Biology Class 12 NCERT Solutions: Chapter 5 Principles of Inheritance and Variation Part 3

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Q.5 Define and design a test − cross?


  • 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.

Image of Cross of Organism

Image of Cross of Organism

Q.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.


  • 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 F1 generation will be same i.e., 1:1.

Image of Cross Between a Homozygous Female and Male

Image of Cross between a Homozygous Female and Male

Q.7 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.


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

Image of Cross Between Tall Plant and Dwarf

Image of Cross between Tall Plant and Dwarf

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