Dihybrid Cross

What a dihybrid cross studies

A dihybrid cross follows two genes at once, each with two alleles, provided the genes are on different chromosomes so their alleles assort independently. It shows how two characteristics are inherited together.

Worked problem 2 (medium): seed shape and seed colour in pea plants

Consider two genes in pea plants: shape (T = round, t = wrinkled) and colour (Y = yellow, y = green), with round and yellow both dominant. Two plants heterozygous for both genes, TtYy, are crossed.

Parental genotypes: TtYy x TtYy.

Gametes: each TtYy parent produces four gamete types in equal number, TY, Ty, tY, ty.

The 4 x 4 Punnett square

Reading the result

Grouping the 16 boxes by phenotype gives round-yellow : round-green : wrinkled-yellow : wrinkled-green in the ratio 9:3:3:1. This is the standard dihybrid ratio produced whenever two heterozygotes for two independently assorting genes are crossed.

How it is examined

SPM questions typically give a scenario with two characteristics, ask you to define the alleles, work out the four gamete types, complete a 4 x 4 Punnett square, and state the phenotype ratio.

Worked problem 1 (easy): pure-breeding parents

A pure-breeding round yellow plant (TTYY) is crossed with a pure-breeding wrinkled green plant (ttyy). The first parent makes only TY gametes and the second makes only ty gametes, so there is one possible combination.

All the F1 offspring are TtYy and show both dominant traits, round and yellow. This F1 uniformity is the starting point before two heterozygotes are crossed to give the 9:3:3:1 ratio.

Worked problem 3 (SPM level): dihybrid test cross

A plant heterozygous for both genes (TtYy) is test-crossed with a fully recessive plant (ttyy). The TtYy parent still makes four gamete types (TY, Ty, tY, ty), while the ttyy parent makes only ty gametes.

The offspring are round yellow (TtYy), round green (Ttyy), wrinkled yellow (ttYy) and wrinkled green (ttyy) in a 1 : 1 : 1 : 1 ratio. A test cross like this exposes the gamete types of the double heterozygote directly, because each recessive gamete simply reveals whatever allele the other parent contributed.

Practice problems

Work through each cross, then check your reasoning against the answers below.

• 1. Cross TtYy x Ttyy. State the phenotype ratio of the offspring.
• 2. In a TtYy x TtYy cross that produces 320 seeds, how many are expected to be wrinkled and green?
• 3. A round yellow plant is test-crossed and gives offspring in the ratio 1 round yellow : 1 round green : 1 wrinkled yellow : 1 wrinkled green. What is the genotype of the round yellow plant?

Answers

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions