An extra finger in humans is rare but is due to a dominant gene. When one parent is normal and the other parent has an extra finger but is heterozygous for the trait, what is the probability that the first child will be normal?

Correct Answer: C

Rationale: Heterozygous (Aa) x normal (aa) gives 50% Aa (extra finger) and 50% aa (normal).

Correct Answer: C

Rationale: Huntington disease is caused by CAG repeat expansion in the HTT gene.

Correct Answer: D

Rationale: 45,Y (only Y chromosome) is not viable; 47,XY,+13 (Patau), +18 (Edwards), +21 (Down) can survive to birth.

Correct Answer: D

Rationale: Fragile X syndrome shows anticipation (worsening with generations) via triplet repeat expansion, often more pronounced in paternal transmission.

Correct Answer: C

Rationale: Congenital adrenal hyperplasia (e.g., 21-hydroxylase deficiency) causes excess androgens, masculinizing XX females.