Question

In a plant, tallness is dominant over dwarfness and red flower is dominant over white. Starting with the parents work out a dihybrid cross. What is the standard dihybrid ratio? Do you think the values would deviate if the two genes in question are interacting with each other?

Accepted Answer

A true-breeding homozygous red flower and tall plant as dominant characters are crossed with a true-breeding homozygous white flower and short plant as recessive characters. In the F₁ generation, only the dominant allele will be expressed, and all the progeny will be heterozygous for both the genes. Let 'R' be the allele for red flower, and 'r' be the allele for the white flower. Let 'T' be the allele for tall plant and 't' for the dwarf. The genotype of a red flower and tall plant will be RRTT and genotype of white flower, and the short plant will be rrtt.

The genotype of F₁ progeny will be RrTt. So, the phenotype of all F₁ generation plants is Red flower and tall plant. So each progeny plant will produce four different types of gamete, which will be RT, Rt, rT, and rt.

Phenotype ratio of F₂ generation will be : Homozygous red flower and tall plant: White flower and tall plant: Red flower and short plant: White flower and short plant are 9:3:3:1.

Yes, I think the values would deviate if the two genes in question are interacting with each other. Interaction between two independent genes leads to the formation of new phenotypes. Examples of such gene interactions are epistasis, complementary gene, supplementary genes and duplicate genes. All these types of gene interactions deviate from the normal Mendelian dihybrid ratio of 9:3:3:1.

NCERT Solutions for Chapter: Principle of Inheritance and Variation, Exercise 4: LONG ANSWER TYPE QUESTIONS

NCERT Biology Solutions for Exercise - NCERT Solutions for Chapter: Principle of Inheritance and Variation, Exercise 4: LONG ANSWER TYPE QUESTIONS

Questions from NCERT Solutions for Chapter: Principle of Inheritance and Variation, Exercise 4: LONG ANSWER TYPE QUESTIONS with Hints & Solutions

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