In a monohybrid cross Aa x Aa, what is the probability that offspring show the dominant phenotype?

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Multiple Choice

In a monohybrid cross Aa x Aa, what is the probability that offspring show the dominant phenotype?

Explanation:
When both parents are heterozygous, expressing the dominant trait requires at least one dominant allele from either parent. Each parent contributes A or a with equal probability, so the only way to get a recessive phenotype is for both to contribute a. That happens in 1 out of 4 equally likely combinations (1/2 × 1/2 = 1/4). Therefore, the probability of the dominant phenotype is 1 − 1/4 = 3/4, i.e., 75%. This reflects the classic 3:1 phenotypic ratio for a monohybrid cross of Aa × Aa.

When both parents are heterozygous, expressing the dominant trait requires at least one dominant allele from either parent. Each parent contributes A or a with equal probability, so the only way to get a recessive phenotype is for both to contribute a. That happens in 1 out of 4 equally likely combinations (1/2 × 1/2 = 1/4). Therefore, the probability of the dominant phenotype is 1 − 1/4 = 3/4, i.e., 75%. This reflects the classic 3:1 phenotypic ratio for a monohybrid cross of Aa × Aa.

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