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NCERT Solutions for Class 12 Biology Chapter 5 - Principles of Inheritance and Variation

Aakash NCERT Solutions for Chapter 5 of Class 12 Biology NCERT book teaches students about topics related to Inheritance and Variation. It also talks about Mendel’s Laws of Inheritance, Inheritance of One Gene, Inheritance of Two Genes, Sex Determination, Mutation, and Genetic Disorders.

1.Mention the advantages of selecting pea plant for experiment by Mendel.
The advantages of selecting garden pea ( Pisum sativum ) for experiments by Mendel were-
(a)Pea has many visibly distinct contrasting characters.
(b)The life span of the pea plant is short and they produce many  seeds.
(c)Pea flowers are bisexual and show self-pollination, reproductive whorls being enclosed  by corolla.
(d)It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile.

2.Differentiate between the following
(a)Dominance  and Recessive



One alleles in gene pair will be dominant over the other allele and it is called the dominant allele

The recessive allele will not express its effect over dominant allele

The dominant allele is represented in capital letters

The recessive allele is represented in small letters.

(b)heterozygous and homozygous



It contains different alleles for traits.

Both dominant and recessive

It contains only one type alleles either dominant or recessive.

Two types of gametes are produced

Only one type of gamete is produced.

Heterozygous alleles show complete dominance, codominance or incomplete dominance

It can be either homozygous dominant or homozygous recessive.

It contains two different copies of alleles coding for a particular trait.

It has two same copies of the same allele coding for a particular trait.

(c)Monohybrid and Dihybrid



It involves a cross between

individuals differing in only

one pair of characters

It  is  a  cross  between  individuals

differing in two pairs of contrasting


The cross between tall (TT)

and dwarf (tt) pea plants is


The  cross  between  plants  having

yellow  round  (YYRR)  seeds  with

those having green wrinkled (yyrr)

seeds is dihybrid

3.A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Loci refer to the place of the chromosome where genes are positioned. So, we can say that loci and genes are synonyms of each other. If a diploid organism is heterozygous for 4 loci then it will have four contrasting traits Aa, Bb, Cc, Dd and during meiosis 16 different kinds of gametes will be formed.

4.Explain the Law of Dominance using a monohybrid cross.
According to Mendel’s law of dominance, traits are controlled by two different forms of the same gene. These alternative forms of genes are called alleles. Among the two alleles, one is dominant while the other one is recessive. The dominant allele suppresses the expression of the recessive allele. Therefore whenever the two alleles are present together, the dominant one masks the expression of the recessive allele.
However, it doesn't mean that the recessive allele has lost its existence. It remains hidden in the F1 generation and reappears in the next generation. A monohybrid cross is given below.In this cross, it can be seen that in F1 generation only tall plants were seen, no plant was dwarf. However in the F2 generation, the F1 progeny was self crossed, three genotypes were observed, among these, the hybrids were showing the dominant trait.

5.Define and design a test-cross
A test cross is a cross between an individual showing a dominant trait with the one having its homozygous recessive trait in order to know whether the dominant trait is homozygous or heterozygous. If the ratio of a test cross is 1:1, it shows that the dominant trait is heterozygous.

6.Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
In guinea pigs, a homozygous female with white coat color (bb) is cross with the heterozygous male having black coat color (Bb). The male gamete will produce two types of gametes i.e. B and b while females will produce only one type of gamete which is b. The F1 progeny will show both individuals with black coat color and white coat color in ratio of 1:1. Here, the phenotypic, as well as the genotypic ratio, will be 1:1.

Parents(male/female) Bb bb
Gametes B b b
Crossing Bb : bb
Black coat color : White coat color
Genotypic ratio- Bb:bb 1:1
Phenotypic ratio- Black coat color : white coat color 1:1

7.When a cross in made between tall plant 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 and (b) dwarf and green
The cross between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), will look like

-Hence, there will be -
(a) 3 plants showing the trait tall plant and green plant
(b) 1 plant showing the trait dwarf and green plant.

8.Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?
Link gene refers to the physical association of genes on the same chromosomes. The genes that show linkage are called linked genes. These linked genes show the same kind of inheritance pattern and are inherited together in the next generation. 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 progeny. The percentage of parental types and recombinant types is 98.7% and 1.3% respectively.

9.Briefly mention the contribution of T.H. Morgan in genetics.
Contribution of T.H. Morgan in genetics analysis.
(a)T.H Morgan discovered the chromosomal theory of inheritance.
(b)He carried out many dihybrid crosses on Drosophila to study sex-linked disease.
(c)Morgan discovered the phenomena called linkage to define the physical association of genes and recombination to describe the origin of non-parental gene combination.

10.What is pedigree analysis? Suggest how  much analysis can be useful.
The analysis of inherited traits in several generations of a family is called the pedigree analysis. The inheritance of a particular trait is represented in a family tree over several generations. It is used to trace the inheritance of a particular trait, abnormality and disease. By pedigree analysis, genetic counselors can help in preventing certain genetic disorders like hemophilia, sickle cell anemia in the future generation of that family.

11.How is sex determined in human beings?
Sex determination in human beings:
In humans, out of 23 pairs of chromosomes, 22 pairs are exactly the same in males and females called autosomes. However, a pair of the X chromosome is present in females and XY in males. During spermatogenesis, male produce two types of gametes (sperms) 50% carries Y chromosome and the remaining 50% contain X chromosome. On the other hand, females produce only one kind of gamete (ovum) having X chromosomes only. When sperm having Y chromosome fertilizes the ovum the sex of the baby is male and when sperm carrying X chromosome fertilizes the egg, the sex of the baby is female.

12.A child has blood group O.If father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.
If the child has O blood group then it is evident that parents are heterozygous. Thus, the genotype of the father will be IAi and that of the mother will be IBi . The child having blood group O will have genotype ii . The other children may have genotypes IAi (A blood group),   IBi (B blood group) and IAIB (AB blood group).

13.Explain the following terms with example
(a) Codominance
It refers to the phenomenon of two alleles lacking dominance-recessive relationship and both expressing themselves in the heterozygous condition. In human beings, ABO blood grouping is controlled by gene I . The gene has three alleles IA, IB and i. One person may have any two of these three alleles. Among these alleles, IA, IB are dominant over i . When IA and IB are present together, both express themselves because of codominance.

(b)Incomplete dominance
Incomplete dominance is the phenomenon of neither of the two alleles being dominant so that expression in the hybrid is a fine mixture or intermediate between the expressions of two alleles. In snapdragon ( Mirabilis jalapa) , there are two types of pure breeding plants, red flowered and white flowered. On crossing the two, F1 plants possess pink flowers. Onselfing them, the F2 generation has 1red: 2 pink: 1white. The pink flower is due to incomplete dominance.

14.What is point mutation? Give one example
Point mutation - When a mutation occurs in a single base pair of DNA, it is termed as a point mutation.e.g. Sickle cell anemia.

15.Who had proposed the chromosomal theory of inheritance?
Sutton and Baveri in 1903 proposed the chromosomal theory of inheritance.

16.Mention any two autosomal genetic disorders with their symptoms.
The two autosomal genetic disorders are as follows
1.Sickle cell anemia – It is an autosome linked recessive trait in which mutant hemoglobin molecules undergo polymerization under low oxygen tension causing a change in the shape of the RBC from biconvex disc to elongated sickle like structure. The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the hemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta-globin gene from GAG to GUG. Symptoms include rapid heart rate, breathlessness, delayed growth and puberty, weakness, fever, decreased fertility etc.
2.Down syndrome - It is an autosomal genetic disorder caused by trisomy of chromosome 21. Symptoms of Down syndrome include short stature with a round head, protruding tongue, slanting eyes, broad short hands etc.


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