In humans, pointed eyebrows (B) is a dominant trait over smooth eyebrows (b). A student and the mother have…

Analyze the question

Pointed eyebrows (B) are dominant over smooth eyebrows (b). The father has pointed eyebrows, the mother has smooth eyebrows, and the student (the child) has smooth eyebrows. We need the father's genotype.

Concept

• A dominant phenotype can come from two genotypes: BB or Bb.
• A recessive phenotype (smooth) can only come from bb.
• Each parent gives one allele to the child.

Solution

1. The mother has smooth eyebrows, so she is bb. She can only pass on b.
2. The child has smooth eyebrows, so the child is bb. The child must have received one b from the mother and one b from the father.
3. The father therefore carries at least one b allele.
4. The father has pointed eyebrows, so he must also carry a B allele.
5. Father = Bb.

Check with a cross: Bb × bb

| | b | b | |---|---|---| | B | Bb | Bb | | b | bb | bb |

The offspring are 50% pointed (Bb) and 50% smooth (bb). A smooth-eyebrowed child is therefore possible, which matches the question. If the father were BB, every child would be Bb (pointed), and a smooth child could not occur.

Answer: B. Bb

Exam tip

If a child shows the recessive trait but a parent shows the dominant trait, that parent must be heterozygous. The child's recessive trait means each parent gave a recessive allele. Option D (BBbb) is not a valid genotype for a human gamete-forming diploid, which has only two alleles per gene.