Question

A cross was carried out between a pea plant heterozygous for round and yellow seeds with a pea plant having wrinkled and green seeds.

1. Show the cross in a Punnett square.
2. Write the phenotype of the progeny of this cross.
3. What is this cross known as? State the purpose of conducting such a cross.

Answer 1

(a) 

• Round seed: R (dominant)
• Wrinkled seed: r (recessive)
• Yellow seed: Y (dominant)
• Green seed: y (recessive)

The first pea plant is heterozygous for both traits: RrYy

The second plant is homozygous recessive for both traits: rryy

The gametes produced by RrYy are: RY, Ry, rY, ry

The gametes produced by rryy are only: ry

Punnett square:

	ry	ry	ry	ry
RY	RrYy	RrYy	RrYy	RrYy
Ry	Rryy	Rryy	Rryy	Rryy
rY	rrYy	rrYy	rrYy	rrYy
ry	rryy	rryy	rryy	rryy

(b)

Phenotypes of progeny:

• Round yellow (R_Y_): RrYy = round and yellow
• Round green (R_yy): Rryy = round and green
• Wrinkled yellow (rrY_): rrYy = wrinkled and yellow
• Wrinkled green (rryy): rryy = wrinkled and green

Ratio = 1 round yellow : 1 round green : 1 wrinkled yellow : 1 wrinkled green

So phenotypic ratio is 1:1:1:1.

(c)

This cross is called a test cross. The purpose of a test cross is to determine whether an individual showing a dominant phenotype (here, the heterozygous RrYy plant) is homozygous or heterozygous for the traits by crossing it with a recessive homozygous plant (rryy) and observing progeny phenotypes. A 1:1:1:1 phenotypic ratio indicates the tested plant is heterozygous.