Question

The diagram represents a dihybrid cross between two pea plants heterozygous for both seed color and seed shape.

What is the phenotypic ratio of the offspring?

A) 1:1:1:1:2:2:2:2:4
B) 1:3:3:9
C) 1:4
D) 4:12

Answer 1

Of the choices given B would be the correct choice

Answer 2

The correct answer would be B) 1:3:3:9.

Dihybrid cross refers to a cross between two organisms that are differ in only two observable characters or traits.

For example, Mendel crossed pure breed pea plants for two contrasting traits- one with round and yellow seeds (RRYY) and other with wrinkled and green seeds (rryy).

Offsprings of first generation were all genetically heterozygous (RrYy) with same phenotype that is, round and yellow seed. Hence, yellow (Y) is dominant over green (y) and round shape of seed (R) is dominant over wrinkled shape (r).

On selfing, second generation resulted in the formation of four types of phenotypes in 1:3:3:9, where:

• 1 represents the homozygous recessive for both the trait that is, wrinkled and green seeds (rryy).
• 3 represents those which display first trait as recessive (wrinkled shaped) and second trait as dominant (yellow color seeds) (rrYY + 2 x rrYy).
• Other 3 represents those which display first trait as dominant (round shaped) and second trait as recessive (green colored seeds) (RRyy + 2 x Rryy)
• 9 represents those in which both the phenotypes were dominant that is, round shaped and yellow color seeds (RRYY + 2 x RRYy + 2 x RrYY + 4 x RrYy).

The figure attached in the question also shows the same results as were observed by Mendel. Hence, the phenotypic ratio of this cross will also be equal to 1:3:3:9 (wrinkled and green : round and green : wrinkled and yellow : round and yellow).