WAEC 2017 Biology · Question 4

Assuming that A is the gene for normal skin color and is dominant, while a is the gene for albinism and its…

Question

Assuming that A is the gene for normal skin color and is dominant, while a is the gene for albinism and its recessive, what is the likely genotype of the couple which had 50% normal and 50% albino offspring?

A.AA, aa
B.Aa, aa
C.AA, Aa
D.Aa, Aa

Answer: B. Aa, aa

Explanation written with AI and checked against the answer key

Analyze the question: A couple has offspring in a 1:1 ratio, 50% normal and 50% albino. We need the parental genotypes that give this ratio.

Concept: This is a test cross ratio. A 1:1 phenotypic ratio comes from a heterozygote crossed with a homozygous recessive (Aa × aa). Albino children (aa) must receive one "a" from each parent, so both parents carry at least one "a".

Check each option:

A. AA × aa: All offspring are Aa, so 100% normal. ✗
B. Aa × aa: Gametes are A, a and a, a. Offspring are 2 Aa : 2 aa, which is 50% normal and 50% albino. ✓
C. AA × Aa: Offspring are AA and Aa, so 100% normal. ✗
D. Aa × Aa: Offspring are 1 AA : 2 Aa : 1 aa, which is 75% normal and 25% albino. ✗

Punnett square for B:

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

Aa (normal) = 50%, aa (albino) = 50%.

Answer: B (Aa, aa)

Tip: Memorize the key ratios:

Aa × Aa → 3:1 (dominant : recessive)
Aa × aa → 1:1
AA × anything → all dominant phenotype

When you see 50:50 in a genetics question, think "heterozygote × homozygous recessive."

AI can make mistakes. Report anything that looks wrong.