Question

Referring to the diagram shown, how is it possible for only half of the children to inherit a disease if the father has the disease?


The mother has strong genes, but over time the genes from the father will win out, causing the children to inherit the disease.


The female children will inherit genes only from their unaffected mother, but the male children will inherit the disease.


During sexual reproduction, only one chromosome is contributed by each parent. The father could contribute one unaffected gene.


The first and last child will inherit the mutated gene because genes need to be equal, which means one son and one daughter will be affected.

Answer 1

Option 3 is correct.

Brief explanation: In sexual reproduction each parent contributes one allele (one chromosome of the pair) to the child. If the affected father is heterozygous for an autosomal dominant disease (genotype Aa, where A = disease allele, a = normal), half his sperm carry A and half carry a. The unaffected mother is aa, so each child has a 50% chance of being Aa (affected) and 50% aa (unaffected). That is why only half the children inherit the disease.

(Other inheritance patterns give different ratios: e.g. X‑linked diseases or autosomal recessive diseases produce different sex distributions or carrier/affected outcomes.)