NCERT Solutions For Class 12 | Biology | Chapter 5 | Principles of Inheritance and Variation

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NCERT Solutions for Class 12 Biology Chapter 5 PRINCIPLE OF INHERITANCE AND VARIATION is the essential study material needed to perfect PRINCIPLE OF INHERITANCE AND VARIATION topics.

NCERT Class 12 Biology solutions provided here have correct answers to NCERT textbook questions. Solutions curated in a comprehensive manner will help students to understand the subtopics in this chapter in a better way.


NCERT Solutions for Class 12 Biology Chapter 5 PRINCIPLE OF INHERITANCE AND VARIATION has the following sub-topics as given below:

Sr. noTopics
1Mendel’s Laws of Inheritance
2Inheritance of One Gene
3Sex Determination
4Polygenic Inheritance
7Genetic Disorders


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

Answer – The advantages of selecting a pea plant for the experiment by Mendel were as follows;

  • It is an annual plant with a short life span. Therefore, the generations can be studied faster.
  • The plant does not require much care except during pollination, making it an easy subject of test.
  • F1 hybrids of the pea plant are fertile.
  • Seven pairs of contrasting characters were easily detectable and seen by the naked eye.
  • It is a self-pollinated crop.

2. Differentiate between the following

  • (a) Dominance and Recessive
  • (b) Homozygous and Heterozygous
  • (c) Monohybrid and Dihybrid.

Answer –

(a) Dominance and Recessive –

Sr. no.DominanceRecessive
1The dominant allele expresses itself in both heterozygous and homozygous conditions.A recessive trait can express itself in a homozygous condition only.
2Example- In the pea plant, round seeds are the dominant character.Example- In the pea plant, the white flower is the recessive trait.

(b) Homozygous and Heterozygous

Sr, no.Homozygous Heterozygous
1For a trait, homozygous means two similar allelesFor a trait, heterozygous means two different alleles.
2In this type, one type of gamete is produced during the cross.In this type, different types of gametes are produced during the cross.
3For homozygous, the genotype contains either recessive or dominant but never both alleles.For heterozygous, the genotype contains both recessive and dominant alleles.
Ex. TT or ttTt

(c) Monohybrid and Dihybrid

Sr. no. MonohybridDihybrid
1Cross between parents with difference of only one pair of contrasting characters.Cross between parents with difference of two pair of contrasting characters.
Ex. Cross between tall and dwarf parents.Cross between yellow wrinkled seed and green rounded seed parents.

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

Answer –

  • Locus- A fixed point on a chromosome that is occupied with one or more genes.
  • For an allelic pair, heterogygous entity contain different alleles.
  • Therefore, a diploid organism which is heterozygous for 4 loci; will have four different contrasting characters at four different loci, i.e., 24 =16 gametes.

4. Explain the Law of Dominance using a monohybrid cross.

Answer – Mendel proposed the Law of Dominance from the observations he made in his infamous pea genetical experiment. It states that:

  • Characters are controlled by discrete units called factors.
  • Factors occur in pairs.
  • In heterozygous conditions, one member of the pair is dominant while the other is recessive.

The law of dominance was able to explain the expression of only one character in F1 in a monohybrid cross and the expression of both in F2. It can also explain the proportion of 3:1 in the F2 generation.

For example- A monohybrid cross between two pea plants, one having round seeds (RR) and other having wrinkled seeds (rr) gives all round seeds (Rr) in the F1 generation. If selfing is done with round seeds (Rr) from F1 generation, both the parents character express themselves with round and wrinkled seeds in the ratio of 3:1 respectively. This is because the recessive wrinkled character that got suppressed in the F1 generation resurfaced in the F2 due to homozygousity in one of the offspring.

5. Define and design a test cross.

Answer –

It is a cross betwee the F1 hybrid expressing dominant phenotype with its homozygous recessive known parent to determine if the trait is homozygous or heterozygous.


Therefore, the F1 hybrid is heterozygous in condition as both the dominant and recessive characters express themselves.

6. Using a Punnett Square, work out 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.

Answer – Parents:

  • Homozygous female= either TT or tt
  • Heterozygous male for a single locus= Tt

Phenotypic ratio= All tall
Genotypic ratio= TT:Tt 2:1

7. When a cross is 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.
  • (b) dwarf and green.

Answer – A cross between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy) is as follows;

Phenotypic observation is;

  • Three tall green plants
  • One dwarf 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?

Answer –

  • Linkage is the co-existence of two or more genes in the same chromosomes. Linked genes are inherited together.
  • If two heterozygous parents show linkage, then the cross would be as follows;

9. Briefly mention the contribution of T.H. Morgan in genetics.

Answer – The contribution of T.H. Morgan in the field of genetics is;

  • He proposed and established that genes are positioned on the chromosomes
  • He gave the chromosome theory of linkage.
  • He selected fruit flies as research material in experiments.
  • He with Sturtevant found that recombination between two linked genes is directly proportional to the distance between the two.
  • He worked on sex-linked inheritance and criss-cross inheritance.

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

Answer –

  • Pedigree- It is a record of inheritance of a specific genetic trait for generations which is presented in the form of a diagram or family tree.
  • Pedigree analysis- An analysis of several generations of a family which helps in the detection of genetic disorders.
  • The usefulness of pedigree analysis;
    • analysis of the transmission of character in a family
    • Identifying a genetic disorder due to a recessive or dominant gene.
    • To identify the origination of a trait and its flow in a family or population.
    • Helpful in reasoning that marriage between close relatives is harmful.

11. How is sex determined in human beings?

Answer –

  • The sex determination in human beings is of the genotype XX-XY type.
  • Human beings have 23 pairs of chromosomes, out of which 22 pairs are autosomes and one pair is a sex chromosome which varies in both males and females.
  • Females are homogametic, i.e. they produce one type of gamete with an X chromosome whereas males are heterogametic, i.e. they produce two types of gametes with either X or Y chromosomes.
  • If the sperm containing an X chromosome fertilizes the egg, then the offspring will be female (XX).
  • If the sperm containing the Y chromosome fertilizes the egg, then the offspring will be male (XY).

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 offspring.

Answer – A set of three alleles- IA , IB, and i control the blood group in humans. IA and IB are dominant alleles and i is a recessive allele.

Individuals with GenotypeBlood group 
IA, IA, IB, iA
IB, IB, IB, iB

If the father has blood group A and mother blood group B, then the possible genotypes of the parents will be as follows:

13. Explain the following terms with example

  • (a) Co-dominance
  • (b) Incomplete dominance

Answer –

  • Co-dominance- The phenomenon where two alleles are able to express themselves independently and together in a heterozygous condition is called co-dominance.
    • Example- In the AB blood group, alleles IA and IB are able to express themselves together dominantly.
  • Incomplete dominance- The phenomenon where either of two contrasting pairs of dominant alleles is not able to express themselves together. The F1 hybrid is a mixture of both factors.
    • Example- Mirabilis jalapa produces pink flowers in a hybrid cross between a red and white flower.

14. What is point mutation? Give one example.

Answer –

  • Mutation arising due to a change like inversion or substitution in a single base pair of DNA without any alteration in the succeeding base pairs is known as a point mutation. For example- Sickle cell anaemia
  • Sickle cell anaemia is caused due to substitution of guanine nitrogen base to adenine at the sixth codon of the beta-globin chain of the haemoglobin molecule. A change in RBCs from biconcave discs to elongated sickle-shaped structures.

15. Who had proposed the chromosomal theory of the inheritance?

Answer – Walter Sutton and Theodore Boveri proposed the Chromosomal Theory of Inheritance independently in 1902. They both proposed that the chromosomes were the carriers of Mendelian factors. And its chromosomes segregate and assort independently during meiosis and recombine during fertilisation. This theory was then expanded by Morgan, Sturtevant, and Bridges.

16. Mention any two autosomal genetic disorders with their symptoms.

Answer – When a genetic defect is found in the autosomal chromosomes, the disorder is called an autosomal genetic disorder. Some of the autosomal disorders are- Down’s syndrome, Sickle cell anaemia, and Phenylketonuria.

  • Flat hands, short neck, broad forehead and a partially open mouth with furrowed tongue.
  • Mongolian-type eye fold and stubby fingers,
  • Mental and physical retardation and underdeveloped gonads leading to infertility.
  • Shape of RBCs change from elongated to sickle-shape due to low oxygen
  • Haemoglobin does less oxygen transport.
  • These sickle-shaped RBCs get destroyed more rapidly than normal ones causing anaemia.

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