Question

Suppose you examined a pedigree of a large family, going back 6 generations. In generation 5, a woman ("G5W") has a serious genetic disease--its first incidence in the family. In the next generation, three of her offspring--two boys and one girl--are affected, with one daughter unaffected. What is the most likely explanation?(A) G5W's parent--the one who married into the family--carried the disease allele (but wasn't affected), and it is autosomal dominant.(B) A mutation in one of the G5W's parents, during gamete formation, created an X-linked dominant disease allele.(C) A mutation in G5W created an autosomal dominant disease allele.(D) The disease allele is recessive and X-linked; G5W's spouse also has the allele.

Answer 1

B: A mutation in one of G5W's parents, during gamete formation, created an X-linked dominant disease allele.

You need to analyze the sex gametes.

Boys are XY

Girls are XX

If you have an X-linked dominant disease you need only one affected X gamete to have the disease.

The mother has XX' where X' is de affected and reproduce with a healthy man XY and breed unhealthy boys, but because of the heterozygous gametes you could also have healthy ones XX and XY