How many different types of genotypes and phenotypes are formed in a dihybrid cross in a pea plant?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 9 genotypes and 4 phenotypes
In a dihybrid cross the parents differ in two traits, such as seed shape (round R, wrinkled r) and seed colour (yellow Y, green y). Take the cross RrYy × RrYy (the F1 plants). Each plant makes 4 kinds of gametes: RY, Ry, rY, ry. So 4 × 4 = 16 combinations are possible in the F2. Work out each trait on its own and multiply:
Shape: Rr × Rr gives 3 genotypes (RR, Rr, rr) and 2 phenotypes (round : wrinkled = 3 : 1).
Colour: Yy × Yy gives 3 genotypes (YY, Yy, yy) and 2 phenotypes (yellow : green = 3 : 1).
Genotypes: 3 × 3 = 9.
Phenotypes: 2 × 2 = 4, in the ratio (3 : 1) × (3 : 1) = 9 : 3 : 3 : 1.
Option (3) has the two numbers the wrong way round. Options (1) and (2) have numbers that the cross does not give.
NCERT Extract:
Inheritance of Two Genes – “… In the dihybrid cross (Figure 4.7), the phenotypes round,yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1. Such a ratio was observed for several pairs of characters that Mendel studied.”