
(a) Law of independent assortment
(b) Law of dominance
(c) Law of Paired factors
(d) Law of segregation
4. When a gene pair in an organism contains
two identical alleles, the organism is considered to be
(a)Homozygous
(b) Heterozygous
(c) Phenotypic
(d) Genotypic
B. TRUE/FALSE:
1. Any gene that produces an effect in heterozygous
condition is called dominant.
2. Tt plant is homozygous for controlling one
character (height).
3. The two alleles of a gene pair are located on
homologous chromosomes.
Hybrid is an individual resulting from cross between
two genetically unlike
parents.
4. Recessive characters remain hidden by normal
dominant characters in hybrids.
FILL IN THE BLANKS:
1._ so factor expresses itself even in the presence
recessive aileles.
2.________ individual produces only one type of
gametes.
3. Inmonohybrid cross Csi pair is considered in
cross breeding.
ANSWER KEY: PART- A
A) MCQs:
1. Answer: (b) If it is present in both the parents
only then offspring can have
genotype tti.e., dwarf plants.
2. Answer: (b) The organism in which both the genes
of a character are
unlike is said to be heterozygous.
3. Answer: (a) Laws of dominance, paired factors and
segregation are deduced
from monohybrid cross.
4. Answer: (a) The organism in which both the genes
of a character are identical
is said to be homozygous.
B) TRUE/FALSE:
1. TRUE
2. FALSE
3. TRUE
4. TRUE
5. TRUE
C)FILL IN THE BLANKS:
1. Dominant
2. Homozygous
3. One
PART-B SHORT ANSWER TYPE QUESTIONS:-
1. What do you understand by alleles?
2. What is principle of paired factors?
3. What is principle of dominance?
PART- C LONG ANSWER TYPE QUESTIONS
1. Explain monohybrid cross taking plant height as a
trait in Pea plant. Work
out the cross up to F2 generation.
2. State the laws of inheritance that can be derived
from a monohybrid cross.
A38
INTRODUCTION
MENDEL’S DIHYBRID CROSS :
A dihybrid cross is a breeding experiment dealing
with two heritable characters at the same time. In such cross the parent plants
differed in two pairs of contrasting traits.
In the dihybrid cross we are discussing here, we
took two Contrasting Characters, simultaneously;
1. Colour of the seed --- Yellow seed plant and Red
seed plant.
2. Shape of the seed --- Round seed plant and
Wrinkled seed plant.
MENDEL’S EXPERIMENT ON DIHYBRID CROSS:
Mendel crossed a pea plant having round and yellow
seeds with a pea plant with wrinkled and green seeds, All the plants of F1
generation had round and yellow seeds suggesting that round was dominant over
wrinkled and yellow was dominant over green. Plants of F; generation, on
selfing (i.e., pollinating among themselves) produced four kinds of plants in
Fz generation.
RESULTS OF MENDEL’S DIHYBRID CROSS :
Four Types of Plants: Four types of plants were
produced in F2 generation in
the ratio of 9 (both dominant): 3 (one dominant
second recessive): 3 (one
recessive second dominant): 1 (both recessive).
New combinations : Two new combinations of traits:
round green and wrinkled yellow, had appeared in a dihybrid cross.
Independent Assortment of a Factors: The formation
of four types of individuals in the F2 generation of a dinybrid cross shows
that the factors of the two characters assort (random separation) independently
which leads to the formation of independent assortment of factors.
LAW OF INDEPENDENT ASSORTMENT:
It states that “when two pairs of traits, of two
characters, are combined in a hybrid,segregation or random separation of one
pair of traits is independent of the other pair of traits”.
LET US KNOW WHAT WE HAVE LEARNT!
PART-A: VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQ:
1. The cross in which parents differ in two
pairs of contrasting characters is called?
a. Monohybrid cross
b. Dihybrid cross
c. Back cross
d. Test cross
2. The phenotypic dihybrid ratio is?
a.9:3:2:1
b.9:3:2:2
c.1:1
d.9:3:3:1
3. Which law is based on dihybrid cross?
a. Law of independent assortment
b. Law of dominance
c. Law of segregation
d. Both B and C
4. What will be the dihybrid ratio for
cross between HHTT and hhtt?
a. 1:2:1
b. 9:3:3:1
c. All the same
d. 3:1
b) FILL IN THE BLANKS:
1. In mendel’s experiment in F1 generation and seeds
were dominant.
2. Plants of F1 generation, on selfing produced
kinds of plants in F2 generation.
ANSWER KEY:PART-A
a.) MCQS
1. b (dyhybrid cross)
2.d (9:3:3: 1)
3. a (Law of independent assortment)
4.c (all the same)
b.) FILL IN THE BLANKS
1. round, yellow
2. four
PART- B SHORT ANSWER TYPE QUESTIONS:
1. What is Law of independent assortment?
2. What is F1 generation in a dihybrid cross?
PART-C LONG ANSWER TYPE QUESTIONS:
1. What is difference between monohybrid and
dihybrid cross?
2. Explain the Mendel’s experiment on dihybrid
cross?
A39(a)
INTRODUCTION
TEST CROSS, as the name indicates, is for testing.
It is done for testing the genotype of dominant individual, whether it is HOMOZYGOUS dominant or HETEROZYGOUS dominant?
In this theF2 dominant is crossed with homozygous recessive parent.
BACK CROSS as the name indicates, it is a cross
between F,dominant hybrid with any of the parent. The purpose of back cross
is to get maximum good parental characters in the
population. Thispopulation is called pure line population.
TEST CROSS:-
To determine the genotype of a tall plant from F2
generation, Mendel crossed
tall plant from F2 with a homozygous dwarf parent
plant. This he called a test
cross. So, test cross is used to know the genotype
of an organism.
It tells whether an individual is homozygous or
heterozygous for a dominant
trait.
Monohybrid cross gives a test cross ratio of 1:1
Dihybrid cross gives a test cross ratio of 1:1:1:1
Back Cross:
In a back cross F, hybrid is crossed with any of its parents. It is used in horticulture and animal breeding to develop good hybrid varieties & to maintain pure line having good characters.
For a back cross
the phenotypic ratio may or may not be 1:1.
PART: A_ VERY SHORT ANSWER TYPE
QUESTIONS:
(a) MCQs:
Q1. A cross between a F; hybrid and any of
its parents is:
(a) Back cross
(b) Monohybrid
(c) Dinybrid cross
(d) Test cross
Q2.A cross between F: hybrid and homozygous
recessive parent is:
(a) Back Cross
(b) Dihybird cross
(c) Test cross
(d) Monohybrid cross
Q3. The monohybrid test cross ratio is:
(a) 3:1
(b) 9:3:3:4
(c) 1:1
(d) 1:1:1:1
Q4.If test cross gives all dominant
characters it means parent is:
(a) Homozygous recessive
(b) Heterozygous
(c) Homozygous dominant
(d) Heterozygous dominant
Q5.The best method to determine the
homozygosity and heterozygosity of an individual is:
(a) Self fertilization
(b) Back cross
(c) Test cross
(d) In breeding
(b) TRUE/FALSE:
1. To determine the genotype of F2, Mendel crosses
tall plant from F2 with a dwarf
plant.
2. In test cross F; hybrid is crossed with any of
the parent.
3. Dihybrid test cross ratio is 9:3:3:1.
(c) FILL UPs:
Q1. Test cross is used to determine the .
Q2. introduced test cross and back cross.
ANSWER KEY: PARTA
(A) MCQs:
1. (a) In back cross F, hybrid is crossed with any
of its parents to get good hybrid
varieties.
2. (c) In test cross F; hybrid is crossed with
homozygous recessive parent to know the genotype of an individual.
3. (c) Ratio 1: 1 proves that parent is heteroz4. (c) All dominant characters show that the parent is homozygous dominant.
5. (c) Test cross is used to know whether individual
is homozygous or heterozygous
genotype.
(B)TRUE/FALSE:
1. True
2. False. In test cross F, hybrid is crossed with
homozygous recessive parent only.
3. True
(C) FILL UPs:
1. Genotype of an individual.
2. Gregor Mendel.
PART: B - SHORT ANSWER TYPE QUESTIONS:
Q1.What is the use of test cross?
Q2. What is the use of back cross?
PART: C -LONG ANSWER TYPE QUESTIONS:
Q1 Define test cross and design a test cross.
Q2. How does a back cross differ from test cross?
A39(b)
INTRODUCTION
In 1900 three scientists DE-VRIES ,CORRENS and
VONTSCHERMAK rediscovered Mendels work on inheritance of characters. By then
there had been advancements in microscopy and scientists were able to observe
cell divisions,nucleus and chromosomes etc. By1902 Walter Sutton and Boveri
observed the chromosomal movement and separation of chromosomes during cell
division
meiosis. All these observations led to the
development of chromosomal theory of
inheritance This theory identified chromosomes as
genetic material for inheritance.
CHROMOSOMAL THEORY OF INHERITANCE
Mendel published his work on inheritance of
characters in 1865 but his work remained unrecognized till 1900 due to
following reasons:-
1. Communication was not easy in those days.
2. His concept of factors as stable and discrete
units was not accepted by other
scientists.
3. Mendel used mathematics to explain biological
phenomenon which was also not
accepted by others.
4. He could not provide any physical proof for the
existance of factors.in1900 De Vries, Correns and Von Tschermak rediscovered
Mendels results.In 1902 Walter Sutton and Boveri observed the chromosomal
movement during meiosis and noted that the behaviour of chromosomes was
parallel to the behaviour of the genes. Based on these observations they gave
chromosomal
theory of inheritance.
ACCORDING TO THIS THEORY:
1. Genes are located on chromosomes.
2. Chromosomes as well as genes occur in pairs.
3. Homologous chromosomes separate during meiosis
which leads to separation
of pair of factors they carry.
4. During anaphase of Meiosis -1, one pair of
chromosomes aligns at metaphase
plate independently of the other pair of chromosomes
as shown in figure:
T.H.Morgan and his Colleagues gave experimental
verification of the chromosomal theory of inheritance while working on
Drosophila Melanogaster.
They also discovered the basis of variations that
were produced during sexual
reproduction.
PART: A - VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQs:
1. In which year Mendel’s Work was
rediscovered?
a) 1900
b) 1904
c) 1902
d)1903
2. In which phase of meiosis-1, the two
chromosomes can align at the metaphase plate independently of each other:
a)Metaphase-Il
b) Metaphase-l
c) Anaphase-!
d) Telophase-l
3. Who argued that pairing and separation
of chromosomes would lead to the segregation of a pair of factors they carried?
a) Sutton
b) Boveri
c) Both (a) &(b)
d) Morgan
4. The parallelism between factors and
chromosomes led to:
a) Cell Theory
b) Chromosome theory of inheritance
c) Pangenesis
d) Mapping of genes
5. Who gave experimental verification of
chromosomal theory of inheritance?
a) de Vries
b) Mendel
c) Sutton and Boveri
d) Morgan.
b) TRUE/FALSE:
1. Mendel used mathematics to explain Biological
phenomenon.
2. Genes are located on chromosomes.
3. Chromosomes do not separate during meiosis.
c) FILL UPS:
1. ------------ as well as genes both occur in
pairs.
2. ------------ postulated the chromosome theory of
inheritance.
1. (a)
2. (Cc) chromosomes align at metaphase plate
independently during Anaphase
of meiosis-1
3.(c)
4. (b)
5. (d)
1. True
2. True
3. False, chromosomes separate during meiosis.
1. Chromosomes
2. Sutton and Boveri.
PART: B - SHORT ANSWER TYPE QUESTIONS:
1. Who rediscovered the Mendel’s work?
2. Why Mendel’s work remained unrecognised?
3. Who discovered chromosome theory of inheritance?
PART: C- LONG ANSWER TYPE QUESTIONS:
1. Give comparison between behaviour of chromosomes
and genes?
2. Give main points of chromosomal theory of
inheritance?
A40
INTRODUCTION
which codes for specific polypeptide
chain. It is basic structural and functional unit of
heredity and control
the expression of a character. Each person has
copies of each gene, one inherited from each parent. Most genes are same in all
people, but small number of copies is slightly
different. This leads to formation of different products and traits.
SIGNIFICANCE:
It brings about variations which are necessary for evolution and adaptation.GE.
The various forms of gene are called alleles. The dominant
and recessive factors of Mendel are alleles of a
gene.Most of the traits of living organism are controlled by collaboration
of several genes.Two alleles need not always be
indentical as in heterozygote.
One of them may be different due to some changes
that it had.
GENE INTERACTION:
It is the influence of allele and non - allele on the normal phenotypic
expression of genes.
TYPES OF GENE INTERACTION:-
There are two types.
1. INTRAGENIC [Allelic Interaction].
2. INTERGENIC [Non-Allelic Interaction].
We are discussing ALLELIC INTERACTION here.
When two alleles are present on the gene locus on
homologous chromosome of a gene, they interact in such a way to produce a
phenotype different from typical recessive
phenotype.
e. g. Incomplete Dominance.
INCOMPLETE DOMINACE:-
Reported by CORL CORNERS in Mirabilus jalapa and
Antirrhinum majus.
In both the plants, two types of flower colour in
pure state, || and
WHITE, when crossed, Hybrid Plants of F: generation
Here dominant factor is unable to express its character results in
formation of intermediate type of generation which
is different from both
parents.
This is not blending because parental types are not
recovered in F2 generation.
Here the genotypic ratio is exactly as we expect in
any Mendelian Monohybrid ratio is J But the phenotype ratio had changed from
3:1 dominant: recessive to a Here the two alleles need not always be identical
in heterozygote.
One of them maybe different due to some changes that
it has undergone which modifies the information that particular allele is
contained.
The normal allele, produce the normal enzyme.
Modified allele is equivalent to the un-modified
allele, it will produce the
same phenotype but if the allele produce,
non-functional enzyme the phenotype may get affected.
Modified Allele which represent new phenotypes is
generally the recessive allele.
LET US KNOW WHAT WE HAVE LEARNT!
PART: A VERY SHORT ANSWER TYPE
QUESTION:
A) MCQs:
1. In across, if you get 1:2:1 ratio, it
denote that;
A. It is a case of multiple allele.
B. It is a multigenic inheritance.
C. It is incomplete dominance.
D. It is codominace.
2. Fe Phenotypic raito in, Snapdragon plant
is ;
A. 1:1.
B. 2:1
C. 3:1
D. 1:2:1
3. The number of gametes produced in
dihybrid cross ;
A. One
B. Two
C. Three
D. Four
4. Intragenic interaction is also called ;
A. Allelic- Interaction.
B. Intragenic interaction.
C. Non-allelic interaction.
D. None of these.
5. In poultry, there is no need to destroy
black and white fowl because if they crossed;
A. 25% offspring are white.
B. 25% offspring are black,
C. S0%offspring are blueygous.
D. 100% offspring are blue.
B. FILL UPs:-
1. Across between parents differing in two traits is
called---—-—.
2. It is an organism in which two members of an
allelic pair are -—--gametes.
3. The type of inhertiance in which geno type ratio
is the same as -—--
C. TRUE/FALSE:-
1. Recessive trait is seen due to nonfuctional
enzyme.
2. Incomplete domiance does not favour blending.
ANSWER KEY: PART- A
A. MCQs:
1. C incomplete domiance.
Hint: because genotypic ratio is exactly same as
other hybrid cross.
2. D 1:2:1
Hint: because both phenotype and genotype ratio are
same in incomplete ratio.
3. D Four
Hint: because each produce two gametes.
4. A Allelic interaction.
5. D 100% offspring are blue.
Hint: because in F, generation all produced fowl are
superior quaility which is due to crossing.
B. FILL UPs:-
1. dihybrid.(two traits are considered in it.)
2. unlike.(because heterozygous produce two diff.
types of gametes.)
3. phenotypic ratio.(like in incomplete domiance.)
C. TRUE/FALSE:
1. True.
2. True
PART-B SHORT ANSWER TYPE QUESTIONS
1. How are alleles of particular gene differ from
each other ? Explain its
significance.
2. Differentiate between phenotype and genotype?
3. How does Incomplete domiance differ from
Co-domiance.?
PART-C LONG ANSWER TYPE QUESTIONS
1. Whatis incomplete dominace ? Explain it with
cross in Andalusian fowl ?
2. When ared flowered Antirrhinum majus plantwas
crossed with a white flowered, the F1 offspring had pink flower.Mention:-
a) The genotype of F1 plant.
b)The reason why it did not bear parental red or
white flower colour.
A41
INTRODUCTION
GENE INTERACTION:
Itis the influence of Allele and Non-Allele on
normal phenotypic expression of genes.
Itis classified into two types:
1. Allelic or Interallelic Interaction
2. Non-Allelic Interaction
ALLELIC OR INTER-ALLELIC INTERACTION:
When two Alleles (present on same gene locus on
homologous chromosome) of a
gene, interact in such a way to produce a phenotype
different from typical
dominant-recessive phenotype.
Examples are Incomplete Dominance, Co-Dominance and
Multiple Allelism.
Students, you have studied incomplete dominance in
the previous assignment.
Now we will discuss here Co-Dominance and Multiple
Allelism.
1. CO-DOMINANCE:-
Itis the phenomenon in which TWO ALLELES OF A GENE
ARE EQUALLY
DOMINANT and expresses themselves in the presence of
the others when
they are together in an organism.
Both alleles which, when present together, in
heterozygous individual,express their traits independently instead of showing
dominant-recessive relationship are called co-dominant alleles.
ABO blood groups are controlled by gene |. Gene |
has three alleles,
I, IP and I?
In this, the alleles I and I? are completely
dominant over I°- That means,
When I‘ and |° are present in an individual only I4
expresses and
When I® and I° are present only IP expresses because
|° does not produce
any glycoprotein.
When an individual possesses |" and |",
both express themselves producing
the glycoprotein A and B, respectively on the
surface of red blood cells,hence the blood group is AB.
Another example of co-dominance is:
Cattle can be Jj (RR=All red hairs),
GE (WW=All white hairs) or MM (RW2=Red and white
hairs together) as shown in figure 1.
2. MULTIPLE ALLELISM:
¢ Itis the phenomenon cf
HEE «on a chromosome or its homologue.
The ABO blood group in humans are determined by
three different allelic
forms I4, I? and I°.
Any individual carries two of the three alleles.
The allele I4 codes for glycoprotein A and the
allele IP codes for glycoprotein
B, which are found on the surface of RBCs; the
allele I° does not produce any glycoprotein.
The allele Ais dominant over I° and |? is also
dominant over 1°.
When the alleles |’ and IB are together, they are
equally dominant and both
the glycoproteins A and B are produced, making the
blood group AB.
With three alleles, there are possible six genotypes
and four phenotypes as shown in figure below.
The inheritance of blood group character [J follow
Mendelian pattern of inheritance.
LET US KNOW WHAT WE HAVE LEARNT!
PART: A VERY SHORT ANSWER TYPE QUESTIONS:
b) True/False:
Q.1. Like ABO blood group, MN blood group also shows
Multiple Allelism.
Q.2. Co-Dominance and Incomplete Dominance is the
same thing.
c) Fill in the blanks:
Q.1. If the blood group of personis AB thenits
genotypeis
Q.2. ABO blood group is a good example of.
PART:A VERY SHORT ANSWER TYPE
QUESTIONS:
a) Multiple Choice Questions:
Ans.1: a) Co-dominants
Ans.2: a) Co-dominance (both of the two alleles are
dominant and show their
expression)
Ans.3: c) A,B, AB and O
Ans.4: d) Both alleles independently expressed in
the heterozygote
Ans.5: c) At the same locus of the chromosome
b) True/False:
Ans.1: False (In MN blood group system i which are
co-dominant. Thus, this is not an example of multiple alleles).
Ans.2: Fa\se
where as in case of incomplete dominance one of the
allele is expressed incompletely in case of heterozygous condition).
c) Fill in the blanks:
Ans.1. IAI? (Both alleles express their traits
independently).
Ans.2. Multiple Allelism (ABO blood group is
controlled by gene I.
Gene | has more than two alleles I4, I® and 1°).
PART: B SHORT ANSWER TYPE QUESTIONS:
Q. 1. What do you mean by inter allelic interaction?
Q. 2. Define Co-dominant alleles.
Q. 3. Write the difference between co-dominance and
incomplete dominance.
Q. 4. Write the possible genotypes for the blood
group A and B.
PART-C LONG ANSWER TYPE QUESTIONS:
Q. 1. Explain co-dominance with the help of an
example.
Q. 2. Explain the phenomenan of Multiple Allelism
taking ABO blood group as an example.
A42
INTRODUCTION
Non-allelic or inter-allelic interactions occur
where the development of single character is due to two or more genes affecting
the expression of each other in various ways. Thus, the expression of gene is
not independent of each other and dependent on the presence or absence of
other gene or genes.
Non Allelic Gene Interactions -Epistasis:
When a gene or gene pair masks or prevents the
expression of other non-allelic gene, called epistasis. The gene which produces
the effect called epistatic gene and the gene whose expression is suppressed
called hypostatic gene.1, Recessive Epistasis or
Supplementary Factor (9:3:4)
In this case, homozygous recessive condition of a
gene determines the
phenotype irrespective of the alleles of other gene
pairs, i.e., recessive
allele hides the effect of the other gene. The coat
colour of mice is controlled by two pairs of genes.Dominant gene C produces
black colour, absence of it causes albino.
Gene A produces agouti colour in presence of C, but
cannot express in absence of it (with cc) resulting in albino. Thus recessive
allele c (cc) is
epistatic to dominant allele A. (Fig. 7.8).
2. Dominant Epistasis (12:3:1):-Sometimes
a dominant gene does not allow the expression of other non-allelic gene called
dominant epistasis.In summer squash, the fruit colour is governed by two genes.
The dominant gene W for white colour, suppresses the expression of the gene Y
which controls yellow colour. So yellow colour appears only in absence of W.
Thus
W is epistatic to Y. In absence of both W and Y,
green colour develops (Fig.
7.10).
Only when the genotype of the individual is
homozygous recessive at epistatic locus(aa) can alleles of hypostatic locus(B
or b) can be Expressed.Thus the genotypes A-B and A-bb produce same phenotype,
whereas aaB
and aabb produces two additional phenotypes. The
original 9:3:3:1 ratio
becomes modified into 12:3:1 ratio.
LET US KNOW WHAT WE HAVE LEARNT!
PART - A VERY SHORT ANSWER TYPE
QUESTIONS
a) MCQs:-
1. What is epistasis?
a) Type of linkage
b) Masking or modifying gene effect
c) Upper portion of achromosome
d) Group of genes
2. Which of the following is NOT the type
of epistasis?
a) Dominant
b) Recessive
c)Collabrative
d) Supplementary
3. Which one of the following is not the
case of epistasis?
a) Fur colour in mouse
b) Fruit colour in summer squash
c) Fruit shape in summer squash
d) Coat colour in Labrador
4. Choose the correct ratio, which shows
the masking gene interaction?
a) 12:3:1
b) 9:3:4
c) 1:2:1
d) 9:7
5. In case of dominant epistasis which of
the following will have the same
expression, when A is the epistatic locus?
a) A/a B/b and a/a B/b
b) AVA b/b and A/A B/b
c) a/a b/b and A/a b/b
d) a/a b/b and a/a B/B
b) TRUE/FALSE:--
1. Epistasis is the interaction between two genes
producing a new phenotype.
a) True b) False
2. Epistasis can produce a ratio of 12:3:1.
a) True
b) False
3. The cross between a white mouse and a black mouse
can give all combinations
a) True
b) False
c) FILL IN THE BLANKS:-
1. In case of two gene interaction, the gene which
is masking the expression of another is called and the gene whose
expression is masked is called
2. Epistasis is the interaction between genes.
ANSWER KEY: PART-A
a) MCQs:--
1. Ans: b
Explanation: It is a non-allelic suppression which
involves two pairs of
alleles. It can suppress both dominant or recessive
alleles
2. Ans: c
Explanation: Epistasis can be of many types based on
the ability of an allele in masking and modification of gene effects. It can be
of dominant,recessive, complementary or supplementary
3. Ans: c
Explanation: White Fur colour in mouse and coat
colour in Labrador is the case of recessive epistasis, and Fruit color in
summer squash being a case of dominant epistasis, fruit shape of summer squash
is gene interaction and not epistasis.
4. Ans: a
Explanation: Masking gene interaction is also
recognized as dominant epistasis. Here, dominant allele of one gene can mask
the effect of either allele of the second gene
5. Ans: b
Explanation: According to the description of
dominant epistasis given, A is the epistatic locus, thus presence of even one
set of dominant gene at A locus will give the same effect irrespective of the B
locus.
b) TRUE/FALSE:-----
1. Ans: b
Explanation: In case of epistasis there is
suppression of expression of one phenotype but there is no production of new
phenotype. This is how it differs from gene interaction.
2. Ans: a
Explanation: This is the phenotypic ratio observed
in case of dominant epistasis. Then the presence of a dominant allele in one
locus will produce the same effect irrespective of the allele at the other
locus.
3. Ans: a
Explanation: Agouti gene is hypostatic to the color
locus, in black mouse the combination is a/a C/- and in albino or white it’s
-/- c/c. So if there is
any dominant agouti gene in white mouse it can be
expressed in the F1 where C locus would be C/-, so all combinations are
possible.
c) FILL IN THE BLANKS:-----
1. Ans: Epistatic, hypostatic
Explanation: In case of single gene two alleles
which suppress one another we use dominant/ recessive relations. But in case of
interaction between two genes we use epistatic for the one that suppresses the
expression of another and hypostatic for the one
whose expression is suppressed.
2. Ans: Two
Explanation: Epistasis is the interaction between
two genes where the genotype of one location affects the expression of genotype
at the other location.
PART — B SHORT ANSWER TYPE QUESTIONS
1. What do you mean by epistasis?
2. What do you mean byDominant Epistasis?
3. What do you mean by Recessive Epistasis?
PART — CLONG ANSWER TYPE QUESTIONS
1. What is epistasis and its types. Explain in
detail with examples?
A43
INTRODUCTION
Recapitulation: Non-Allelic Interactions
Alleles of two or more independent genes interact to
produce a phenotypic
expression different from normal expression. The
characteristics of non-allelic interactions are:
1. Interaction produces a distinct phenotype
different from the normal.
Interacting genes show normal dominance
recessiveness.
2. They assort independently.
COMPLEMENTARY GENES:
They are those non allelic genes which independently
show a similar effect but
produce a new trait when present together in the
dominant form.
Bateson and Punnet (1906) crossed two white flowers
strains (CCpp, ccPP) of sweet
pea and obtained purple flower plants (CcPp) in the
F: generation.
Which after self-breeding produce purple and white
flower plants in F2 generation in
the ratio of JJ, which is the modification of
It is believed that,
Dominant gene C produces an enzyme which converts the raw
material into chromagan.The dominent gene P gives rice to an oxidase enzyme
that changes chromagan into purple anthocyanin pigment Thus purple colour
formation is two-step reaction and the two genes
cooperate to form the ultimate product.
SUPPLEMENTARY GENES:
Supplementary genes are a pair of non-allelic genes,
one of which produces its
effect independently in the dominant state while the
dominant allele of the second
gene (supplementary gene) needs the presence of
other gene for its expression.
Supplementary Genes in Lablab: Lablab has two genes,
K and L. In the recessive
state the second or supplementary gene (Il) has no
effect on seed coat colour.
Dominant K independently produces Khaki colour while
its recessive allele give rise
to Buff colour irrespective of the supplementary
gene being dominant or recessive. In the dominant state the supplementary gene
(L-) changes that effect of dominant allele of pigment forming gene (K) into
chocolate colour . F2 ratiois9:3:4.
PART.A: VERY SHORT ANSWER TYPE QUESTIONS:
A) MCQs:
1. The phenomenon of two or more than two
genes affecting the expression of each other is called:
a) Crossing over
b) Pairing
c) Gene interaction
d) Linkage
2. Which of the following ratio shows
complementary gene interaction ?
a)9:7
b)15:1
c)1:2:1
d)9:3:3:1
3. Two or more independent genes present on
different chromosomes which determine nearly same phenotype are called a)
Supplementary genes
b) Complementary genes
c) Duplicate genes
d) None of these
4. Which is the example of supplementary
gene ?
a) Cucurbitta pepo
b) Skin colour of mice
c) Comb shape of poults
d) None of the above
5. The fruit shape in tomato is an example
of.
a) Complementary genes
b) Supplementary genes
c) Polygenic inheritance
d) Duplicate genes
B) TRUE / FALSE:
1. Crossing of two white flowers gives a colored
flower, this shows complementary
gene interaction.
2. Gene interaction always modifies the normal
Mendelian ratio.
C) FILL UPS:
1. Complementary genes were first discoveredby_
2. Theratio of supplementary genesis.
PART A: VERY SHORT ANSWER TYPE
QUESTIONS
A) MCQs:
Ans. 1:¢
EXPLANATION: Mendel described every character is
influenced by the
particular expression of the gene but when the
characters are controlled by the
combination of more than one gene, called gene
interaction.
Ans. 2: a
EXPLANATION: 9:7 ratio shows that 9 offspring have
both dominant genes,
while 7 is having either one dominant or both
recessive.
Ans. 3:¢
EXPLANATION: Duplication is a type of mutation that
involves the production of
one or more copies of a gene or region of a
chromosome.
Ans. 4: b
EXPLANATION: When black mice CCaa are crossed with
ordinary albinos ccAA, the progeny are agouti CcAa.
Ans. 5: d
EXPLANATION: When two or more genes localized on
different chromozomes
show the same phenotypic action, they are duplicate
genes.
B) TRUE / FALSE:
1. True
2. False
C) FILL UPs:
1. Bateson and Punnet
2. 9:3:4
PART:B SHORT ANSWER TYPE QUESTIONS:
1. What is non-allelic interaction?
2. What are the differences between complementary
and supplementary genes?
PART:C LONG ANSWERS TYPE QUESTIONS:
1. Briefly explain the supplementary genes with the
help of an example.
A44
INTRODUCTION
Genetic linkage describes the way in which two genes
that are located close to
each other on a chromosome are often inherited
together.This is deviation from the Mendelian Principle of independent
assortment, as it is applicable to the genes that are situated in separate
chromosomes.Linkage group is a linearly arranged group of linked genes which
are normally inherited together except for crossing over.
Example- Genes for hair color and eye color are
linked. So certain hair and eye
color genes tend to be inherited together. Such as
blonde's hair with blue eyes and
brown hair with brown eyes.
The term linkage was coined by Morgan (1910) by
performing dihybrid cross
experiments on fruit fly Drosophila melanogaster.
IMPORTANCE. linkage reduces the chance of
recombination of genes and thus
helps to hold parental characteristics together. It
thus helps organism to maintain its parental, racial and other character
linkage.
TYPES OF LINKAGES:---
(il) COMPLETE LINKAGE:
If two or more character are inherent together and
consistently appear in two or
more generation in their original or parental
combination , it is called complete linkage. These genes do not produce non-
parental combination.Genes showing complete linkage are closely located in the
same chromosome.Genes for grey body and long wings in male drosophila show
complete linkage.
(Il) INCOMPLETE LINKAGE:
Incomplete linkage is exhibited by those genes which
produce some percentage of
non-parent combination. Such genes are located
distantly on the chromosome. It is
due to accidental or occasional breakage of
chromosomal segments during crossing over.
FEATURES OF THEORY OF LINKAGE.
Morgan and Castle formulated ‘The chromosome theory of linkage’ Salient
features are:
1. Genes that show linkage are located in same
chromosome.
2. Linkage of genes is linear.
3. The distance between the linked genes is
inversely proportional to the strength
of linkage .Genes which are closely located show
strong linkage ,where as those
which are widely separated have more chance to get
separated by crossing over,is weak linkage.
4. Linked genes remain in their original combination
during course of inheritance.
5. Types of arrangement-
CIS - ARRANGEMENT- If the dominant and recessive
alleles are present on
different chromosomes of homologous pair.
TRANS - ARRANGEMENT- if the dominant and recessive
alleles are present on
both the chromosomes of the homologous pair.
SIGNIFICANCE OF LINKAGES:
In the improvement of plants, linkage plays great
role in determining the nature and scope of hybridization and selection
programs.
It reduces the possibility of variability in gametes
unless crossing over
occurs.
Linkage helps to hold the parental characteristics
together.
Linkage serves as a factor of racial and specific
cohesion rather than as a
barrier between species and between races.
Practical study of sex-linked character for early
determination of sex in the
domesticated fowl has its own significance in the
poultry.
PART (A) VERY SHORT ANSWER TYPE
QUESTIONS
a) MCQs:.-
Q.1. In Drosophila males there is complete
linkage. What is the reason behind this?
a) The genes are very closely located
b) Coupling theory
c) No synapsis
d) Unknown reason
Q.2. Repulsion and coupling are two faces
of:
a) Mutation
b) Chiasmata
c) linkage
d) Crossing
Q.3. Linkage result in
a) Formation of more dominant phenotype
b) formation of more wild phenotype
c) formation of more parental phenotype
d) formation of more recombinant phenotype
b). FILL Ups:
1. Linkage as the distance between two genes .
c) TRUE/FALSE:
1. The tendency of linkage is directly proportional
to the rate of crossing over
between two genes.
a) MCQ:
1. c. No synapsis no recombinations.
2. c. Linkage and Crossing over
3.c. More the linkage more will be the parental
types.
b) FILL UPS:
1. Decrease, Increase
c) TRUE/FALSE
1. False; the tendency of linkage is INVERSELY
proportional to the rate of
crossing over between two genes. More the linkage,
lesser the crossing
over.
PART (B) SHORT ANSWER TYPE QUESTIONS:
Q.1 Explain linkage groups.
Q.2 Differentiate between complete linkage and
incomplete linkage.
Q.3 Give examples of sex linked inheritance in
Drosophila.
PART (C) LONG ANSWER TYPE OF QUESTIONS:
Q. 1 Write salient features of ‘The Chromosome
Theory of Inheritance’.
A45
INTRODUCTION
Recombination is a new arrangement of genes present
in offspring that is different from those of parents due to independent
assortment, crossing over and random combination during fertilization. Thus, it
is clear from its definition that the recombination may be caused by crossing
over.Crossing over is the mutual exchange of segments between non-sister
chromatids of homologous chromosomes in the
pachytene of meiosis-1,producing new combinations of alleles of linked genes.
The non-sister chromatids, in which exchange of
segments has occurred,
are called re-combinants or cross-overs while the
other chromatids in which crossing over has not taken place are known as
parental chromatids or non cross-overs.
Mechanism of Crossing Over Chromosomes replicate
during S-phase of interphase. Therefore,leptotene chromosomes are double
stranded though the two strands are not visible due to the presence of
nucleoprotein complex in between the
chromatids.
(i) Synapsis- Replicated but apparently single
homologous chromosomes come to lie side by side with similar gene loci of the
two chromosomes exactly opposite. It occurs in the
zygotene stage of Prophase !. The phenomenon is called synapsis. The synapsed
pairs of homologous chromosomes are called
bivalents. The small amount of unreplicated chromosome (0.3%), if present, also
undergoes replication (Stern and Hotta, 1973). The
two homologous chromosomes are held together by a synaptonemal complex.
(ii) Tetrad Formation- Soon after completion of
synapsis, the cell
enters Pachytene stage. Each of the homologous
chromosome in a bivalent split longitudinally into two sister chromatids. Thus,
the bivalent now consists of four chromatids and is known as tetrad.
(iii) Crossing over and Chiasma Formation- It occurs
in thePachytene stage. The non-sister chromatids remain in contact at
one or more points. These points of contact are
known as chiasmata (sing. Chiasma). There is breakage of chromatid segment and
then rejoin with the exchange of segments of two non-sister chromatids. After
the completion of crossing over, the
homologous chromosomes move apart.
Types of Crossing Over
Crossing over can be single, double or multiple.
(i) Single Crossing Over- Crossing over occurs at
one point between two non-sister chromatids of a homologous chromosome pair.
There are two parental types and two
recombinants.
(ii) Double Crossing Over- Crossing over occurs at
two points in a homologous pair of chromosomes.
(a) Reciprocal Double Crossing Over. Two points of
crossing over occur between
the same non-sister chromatids.
(b) Complementary Crossing Over. The two crossing
overs involve three or all
the four chromatids so that the number of cross
overs is three or four with the occurrence of one or no parental type.
(iii) Multiple Crossing Over- Three or more points
of crossing over occur in the same homologous chromosome. Double cross-overs
and parental types may or may not occur.
importance of Crossing Over
1. Crossing over is a means of introducing new
combinations of genes and hence traits.
2. It increases variability which is useful for
natural selection and under the changed environment.
3. Since the frequency of crossing over depends upon
the distance between the two genes, the phenomenon is used for preparing
linkage chromosome maps.
4. It has proved that genes lie in a linear fashion
in the chromosome.
5. Breeders have to select a small or large
population for obtaining the required cross-overs. For obtaining cross-overs
between closely linked genes, a very large population is required.
6. Useful recombination produced by crossing over
are picked up by breeders to develop useful new varieties of crop plants and
animals. Green revolution has been achieved in India due to this selective
picking up of useful recombination. Operation flood or white revolution is also
being carried out on the similar lines.
LET US KNOW WHAT WE HAVE LEARNT!
PARTA: VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQs.:-
1. Crossing over in diploid organisms is
responsible for
a) Dominance of genes
b) Linkage between genes
c) Segregation of alleles
d) Recombination of linked alleles
2. Synapsis occurs between
a) Amale and a female gamete
b) mRNA and ribosomes
c) Spindle fiber and centromere
d) Two homologous chromosomes
3. The exchange of genetic material between
Chromatids of paired homologous chromosomes during first meiotic division is
called
a) Transformation
b) Chiasmata
c) Crossing Over
d) Synapsis
4. A bivalent consists of
a) Two chromatids and one centromere
b) Two chromatids and two centromeres
c) Four chromatids and two centromeres
d) Four chromatids and Four centromeres
5. Presence of recombinants is due to
a) Crossing Over
b) Linkage
c) Lack of independent assortment
d) All of the above.
b) FILL UPS. :-—
1. Linked Genes can be separated by .
2. Crossing over produce genetic among offsprings.
c) TRUE/ FALSE:
-
1. Crossing over can occur during either metaphase |
or metaphase Il
2. Synapsis and crossing over occur only in germ
cells.
3. The bivalent with four chromatids is called a
tetrad.
ANSWER KEY- PART-A
a) MCQs::-
1. (d) Hint: Crossing over in diploid organisms is
responsible for recombination of linked alleles.
2. (d) Hint: In Zygotene of prophase+ homologous
chromosomes pair up. This process is called synapsis
3. (c) Hint: In pachytene stage of prophase4 of
meiosis there is breakage and reunion of chromatids, it results in exchange of
segments between non-sister chromatids of a bivalent.
4. (c) Hint: Each pair of homologous chromosomes
carrying one maternal and one paternal chromosomes of similar type is called
bivalent.
5. (a) Hint: Recombination is new arrangement of
genes present is offspring that is different from those of parents due to
independent assortment, crossing over.
b) FILL UPS.:-
1. Crossing Over
Hint: Linked genes can be separated by crossing
over. The genes that occur on the same chromosome are said to be linked.
2. Variations
Hint: The process provides genetic variability in
sexually
reproducing organisms.
c) TRUE/FALSE:--
1. False
Hint : Crossing over occurs in the achytene stage during prophase |
2. True
Hint : Both the process occur during meiosis and
lead to formation of gametes.
3. True
Hint: After Synapsis each of the Homologous
chromosomes ina bivalent split longitudinally into two sister chromatids.
PARTB: SHORT ANSWER TYPE QUESTIONS:
1. What is the importance of crossing over?
2. Explain briefly various types of crossing over.
PARTC: LONG ANSWER TYPE QUESTIONS:
1. a) Define crossing over.
b) Explain the process of crossing over.
A46
INTRODUCTION
SEX DETERMINATION:The cytological observations made
in a number of insects led to the development of the concept of chromosomal
basis of sex determination.
could trace co EE al through the
spermatogenesis, in a few insects and he also
observed that [I He named this structure as X-body but could not explain its
significance.Further investigation by other scientists led to the conclusion
that the X-body described by Henking was in fact a chromosome and it was named
the X
chromosome. Later, EE put forward the a
SEX CHROMOSOMES:These
are the chromosomes, which singly or in a pair determine the sex of individual
in dioecious or unisexual organisms. They are called Allosomes or
Heterosomes.
The normal chromosomes except sex chromosomes, in an
organism are known as
AUTOSOMES.
The latter carry genes which control the SOMATIC TRAITS and have no bearing on
the sex. Two members of each pair of Homologous Autosomes are similar in size
and shape, but this may not be true of sex chromosomes.Chromosome of sex determination is based on
hetero-gametogenesis (formation of two
types of gametes in one sex). Sex of individual is determined at time of
fertilization.
MAIN MECHANISMS OF SEX DETERMINATION
ARE:
(a) XX-XY TYPE:
(i) E.g., Drosophila(Fruit fly) and Mammals (like
Human Beings)
(ii) The females are XX and males are XY. Both
Drosophila and Human chromosome complements consist of Autosomes (AA) and
SexChromosomes (XX in females and XY in males). Female with AA+XX and male with
AA+XY.Min Humans and Drosophila, as PE 2nd is termed ES while Pe Ps and is
called
Pe
(iv) Sex of Offspring is determined at the time of
fertilization. It cannot be changed
later on. Female being homogametic produces only one
type of eggs (22+X).The male gametes are of two types, Androsperms (22+Y) and
Gynosperms (22+X). They are produced in equal proportion.
(v) Fertilization of egg (22+X) with Gynosperm
(22+X) will produce a female child(44+XX) while fertilization with Androsperm
(22+Y) produces a male child
(44+XY).
(vi) Since Y-Chromosome determines the male sex,
it’s also called as Androsome.
(b) XX-XO TYPE:In some insects (e.g., Grasshopper,
Cockroach), the females have two sex chromosomes, XX is Homogametic as it
produces only one type of eggs
(A+X).The male has only one sex chromosome (XO) and
is Heterogametic as it
produces two types of male gametes (sperms).Half of
these carry X-chromosome (A+X) and remaining half are without X-
chromosome (A+0).Fertilization of an egg by X-
bearing sperm, yields female offspring and fertilization by No-X bearing sperm
(O), yields male offspring. The sex ratio
produced in progeny is 1:1
(c) ZW -ZZ TYPE:20th the sexes possess two sex
chromosomes.The female has two Heteromorphic sex chromosome (ZW) and is
Heterogametic, producing two types of eggs, I and I
The male has two Homomorphic sex chromosomes (ZZ) and is Homogametic, produces
one type of sperms (A+Z).
depends on ES thc
(d) ZO-2ZZ TYPE:
Contrary to the ZW — ZZ type, in [iS tic SES So that
the condition in the female is Heterogametic, producing two kinds of eggs,
half with Z chromosome (A+Z) and half without Z
chromosome (A+0).The male has two Z chromosomes (AA+ZZ) and Homogametic
producing one type of sperms
(i) Ithas already been mentioned that the sex
determination in humans is of
MEE ypc.
(ii) | Out of 23 pairs of Chromosomes, MM are
exactly same in both males and females, these are the
(ii) (2 pair of X chromosome is present in the
female (XX).Whereas the presence of an X and a Y chromosome in male (XY) are
determinant of male characteristics.
The females are Homogametic and males are
Heterogametic.
(iv) RRR formed by fern ale Therefore, females are
(v) The males produce two types of gametes or
sperms, (22+X) and (22+Y).Thus, human males are Heterogametic.
Thus the genetic makeup of sperm that determines the
sex of child in
Human beings.
PART: A VERY SHORT ANSWER TYPE
QUESTIONS:
(a) MCQs:
(i) Sex chromosomes of female bird are
represented by:
(a) XO
(b) ZW
(c)XY
(d) ZZ
(ii) Which one of the following conditions
of the zygote would lead to birth of a normal human female child?
(a) Two X chromosomes
(b) Only One Y chromosome
(c) Only One X chromosome
(d) One X and One Y chromosome
(iii) A woman has an X-linked genetic
condition on one of her X chromosomes. This chromosome can be inherited by:
(a) Only daughters
(b) Only sons
(c)Only grandchildren
(d) Both sons and daughters
(iv) Howmany autosomes are found ina single
human sperm?
(a) 23
(b) 24
(c) 22
(d) 22 pairs
(v) Howmany sex chromosomes are found ina
single human sperm?
(a) 1
(b) 2
(c) 3
(d) 4
(b) TRUE/FALSE:
(i) | Henking (1891) could trace a specific nuclear
structure all though spermatogenesis in a few insects.
(ii) Butterflies and moths have XX-XY type mechanism
of sex determination.
(iii) The normal chromosomes, other than sex
chromosomes in an individual are
known as autosomes.
(c) FILL IN THE BLANKS:
(i) The normal chromosomes except the sex
chromosomes in an organism is known as
(ii) The Y-chromosome determines the male sex, it is
also called as .
Ans. 1: (b) ZW
Explanation: Birds have ZW — ZZ type of sex determination.
Females are heterogametic and produce eggs of (A+Z) or (A+W) type. Males are
homogametic and produce only (A+Z) type of sperm.
Ans.2: (a) Two X chromosomes.
Explanation: Humans have XX — XY type of sex
determination. Females are
homogametic i.e., only one type of egg (22+X) and
males are heterogametic
and produce either (22+X) or (22+Y) type of sperms.
Ans.3: (d) Both sons and daughters.
Explanation: Human female genotype is 44+XX and
human male genotype is 44+XY. Females are homogametic (egg is 22+X), thus in
either case, the X-chromosome is inherited from the mother and so is the
genetic condition.
Ans.4: (c) 22
Explanation: Sperm is the male gamete. And gametes
are monoploid i.e.,have only one set of chromosomes. So, they have 22 autosomes
and 1 sex chromosome.
Ans.5: (a) 1
Explanation: Human genotype is diploid, either
(44+XX) or (44+XY). Human sperms are heterogametic having genotype as either
22+X or 22+Y i.e., they have 1 sex determining chromosome.
(i) True
(ii) False
(iii) True
(i) Autosome
(ii) Androsome
PART: B_ SHORT ANSWER TYPE QUESTIONS:
(i) Why is human female referred to as homogametic?
(ii) A man blames his wife for giving birth to a
female child. Is he wrong, if yes,
who's responsible for the sex of the child? Explain.
(iii) Explain the ZW-ZZ type of sex determination?
Give 4 points.
PART:C LONG ANSWER TYPE QUESTIONS:
(i) Explain the mechanism of sex determination in
insects like Drosophila and Grasshopper.
A47
INTRODUCTION
A chart of an individual's Ancestors used in Human
Genetics to analyze Mendelian Inheritance of certain traits, especially of
family diseases.In simple words [MM is alist of the parents and other relations
of an animal or a line of ancestors; a lineage.Pedigrees are used to analyze
the pattern of inheritance of a particular trait throughout a family. Pedigrees
show the presence or absence of a trait as it
relates to the relationship among parents,
offspring, and siblings.Scientists have devised an approach, called pedigree
analysis, to study the inheritance of genes in humans.
Scientists have devised an approach, called pedigree
analysis, to study the
inheritance of genes in humans. Pedigree analysisis
also useful when studying any population when progeny data from several
generations is limited. Pedigree analysis is also useful when studying species
with a long generation time.
READING A PEDIGREE:Pedigrees represent family members and relationships using standardized symbols.
By
analyzing a pedigree, we can determine Genotypes, identify Phenotypes,and
predict how a Trait will be passed on, in the future.
The information from a pedigree makes it possible to
determine, how certain
alleles are inherited: whether they are dominant,
recessive, autosomal,
or sex-linked.
To start reading a pedigree:
1. Determine whether the trait is dominant or
recessive.If the trait is dominant, one of the parents must have the trait.
Dominant traits will not skip a generation. If the trait is recessive, neither
parent is required to have the trait since they can be heterozygous.
2.Determine if the chart shows an autosomal or
sex-linked (usually X-linked) trait.
In X-linked recessive traits, males are much more
commonly affected than
females.In autosomal traits, both males and females
are equally likely to be affected
(usually in equal proportions).
Autosomal Dominant
Trait The diagram shows the inheritance of FRECKLES ( pigmented cells) in a
family. The allele for freckles (F) is dominant to the allele for no freckles
(f).
At the top of the pedigree is a Grandmother
(individual I-2) who has freckles.Two of her three children have the trait
(individuals II-3 and II-5) and three
of her grandchildren have the trait (individuals
III-3, IIl-4, and III-5).
The diagram shows. the inheritance of colorblindness
in a family.Colorblindness is a recessive and X-linked trait XX°The allele for
normal vision is dominant and is represented by XX, and XY.In generation |,
neither parent has the trait, but one of their children (II-3) is colorblind.
Because there are unaffected parentsthathave affected offspring,
it can be assumed that thetrait is recessive. In
addition, the traitappears
toaffect males more than females(in this case,
exclusively malesare
affected), suggesting that thetrait may be X-linked.
COMMON MISTAKES AND MISCONCEPTIONS
1. The presence of many affected individuals in a
family does not always
mean that the trait is dominant.
Seas The terms dominant and recessive refer to the
way that a trait is expressed,
not by how often it shows up in a family. In fact, although
it is uncommon, a
trait may be recessive but still snow up in all
generations of a pedigree.
2. You may not always be able to determine the
genotype of an individual based on a pedigree.pe Sometimes an individual can
either be homozygous dominant or
heterozygous for a trait. Often, we can use the
relationships between an individual and their parents, siblings, and offspring
to determine genotypes.However, not all carriers are always explicitly
indicated in a pedigree, and it may not be possible to determine based on the
information provided.
LET US KNOW WHAT WE HAVE LEARNT!
PART A: VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQs:
b) TRUE/FALSE:
1. Pedigree analysis is study of family record and
inheritance of genetic traits
of humans only.
2. Pedigree analysis is useful for the genetic
counselors to advise intending
couple about the possibility of having children with
genetic defects.
3. There is no use of principle of probability in
pedigree analysis.
c) FILL UPs:
1. Mating is shown by line.
2. Pedigree is tree.
ANSWER KEY: PART-A
a) MCQs:
1.d (These are the Symbols used in pedigree
analysis)
2.a (Thisis the inheritance of Autosomal Recessive
Traits)
3.a (These results will come if female parent ,
(blank circle),is heterozygous)
4.a (Dominant X-linked inheritance)
5.c (Parents are Heterozygous Normal otherwise II-2
could not be Albino. Now cross between 2-heterozygous (Aa) produces, AA(1);
Aa(2); AA(1). Since II-1 is normal,
the chances of a heterozygous (normal) child will be
2 out of 3, i.e. 2/3)
b) TRUE/FALSE:
1. False: lt can be applied on other organisms also.
2. True: This helps the intending couples to decide
further about their
marriage.
3. False: The principle of probability is also
applicable in pedigree analysis.
c) FILL UPs:
1. Horizontal
2. Family
PART B:SHORT ANSWER TYPE QUESTIONS
1. Define Pedigree?
2. What is Pedigree Analysis?
3. Why pedigree charts are used?
4. In the following Pedigree chart, state if the
trait is autosomal dominant,
autosomal recessive or sex linked? Give a reason for
you answer.
PART C: LONG ANSWER TYPE QUESTIONS:
1. Write common mistakes and misconceptions related
to pedigree?
A48
Mutations are unpredictable, stable and inheritable
changes that occur in the
organisms due to permanent changes in the chromosome
number,Chromosome structure and genes. Darwin called them SPORTS and Bateson
called them as Discontinuous Variations.
MUTATION:Mutation
is a phenomenon which results in alteration of DNA sequences and
consequently results in changes in the genotype and
phenotype of an organism. So in simple mutation, there is a change to genetic
material that leads to variation in DNA.Therefore loss (Deletions) or gain
(insertion/duplications) of a segment of
DNA, results in alteration in chromosomes. Since
genes are known to be located on chromosomes, alteration in chromosomes results
in abnormalities or aberrations. Chromosomal aberrations are commonly observed
in Cancer cells.
MUTAGENS:There
are many physical and chemical factors that introduce mutation. These are
referred to as mutagens. For example:
|. Ultra violet radiations
Il. Chemicals
Ill. Infection Agents
TYPES OF MUTATION: Mainly there are
three types of mutation:
A) GENE MUTATIONS
1. Point Mutation or Substitution or Replacement
Mutation.It is called Transition Mutation when Purine is replaced by Purine and
Pyrimidine is replaced by Pyrimidine.It is called Transversion Mutation when
Purine is replaced by Pyrimidine or Pyrimidine is replaced by Purine.A point
mutation is a genetic mutation where a single nucleotide base is
changed, inserted or deleted at a point from a DNA
or RNA sequence of organism’s genome. e.g. Sickle cell Anaemia
In sickle cell anaemia there is a change of single
base pair in the gene for beta
globins chain that results in the change of amino
acid residue glutamate to
vaiine.
2. Erame-shift mutation: In this mutation reading of
the codons is changed due to
insertion or deletion of the nucleotides.
When there is a deletion or insertion of base pairs
of DNA, it causes frame shift mutation. e.g. Tay - Sachs disease.
B) GENOMIC MUTATIONS
These are the mutations caused by the changes in the [J of Chromosomes. eving
ees © one set(n)
of chromosome. Monoploid (1n), Diploid (2n),
Polyploid (3n,4n.....)a saving EE in the chromosome number in the genome
or genomes of the individual. (2n — 1), or (2n+1).
C) CHROMOSOMAL ABERRATIONS
They are the changes in the morphology of
chromosomes, which brings about change in number and sequence of genes over the
chromosomes, without changing Gene structure or ploidy of the chromosomes.
Mutation good or bad?
Beneficially mutations produce genetic variations
which can be beneficial to
population. New proteins or altered activity can be
useful in a changing environment e.g. HIV resistance.
On other side, Mutation that causes dramatic changes
in protein structure of gene activity often disrupts normal biological
activities. e.g.
a) Sickle cell anaemia
b) Cystic fibrosis
PART A: VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQ
1. Point mutation involves:
b) Deletion
c) Insertion
d) Duplication
e) Change in single base pair
2. Transition type of gene mutation is
caused when:
a) GCis released by TA
b) CG is replaced by GC
c) ATis replaced by CG
d) AT is replaced by GC
3. Gene mutation occurs at the time of:
a) DNA repair
b) DNA replication
c) Cell division
d) RNA transcription
4. The action of ultraviolet radiations on
DNA to induce mutation is the:
a) Formation of thymine dimmers
b) Methylation of base pairs
c) Deletion of base pairs
d) Addition of base pairs
5. X- Ray causes mutation by:
a) Deletion
b) Transition
c) Transversion
d) Base substitution
b) FILL IN THE BLANKS:
1) The process of making an RNA copy of a gene is
called .
2) The enzyme used in light repair to split Thymine
Dimers is called,
3) MRNA is one that codes for multiple polypeptides.
c) TRUE / FALSE:
1) Carcinogens are typically mutagenic.
2) More primers are used in lagging strand synthesis
than in leading
strand synthesis.
3) Cells are always producing proteins from every
gene they possess.
a) MCQs:
1. d
2. d
3. b
4. a
5. ab)
FILL IN THE BLANKS:
1. Transcription
2. Photolyase
3. Polycistronic
c) True/false
1. False
2. True
3. False
PART B: SHORT ANSWER TYPE QUESTIONS:
1. Define Mutations?
2. What are Mutagens? Give Example.
3. Is Mutation good or bad?
PART C: LONG ANSWER TYPE QUESTIONS:
1. What are types of Mutations? Explain these.
A49
INTRODUCTION
GENETIC DISORDERS:
Genes are the basic unit of heredity. They hold the
genetic information in the
form of DNA which can be translated into useful
proteins to carry out life
processes. These genes undergo a mutation sometimes,
which changes the instructions to formulate the protein, due to which the
protein does not work properly. Such disorders are known as genetic disorders.
Some disorders are innate, i.e., present by birth,
while others are acquired due
to mutations in a particular gene.
The disorders can be categorized into two types,
namely:
Mendelian Disorders, and Chromosomal Disorders.
WHAT ARE MENDELIAN DISORDERS?
In humans, Mendelian disorder is a type of genetic
disorder primarily resulting
due to alterations in one gene or as a result of
abnormalities in the genome.
Such a condition can be seen since birth and be
deduced on the basis of family
history using the family tree,i.e, Pedigree
Analysis.“Mendelian disorders are the genetic disorders caused at a single
genetic locus.”These genetic disorders are quite rare and may affect one person
in every thousand or a million. Genetic disorders may or may not be inherited.
Inheritable genetic disorders usually occur in the
germ line cells, whereas in
non-inheritable genetic disorders the defects are
generally caused by new
mutations or due to some changes in the DNA.For
instance, Cancer may either be caused by an inherited genetic condition, or by
anew mutation caused by the environmental causes or otherwise.
The different types of Mendelian
disorders include:
1. Autosomal dominant.
2. Autosomal recessive.
3. Sex-linked dominant.
4. Sex-linked recessive.
5. Mitochondrial.
The various types of Mendelian disorders can be
identified easily from the
Pedigree Analysis.Few examples of the Mendelian
disorder in humans are:
Sickle Cell Anaemia * Phenylketonurta
Muscular Dystrophy Colour Blindness
Cystic Fibrosis Skeletal Dysplasia
Thalassemia Haemophilia
HAEMOPHILIA
This is a type of sex-linked recessive disorders.
According to the genetic
inheritance pattern, the unaffected carrier mother
passes on the Haemophilic
Genes to Sons.
It is a very rare type of disease among females
because for a female to get the
disease, the Mother should either be Hemophilic
affected or a carrier but the
Father should be Haemophilic.
In this disorder, iE as the protein which helps in
clotting of blood is affected. Therefore, a person suffering from this disease
usually has symptoms of unexplained and
This type of genetic disorder is caused when the
affected gene is located on the
X- chromosomes. Therefore, males are more frequently
affected.
SICKLE-CELL ANAEMIA
This is a type of Autosomal Recessive Genetic
Disorder.
According to Mendelian genetics, its inheritance
pattern follows inheritance from two carrying parents.
Itis caused when the Giutamic Acid in the sixth
position of the beta-globin chain
of haemoglobin molecule is replaced by Valine. The
Mutant Haemoglobin molecule undergoes a physical change which changes the
Biconcave Shape of RBCs into the Sickle Shape.
This reduces the Oxygen-Binding capacity of the
Haemoglobin molecule.
PHENYLKETONURIA
This genetic disorder is Autosomal Recessive in
nature.
It is an inborn error caused due to the decreased
metabolism level of the
amino acid Phenylalanine.
In this disorder, the affected person does not have
the enzyme that converts
Phenylalanine to Tyrosine. As a result, Phenylalanine
accumulation takes place in the body and is converted into many derivatives
which result in mental retardation.
THALASSEMIA
Itis an Autosomal Recessive disease.
This is a type of disorder in which the body makes
an abnormal amount of
haemoglobin. As a result, a large number of Red
Blood Cells are destroyed that leads to Anaemia.
Facial bone deformities, abdominal swelling, dark
urine are some of the
symptoms of Thalassemia.
CYSTIC FIBROSIS
This is an Autosomal Recessive disorder.
This disease affects the lungs and the digestive
system and the body produces thick and sticky mucus that blocks the lungs and
pancreas.
People suffering from this disorder have a very
short life-span.
PART: A VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQs
1. Which of the following disorder is
called the “Royal disease”?
b) Colour blindness
c) Haemophilia
d) Sickle cell anaemia
e) Alzheimer's disease
2. Which of the following is a type of
autosomal recessive genetic disorder?
a) Haemophilia
b) Skeletal dysplasia
c) Sickle cell anaemia
d) None of these
3. Cystic fibrosis is an autosomal
recessive disorder. What are the chances
that the child would have the disease if
any of the parents is a carrier of
the faulty cystic fibrosis gene (Cc)?
a) 100 %
b) 50 %
C) 25%
d)0%
4. What will be the condition of the
progeny if the father is normal, while the mother has one gene for haemophilia
and one gene for colour blindness on one of the X chromosome?
a) Only daughters are haemophilic and colour blind
b) Both sons and daughters will be haemophilic and
colour blind
c) 50% haemophilic and colour blind sons and 50%
normal sons
d) 50% haemophilic and colour blind daughters and
50% normal daughters
5. If the father in a family has a disease
while mother is normal, the daughters only are inherited by this disease and
not the sons. Name the type of disease?
a) Autosomal recessive
b) Autosomal dominant
c) Sex linked recessive
d) Sex linked dominant
b) True/ False
1. Colour blindness is a sex linked disorder.
2. Haemophilia and colour blindness are Mendelian
disorders.
3. Sickle cell anaemia is caused by a virus.
C) Fillin the blanks
1. Thalassemia is a Disorder.
2. Phenylketonuria is autosomal genetic disorder.
3. Mendelian disorders are the genetic disorders
caused ata genetic-locus.
ANSWER
KEY
PART:
A VERY SHORT ANSWER TYPE QUESTIONS:
a)
MCQs
1. b (Because it affected the royal
families of England, Germany, Russia and Spain).
2. C
3. d (Only one parent is carrier and disorder is
autosomal recessive)
4. c (Itis a type of sex-linked recessive disorder)
5. d
b) True/ False
1. True
2. True
3. False: It is an autosomal recessive genetic
disorder.
C) Fill in the blanks
1. Mendelian
2. Recessive
3. Single
PART: B SHORT ANSWER TYPE QUESTIONS
1. What are genetic disorders?
2. Write a note on Mendelian disorder Sickle cell
anaemia?
3. If aman is haemophilic but woman is normal, what
is possibility of haemophilic son to be born?
ANSWER KEY
PART B: SHORT ANSWER TYPE QUESTIONS
3. Genotype of man is X"Y. He will produce two
types of gametes i) with X" and ii) with Y. Women will produce only one
type of gametes with X chromosome. So sons will born with XY genotype. Hence,
the possibility of haemophilic son to be born is zero percent.
PART: C LONG ANSWER TYPE QUESTIONS
1. What are Mendelian disorders? What are its types?
A50(a)
INTRODUCTION
CHROMOSOMAL DISORDERS:Chromosomal
disorders are caused by the =. 2.0) OF esc Or eons
arrangement of one or more chromosomes.
Aneuploidy - Failure of segregation of chromatids
during cell division resulting in the gain or loss of chromosome is called
aneuploidy.
Total number of chromosomes in humans: 46 (23
pairs).
Total 23 pairs: Autosomes (22 pairs) + Sex
chromosomes (1 pair).
Monosomy: Lack of any one pair of chromosomes.
Trisomy: Inclusion of an additional copy of
chromosome.Some examples of Chromosomal Disorders are as follows:
Down's Syndrome:
The disorder was first described by Langdon Down.
Cause: Down's syndrome occurs due to presence of an
additional copy
of the chromosome number 21.This condition is called
trisomy of 21. So, it is
an example of Autosomal Trisomy.
Here, total number of chromosomes become 47.
Affected individual has short stature, small and
round head, furrowed tongue,
partially opened mouth, palm crease and mental
retardation.
Klinefelter’s Syndrome
Cause: Klinefelters Syndrome is caused due to the
presence of additional copy of X-chromosome, i.e., 47 chromosomes (XXY).So; it
is an example of Trisomy of Sex Chromosome. It is represented by (2n+1).
Affected individual has an overall masculine
development, but feminine development (development of breasts, i.e.
Gynaecomastia) also occur, individual is sterile.
Turner’s Syndrome:
Cause: Turner’s syndrome is a disorder caused due to
the absence of one X- chromosome. In this case, the number of chromosomes is 45
with XO. So, it is an example of Sex Chromosomal Monosomy . It is
represented by (2n-1).
Affected females are sterile, have rudimentary
ovaries, secondary sexual characters are absent, short stature.
LET US KNOW WHAT WE HAVE LEARNT!
PART:(A) VERY SHORT ANSWER TYPE
QUESTIONS:
a) MCQs:-
1) Aneuploidy represents:
A) increase or decrease number of chromosome
B) increase in multiple sets of chromosome
C) deletion of some parts of chromosome
D) addition of some parts of chromosome
2) Missing of one chromosome is known as:
A) monosomy
B) nulliosomy
C) trisomy
D) tetrasomy
3) Increase of one chromosome is known as:
A) monosomy
B) nulliosomy
C) trisomy
D) tetrasomy
4) Main cause of Down syndrome is which of
the following:
A) trisomy of 21st chromosome
B) tetrasomy of 21st chromosome
C) trisomy of 22nd chromosome
D) tetrasomy of 22nd chromosome
5) Representation of trisomic condition is:
A) (2n+1)
B) (2n+2)
C) (2n-1)
D) (2n-2)
b) Fill Ups.:--
1.Down’s syndrome was discovered by .
2. Representation of monosomic condition is
3. causes gynecomastia.
c) True/ False:--
1. Mental retardation in child is main
characteristics of Down's syndrome.
2. Representation of trisomic condition is (2n + 1).
3. Missing of one chromosome is known as trisomy.
ANSWER KEY - PART-A
A) MCQS.:-
1. A) increase or decrease number of chromosome
2. A) monosomy
3. C) trisomy
4. A) trisomy of 21st chromosome
5. A) (2n+1)
b) Fill Ups.:--
1. Langdon Down
2. (2n—- 1)
3. Klinefelter'sSyndrome
c) TRUE/ FALSE:--
1. True
2. True
3. False (Missing of one chromosome is known as
monosomy).
(B) SHORT ANSWER TYPE QUESTIONS:
1. Define Aneuploidy?
2. How monosomic conditions occur?
3. Define Trisomy?
(C) LONG ANSWER TYPE QUESTIONS:
1. What are Chromosomal disorders? Explain any two
Chromosomal disorders?
2. Write a note on Down's syndrome?
A50(b)
RECAPITULATION
Genetics is the study of heredity and variation.
Gene is a unit of heredity equivalent to Mendelian factor,
various forms
of a gene/ pair of contrasting traits is called
alleles.
Mendel, father of genetics studied 7- characters in
Pea and studied inheritance of these characters.
Monohybrid cross: only one character is considered.
Dihybrid cross: two characters are considered.
Back cross: cross between F, generation and either
parent.
> Test cross: cross between F, generation and
recessive parent.
Polygenic inheritance: expression is brought about
by multiple no. of genes. For example: human height.
Pleiotropy: ability of a gene to show multiple
effects. For example:sickle cell anaemia.
Chromosomal theory of inheritance: Sutton and Boveri
argued the pairing and separation of a pair of chromosomes would lead to
segregation of a pair of factors they carried. Sutton united knowledge
of chromosomal segregation with Mendelian principles
and called it Chromosomal Theory of Inheritance.
Linkage is the tendency of genes to remake together
during crossing over. It is an exception to the law of independent assortment.
Linkage group is a haploid set of no. of chromosomes
in an organism.
For example: 23 in man
The more is the distance between two genes, the
lesser is the linkage.
Determination of sex in
Mutation is a phenomenon which results in alteration
of DNA sequences and consequently results in changes in genotype and phenotype
of an organism.
Mutagens are the chemical and physical factors that
induce mutations. For
example: UV radiations.
Pedigree analysis is the study of inheritance of
genetic traits and diseases for two or more ancestral generations of an
individual.
If transmission is from male to male only then trait
is Y linked.
If parents are both none affected but produce both
affected sons, female is
carrier and trait is recessive.
If the trait passes from one parent only then the
trait is dominant.
If transmission is in criss cross pattern i.e, from
male to female and female
to male then trait is X- linked recessive.
Genetic disorders can be Mendelian and Chromosomal.
Mendelian disorders are caused by alteration in
single gene.
Haemophilia is sex linked recessive disease.
Sickle cell anaemia and Phenylketoneuria are
Autosomal Recessive Traits.
Chromosomal disorders are caused due to absence or
excess or abnormal arrangement of one or more chromosomes.
Down syndrome is due to gain of extra copy of
chromosome 21.
Turner syndrome results due to loss of one X-
chromosome in female.
Klinefelter syndrome is caused due to extra copy of
X- chromosome in male.
Now let us do NCERT exercise questions.
NCERT- EXERCISE QUESTIONS. (Q.1 to Q.5)
Question 1:Mention the advantages of
selecting pea plant for experiment by Mendel.
Answer 1:He selected a pea plant because of the
following features.
Peas have visible contrasting characters such as
tall/ dwarf plants.
In pea plants cross pollination can be easily
achieved by emasculation in
which the stamen of the flower is removed without
affecting the pistil.
Pea plants have a short life span and produce many
seeds in one generation.
The traits of each kind of pea plant were presented
in generation after generation because plants had bisexual flowers and normally
resorted to self pollination.
Question 2:Differentiate between the
following:
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.
Question 3:A diploid organism is
heterozygous for 4 loci, how many types of gametes can be produced?
Answer 3:This will be calculated by using formula
2"; where n is the number of loci
i.e. 24 = 16
Question 4:Explain the Law of Dominance
using a monohybrid cross.
Answer 4:Law of Dominance states that one allele of
a pair which control the character usually expresses itself and prevents the
expression of other. This allele which is expressed in a hybrid is called dominant
allele and the one which is unexpressed is called recessive allele.
For example, when pea plants with round seeds (RR)
are crossed with plants with
wrinkled seeds (rr), all seeds in F; generation were
found to be round (Rr). When these round seeds were self-fertilized, both the
round and wrinkled seeds appeared in Fz generation in 3:1 ratio. Hence, in F;
generation, the dominant character (round seeds) appeared and the recessive
character (wrinkled seeds) got suppressed,which reappeared in Fz generation.
Question 5:Define and design a test -
cross?
Answer 5:Test cross is a cross between an organism
with unknown genotype and a recessive parent. It is used to determine whether
the individual is homozygous or
heterozygous for a trait.If the progenies produced
by a test cross show 50% dominant trait and 50% recessive trait, then the
unknown individual is heterozygous for a trait. On the other hand, if the
progeny produced shows dominant trait, then the unknown individual is
homozygous for a trait.
A50(c)
INTRODUCTION
Hello students, we have discussed questions from 1
to 5 in the previous assignment. Now let us work on the next exercise questions
of the chapter; Principles of Inheritance and Variation.
Question 7:When a cross in made between
TALL PLANTS with YELLOW SEEDS (Tt Yy)
and TALL PLANT with GREEN SEED (Tt yy), what
proportions of phenotype in
the offspring could be expected to be?
(a) Tall and green.
(b) Dwarf and green.
Question 8:Two HETEROZYGOUS parents are
crossed. If the two loci are LINKED what would be the distribution of
phenotypic features in Fi generation for a Dihybrid Cross?
Question 9:Briefly mention the contribution
of T.H. Morgan in genetics.
Answer 9:Morgan's work is based on FRUITFLY (Drosophila
melanogaster). He formulated
the chromosomal theory of linkage. He defined
linkage as the co-existence of two or more genes in the same chromosome and
performed Dihybrid crosses in Drosophila to show that linked genes are
inherited together and are located on X-
chromosome. His experiments have also proved that
tightly linked genes show very
low recombination while loosely linked genes show
higher recombination.
Question 10:What is pedigree analysis?
Suggest how such an analysis, can be useful.
Answer 10:Pedigree analysis is a record of
occurrence of a trait in several generations of a family. It is based on the
fact that certain characteristic features are heritable in a family, for
example, eye colour, skin colour, hair form and colour, and other facial
characteristics. Along with these features, there are other genetic disorders
such as Mendelian disorders that are inherited in a family, generation after
generation.Hence, by using pedigree analysis for the study of specific traits
or disorders,generation after generation, it is possible to trace the pattern
of inheritance. In this
analysis, the inheritance of a trait is represented
as a tree, called family tree.
Genetic counselors use pedigree chart for analysis
of various traits and diseases in
a family and predict their inheritance patterns.
Question 11:How is sex determined in human
beings?
Answer 11:Sex determination refers to the mechanisms
employed by organisms to produce off springs that are of two different sexes.
The sex of an individual is determined by the genetic information present in
the individual's sex chromosomes. Sex determination in human is done by XY type
chromosome. In humans, females have two XX chromosomes and males have two
different chromosomes (XY).
Question 12:A child has blood group O. If
the father has blood group A and mother blood group B, work out the genotypes
of the parents and the possible genotypes of the other offsprings.
So the genotype of parents of a child
having blood group O will be
I“i(male) and 1°i (female).
Question 13:Explain the following terms
with example :
(a) Co-dominance
(b) Incomplete dominance
Answer 13:
(A) CO-DOMINANCE
Co-dominance is the phenomenon in which both the
alleles of a contrasting
character are expressed in heterozygous condition.
Both the alleles of a gene are
EQUALLY DOMINANT. ABO blood group in human beings is
an example of co-
dominance. The AB blood group character is
controlled by three sets of alleles,
namely, IA, |B, and i. The alleles, [A and |B, are
equally dominant and are said to be
co-dominant as they are expressed in AB blood group.
Both these alleles do not
interfere with the expression of each other and
produce their respective antigens.
Hence, AB blood group is an example of co-dominance.
(B) INCOMPLETE DOMINANCE
Incomplete dominance is a phenomenon in which one
allele shows incomplete
dominance over the other member of the allelic pair
for a character. For example, a monohybrid cross between the plants having red
flowers and white flowers in
Antirrhinum species will result in all pink flower
plants in F; generation. The progeny
obtained in F; generation does not resemble either
of the parents and exhibits
intermediate characteristics. This is because the
dominant allele, R, is partially dominant over the other allele, r. Therefore,
the recessive allele, r, also gets expressed in the F, generation resulting in
the production of intermediate pink flowering progenies with Rr genotype.
Question 14:What is point mutation? Give
one example.
Answer 14:Point mutation is a change in a single
base pair of DNA by substitution, deletion, or insertion of a single
nitrogenous base.An example of point mutation is Sickle Cell Anaemia.
Question 15:Who had proposed the
chromosomal theory of inheritance?
Answer 15:Sutton and Boveri proposed the chromosomal
theory of inheritance in 1902.
Question 16:Mention any two autosomal
genetic disorders with their symptoms.
Answer 16:Down Syndrome : It is an autosomal
disorder that is caused by the trisomy of chromosome 21.
Symptoms
The individual is short statured with round head,
open mouth, protruding tongue,
short neck, slanting eyes, and broad short hands.
The individual also shows
retarded mental and physical growth.
Phenylketonuria : It is a Pleiotropic inborn error
of metabolism caused by
Autosomal Recessive allele present on Chromosome
no.12. Due to non-formation
of enzyme Phenylalanine hydroxylase. Phenylalanine
does not get converted into
Tyrosine.So Phenyl alanine get accumulated in the
body, causing disease.
Symptoms
Mental retardation (due to accumulation of
phenylalanine in brain), hypopigmentation
of skin & hair, eczema etc.
