Biology Class 12 NCERT Solutions: Chapter 5 Principles of Inheritance and Variation Part 1

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Q.1 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.

  • Peas have many visible contrasting characters such as tall/dwarf plants, round/wrinkled seeds, green/yellow pod, purple/white flowers, etc.

  • Peas have bisexual flowers and therefore undergo self-pollination easily. Thus, pea plants produce offsprings with same traits generation after generation.

  • 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.

  • It has perfect bisexual flowers containing both male and female parts. The flowers are predominantly self-pollinating

  • Because of self-fertilization, plants are homozygous. It is therefore, easy to get pure lines for several generations

  • It is an annual plant. Its short life cycle made it possible to study several generations within a short period.

Image of Pea Plant Mendel

Image of Pea Plant Mendel

Image of Pea Plant Mendel

Q.2 Differentiate between the following:

(a) Dominance and Recessive

(b) Homozygous and Heterozygous

(c) Monohybrid and Dihybrid.

Answer:

(a) Dominance and Recessive

Table of Dominance and Recessive
Table of Dominance and Recessive

Dominance

Recessive

1.

A dominant factor or allele expresses itself in the presence or absence of a recessive trait.

A recessive trait is able to express itself only in the absence of a dominant trait.

2.

It does not require another similar allele to produce its effect on the phenotype.

It produces its phenotypic effect only in the presence of a similar allele.

3.

Dominant allele or enzyme for expressing its effects.

The recessive allele forms an incomplete or defective polypeptide or enzyme so that the expression consists of absence of the effect of dominant allele.

4.

For example, tall plant, round seed, violet flower, etc. are dominant characters in a pea plant.

For example, dwarf plant, wrinkled seed, white flower, etc. are recessive traits in a pea plant.

(b) Homozygous and Heterozygous

Table of Homozygous and Heterozygous
Homozygous and Heterozygous

Homozygous

Heterozygous

1.

It contains two similar alleles for a particular trait.

It contains two different alleles for a particular trait.

2.

Genotype for homozygous possess either dominant or recessive, but never both the alleles. For example, RR or rr.

Genotype for heterozygous possess both dominant and recessive alleles. For example, Rr

3.

It produces only one type of gamete.

It produces two different kinds of gametes

4.

It is pure for a trait and breeds true i.e., gives rise to similar homozygous individuals

Heterozygous individual is seldom pure and produces offspring with different genotypes on selfing, e.g., TT, Tt and tt on selfing of Tt individuals

5.

It does not show extra vigour.

The individual can show extra vigour called hybrid vigour or heterocyst.

(c) Monohybrid and Dihybrid

Table of Monohybrid and Dihybrid
Table of Monohybrid and Dihybrid

Monohybrid

Dihybrid

1.

Monohybrid involves cross between parents, which differs in only one pair of contrasting characters

Dihybrid involves cross between parents, which differs in two pairs of contrasting characters.

2.

It is a cross between two pure organisms in order to study the inheritance of a single pair of alleles.

It is a cross between two pure organisms of a species in order to study the inheritance of two pairs of alleles belonging to two different characters.

3.

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.

4.

It produces a phenotypic monohybrid ratio of 3 : 1 in F2 generation.

It produces a phenotypic Dihybrid ratio of 9 : 3 : 3 : 1 in F2 generation

5.

It produces genotypic ratio of 1 : 2 : 1 in F2.

It produces genotypic ratio of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 in F2.