Question

You create a test cross to determine whether a smooth pea is homozygous dominant or is heterozygous. You raise 4 offspring and obtain all smooth peas.

When the smooth pea is a homozygote the probability of this outcome is 1.

Question: When the smooth pea is a heterozygote the probability of this outcome is ____  Please format your answer as a fraction. Explain your reasoning.

Answer 1

When the smooth pea is a heterozygote the probability of this outcome is 1/2.

Testcross is performed for identifying an individual's genotype. A test cross is performed by crossing the individual with an unknown genotype with a homozygous recessive individual.

(Smooth peas are dominant over wrinkled peas.)

Here, A heterozygous smooth pea (Rr) is test crossed.

• Cross:

Parents: Rr (Smooth, heterozygous) rr (Wrinkled, recessive)

Genotype: R r r r

offspring -

	R	r
r	Rr (Smooth)	rr (Wrinkled)
r	Rr (Smooth)	rr (Wrinkled)

Here, two outcomes i.e., both smooth and wrinkled are possible. Half of the offspring are smooth and half are wrinkled.

The phenotypic and genotypic ratio of the outcome is the same and is equal to 1:1.

So, the probability for the outcome is 1/2. Half are smooth and half are wrinkled.