NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Biology
Chapter : 5
Chapter Name : Principles of Inheritance and Variation

Q1 Mention the advantages of selecting pea plant for experiment by Mendel.

Answer. Mendel selected pea plants to carry out his study on the inheritance of characters from
parents to offspring. He selected a pea plant because of the following features.
(a) Peas have many visible contrasting characters such as tall/dwarf plants, round/wrinkled seeds,
green yellow pod, purple/white owers, etc.
(b) Peas have bisexual owers and therefore undergo self pollination easily. Thus, pea plants
produce offsprings with same traits generation after generation.
(c) In pea plants, cross pollination can be easily achieved by emasculation in which the stamen of
the ower is removed without affecting the pistil.
(d) Pea plants have a short life span and produce many seeds in one generation.

Q2 Differentiate between the following -
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.

Answer. (a) Dominance and Recessive

(b) Homozygous and Heterozygous

(c) Monohybrid and Dihybrid.

Q3 A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Answer. Locus is a xed position on a chromosome, which is occupied by a single or more genes.
Heterozygous organisms contain different alleles for an allelic pair. Hence, a diploid organism,
which is heterozygous at four loci, will have four different contrasting characters at four different
loci. For example, if an organism is heterozygous at four loci with four characters, say Aa, Bb, Cc,
Dd, then during meiosis, it will segregate to form 8 separate gametes. If the genes are not linked,
then the diploid organism will produce 16 different gametes. However, if the genes are linked, the
gametes will reduce their number as the genes might be linked and the linked genes will be
inherited together during the process of meiosis.

Q4 Explain the Law of Dominance using a monohybrid cross.

Answer. Mendel's law of dominance states that a dominant allele expresses itself in a monohybrid
cross and suppresses the expression of recessive allele. However, this recessive allele for a
character is not lost and remains hidden or masked in the progenies of Fl generation and
reappears in the next generation. For example, when pea plants with round seeds (RR) are crossed
with plants with wrinkled seeds (rr), all seeds in Fl generation were found to be round (Rr). When
these round seeds were self fertilized, both the round and wrinkled seeds appeared in F2
generation in 3: 1 ratio. Hence, in Fl generation, the dominant character (round seeds) appeared
and the recessive character (wrinkled seeds) got suppressed, which appeared in F2 generation.

Q5 De ne and design a test — cross?

Answer. 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.

Q6 Using a Punnett square, work out the distribution of phenotypic features in the rst lial
generation after a cross between a homozygous female and a heterozygous male for a single locus.

Answer. In guinea pigs, heterozygous male with black coat colour (Bb) is crossed with the female
having white coat colour (bb). The male will produce two types of gametes, B and b, while the
female will produce only one kind of gamete, r. The genotypic and phenotypic ratio in the
progenies of Fl generation will be same i.e., 1:1.

Q7 When a cross in made between tall plants with yellow seeds (TtYy) and tall plant with green
seed (TtYy), what proportions of phenotype in the offspring could be expected to be
(a) Tall and green.
(b) Dwarf and green.

Answer. A cross between tall plant with yellow seeds and tall plant with green seeds will produce
(a) three tall and green plants
(b) one dwarf and green plant

Q8 Two heterozygous parents are crossed. If the two loci are linked what would be the distribution
of phenotypic features in Fl generation for a dihybrid cross?

Answer. Linkage is de ned as the coexistence of two or more genes in the same chromosome. If
the genes are situated on the same chromosome and lie close to each other, then they are
inherited together and are said to be linked genes. For example, a cross between yellow body and
white eyes and wild type parent in a Drosophila will produce wild type and yellow white progenies.
It is because yellow bodied and white eyed genes are linked. Therefore, they are inherited together
In progenies.

Q9 Brie y mention the contribution of T.H. Morgan in genetics.

Answer. Morgan's work is based on fruit ies (Drosophila melanogaster). He formulated the
chromosomal theory of linkage. He de ned 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.

Q10 What is pedigree analysis? Suggest how such an analysis, can be useful.

Answer. 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 speci c
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. It is useful in preventing hemophilia, sickle cell anemia, and other
genetic disorders in the future generations.

Q11 How is sex determined in human beings?

Answer. Human beings exhibit male heterogamy. In humans, males (XY) produce two different
types of gametes, X and Y. The human female (XX) produces only one type of gametes containing
X chromosomes. The sex of the baby is determined by the type of male gamete that fuses with the
female gamete. If the fertilizing sperm contains X chromosome, then the baby produced will be a
girl and if the fertilizing sperm contains Y chromosome, then the baby produced will be a boy.
Hence, it is a matter of chance that determines the sex of a baby. There is an equal probability of
the fertilizing sperm being an X or Y chromosome. Thus, it is the genetic makeup of the sperm
that
determines the sex of the baby.

Q12 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.

Answer. The blood group characteristic in humans is controlled by three set of alleles, namely, I , A

I , and i. The alleles, I andI , are equally dominant whereas allele, i, is recessive to the other
B A B

alleles. The individuals with genotype, I I and I i, have blood group A whereas the
A A A

individuals with genotype, I I and I i, have blood group B. The persons with genotype I I
B B B A B

have blood group AB while those with blood group O have genotype ii. Hence, if the father has
blood group A and mother has blood group B, then the possible genotype of the parents will be
Father Mother
I
A
I
A
or I i I I or I i
A B B B

A cross between homozygous parents will produce progeny with AB blood group.

A cross between heterozygous parents will produce progenis with AB blood group (IA IB) or O
blood group (ii).

Q13 Explain the following terms with example
(a) Codominance
(b) Incomplete dominance

Answer. (a) Codominance
Codominance 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 codominance. The blood group character is controlled by
three sets of alleles, namely, I , I , and i. The alleles, I and I , are equally dominant and are
A B A B

said to be codominant 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 codominance.
(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 owers and white owers in Antirrhinum species will result in all pink ower
plants in Fl generation. The progeny obtained in Fl 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 Fl.
generation resulting in the production of intermediate pink owering progenies with Rr genotype.

Q14 What is point mutation? Give one example.

Answer. 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. It
involves mutation in a single base pair in the beta-globin chain of haemoglobin pigment of the
blood. Glutamic acid in short arm of chromosome II gets replaced with valine at the sixth position.

Q15 Who had proposed the chromosomal theory of inheritance?

Answer. Sutton and Boveri proposed the chromosomal theory of inheritance in 1902. They linked
the inheritance of traits to the chromosomes.

Q16 Mention any two autosomal genetic disorders with their symptoms.

Answer. Two autosomal genetic disorders are as follows.
(a) Sickle cell Anaemia

It is an autosomal linked recessive disorder, which is caused by point mutation in the beta-globin
chain of haemoglobin pigment of the blood. The disease is characterized by sickle shaped red
blood cells, which are formed due to the mutant haemoglobin molecule. The disease is controlled
by HbA and HbS allele. The homozygous individuals with genotype, HbS HbS, show the symptoms
of this disease while the heterozygous individuals with genotype, HbA HbS, are not affected.
However, they act as carriers of the disease.
Symptoms Rapid heart rate, breathlessness, delayed growth and puberty, jaundice, weakness,
fever, excessive thirst, chest pain, and decreased fertility are the major symptoms of sickle cell
anaemia disease.
(b) Down's 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.