In an Aa × Aa cross, what is the probability of a recessive phenotype?

In an Aa × Aa cross, what is the probability of a recessive phenotype?

• A. 0%.
• B. 25%.
• C. 75%.
• D. 50%.

Answer: (B)

In an Aa × Aa cross, each parent can pass on either the A or the a allele with equal probability. The four equally likely genotype combinations for the offspring are AA, Aa, Aa, and aa. Only the homozygous recessive genotype (aa) shows the recessive phenotype. Since 1 of the 4 possible outcomes is aa, the chance of a recessive phenotype is 1/4, which is 25%. So about a quarter of the offspring would display the recessive trait.

In an Aa × Aa cross, each parent can pass on either the A or the a allele with equal probability. The four equally likely genotype combinations for the offspring are AA, Aa, Aa, and aa. Only the homozygous recessive genotype (aa) shows the recessive phenotype. Since 1 of the 4 possible outcomes is aa, the chance of a recessive phenotype is 1/4, which is 25%. So about a quarter of the offspring would display the recessive trait.