If both parents are heterozygous for a trait. the probability that an offspring will be recessive for that trait is?

Analyze the question: Both parents are heterozygous (Aa × Aa). We need the probability that an offspring is recessive (aa).

Concept: In a monohybrid cross, each heterozygous parent produces two types of gametes, A and a, in equal numbers. Each gamete has a 1/2 chance of being A or a. We combine them using a Punnett square.

Solution:

Parents: Aa × Aa

| | A | a | |---|---|---| | A | AA | Aa | | a | Aa | aa |

Genotypic ratio: 1 AA : 2 Aa : 1 aa Phenotypic ratio: 3 dominant : 1 recessive

The recessive phenotype appears only in the aa genotype, which is 1 out of 4 offspring.

Or by probability: P(a from father) × P(a from mother) = 1/2 × 1/2 = 1/4

Answer: B. 1/4

Why the others are wrong:

• A (1/2): this is the probability of a heterozygous (Aa) offspring, not the recessive one.
• C (1): this would be certain, which happens only if both parents are aa.
• D (3/4): this is the probability of showing the dominant trait.

Exam tip: Remember that Aa × Aa always gives a 3:1 phenotypic ratio, so the recessive is 1/4 (25%). Watch for wording. If the question asks for "dominant phenotype," the answer is 3/4. If it asks for "heterozygous," the answer is 1/2.