What is the probability of obtaining a homozygous recessive offspring from a cross between two heterozygotes?

In a cross between two heterozygotes (for example, individuals with genotypes Aa x Aa), the probability of obtaining a homozygous recessive offspring (aa) can be determined using a Punnett square.

First, the alleles contributed by each parent are represented. Each heterozygote can pass on either the dominant (A) or recessive (a) allele. When setting up the Punnett square, the combinations of alleles from each parent would look like this:

• The top of the square might have A and a (from the first parent)

• The side of the square might have A and a (from the second parent)

Filling this out gives the following possible genotypes for the offspring:

• AA

• Aa

• Aa

• aa

When you tally up these outcomes, you find that there is one instance of aa (homozygous recessive) among the four total boxes. Therefore, the probability of obtaining a homozygous recessive offspring from the cross is calculated as:

Number of homozygous recessive outcomes (aa) / Total number of outcomes = 1/4 = 25%.

Thus, the probability of obtaining a homozygous recessive offspring from a cross