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

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NCERT Biology Chapter 5 Principles of Inheritance and Variation Solved Textbook Questions

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

Ans- The advantages of selecting garden pea (Pisum sativum) for experiments by Mendel were –

  1. Pea has many visibly distinct contrasting characters.
  2. The life span of the pea plant is short and they produce many seeds in one generation.
  3. It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile.
  4. Pea flowers are bisexual and show self-pollination, reproductive whorls being enclosed by corolla.

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

Ans- (a) Dominance and Recessive

The phenomenon wherein a factor (allele) expresses itself in the presence or absence of its dominant factor, is called dominance.It can only express itself in the absence of its dominant factor.
It forms a complete functional enzyme that perfectly expresses it.It forms an incomplete defective enzyme which fails to express itself when present with its dominant allele, i.e., in heterozygous condition.

(b) Homozygous and Heterozygous

It contains two similar alleles for a particular trait.It contains two different alleles for a particular trait.
Genotype for homozygous possess either dominant or recessive, but never both the alleles. For example, RR or rrGenotype for heterozygous possess both dominant and recessive alleles. For example, Rr
It produces only one type of gamete.It produces two different kinds of gametes.

(c) Monohybrid and Dihybrid

Monohybrid CrossDihybrid Cross
Monohybrid Cross involves cross between parents, which differs in only one pair of contrasting characters.Dihybrid Cross involves cross between parents, which differs in two pairs of contrasting characters.
For example, the cross between tall and dwarf pea plant is a monohybrid cross.For example, the cross between pea plants having yellow wrinkled seed with those having green round seeds is a dihybrid cross.

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

Ans- Locus is a fixed 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.

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

Ans- According to Mendel’s law of dominance. traits are controlled by two different forms of the same gene. These alternative forms of a gene 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 F1 generation and reappears in the next generation. A monohybrid cross is given below.

Principles of Inheritance and Variation

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.

5. Define and design a test-cross.

Ans- 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 homozenous for a trait.


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.

Ans- In guinea pigs, homozygous female with white coat colour (bb) is cross with the heterozygous male having black coat colour (Bb). The male gamete will produce two types of gametes i.e. B and b while female will produce only one type of gamete which is b. The F1 progeny will show both individuals with black coat colour and white coat colour in a 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 colour : White coat colour
  • Genotypic ratio- Bb:bb 1:1
  • Phenotypic ratio- Black coat colour : white coat colour 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.
(b) dwarf and green.

Ans- The cross between a tall plant with yellow seeds (TtYy) and tall plant with green seeds (Ttyy) is as follows:

Principles of Inheritance and Variation

Hence, there will be plants showing three tall and green seed trait.

(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 only one plant with dwarf and green seed trait.

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 dibybrid cross?

Ans- In case of two heterozygous parents, showing linkage, result will be:

                Parents:       BpLl   X    BpLl                    

    Genotype                 (Blue long)          (Blue long)     

Phenotype In f1 all the combinations may show parental characters as the genes are completely linked. With all possible genotypes in f1, progeny may exhibit blue long type of phenotype in above case. However in case of incomplete linkage, parental combinations will be more and new combinations will be less in number.

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

Ans-  Morgan’s work is based on fruit flies (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.

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

Ans- 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. It is useful in preventing hemophilia, sickle cell anemia, and other genetic disorders in the future generations.

11. How is sex determined in human beings?

Ans- Chromosomal determination of sex in human beings is of XX-XY type. Human beings have 22 pairs of autosomes and one pair of sex chromosomes. The female possess two homomorphic (= isomorphic) sex chromosomes, named XX. The males contain two heteromorphic sex chromosomes, i.e., XY. All the ova formed by female are similar in their chromosome type (22 + X). Therefore, females are homogametic. The male gametes or sperms produced by human males are of two types, gynosperms (22 + X) and androsperms (22 + Y). Human males are therefore, heterogametic. Sex of the offspring is determined at the time of fertilization. Fertilization of the egg (22 + X) with a gynosperm (22 + X) will produce a female child (44 + XX) while fertilization with an androsperm (22 + Y) gives rise to male child (44 + XY). As the two types of sperms are produced in equal proportions, there are equal chances of getting a male or female child in a particular mating. As Y-chromosomes determines the male sex of the individual ,it is also called as androsome.

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.

Ans– The blood group characteristic in humans is controlled by three set of alleles, namely, IA, IB, and i. The alleles, IA and IB, are equally dominant whereas allele, i, is recessive to the other alleles. The individuals with genotype, IA IA and IA i, have blood group A whereas the individuals with genotype, IB IB and IB i, have blood group B. The persons with genotype IA IB 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

IA IA or IA i IB IB or IB i

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

A cross between heterozygous parents will produce progenies with AB blood group (IAIB) and O blood group (ii)

13. Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance

Ans- (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 blood group character is controlled by three sets of alleles, namely, IA, IB, and i. The alleles, IA and IB, 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 F1 generation. The progeny obtained in F1 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 F1 generation resulting in the production of intermediate pink flowering progenies with Rr genotype. 

14. What is point mutation? Give one example.

Ans- 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

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

Ans- Sutton and Baveri in 1902 proposed the chromosomal theory of inheritance.

16. Mention any two autosomal genetic disorders with their symptoms

Ans- 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.

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