What is the probability that the first son of a woman whose brother is affected will be affected? What is the probability that the second son of a woman whose brother is affected will be affected if her first son was affected?

Question

Duchenne muscular dystrophy (DMD) is cause by a relatively rare X-linked recessive allele. It causes progressive muscular wasting, and usually leads to death before age 20.

Elias Aboujaoude · Accepted Answer

#P("first son has DMD")=25%#
#P("second son has DMD"|"first son has DMD")=50%# If a woman's brother has DMD then the woman's mother is a carrier of the gene. The woman will get half of her chromosomes from her mother; so there is a #50%# chance that the woman will inherit the gene. If the woman has a son, he will inherit half of his chromosomes from his mother; so there would be a #50%# chance if his mother was a carrier that he would have the defective gene. Therefore if a woman has a brother with DMD there is a #50%XX50%=25%# chance that her (first) son will have DMD. If the woman's first son (or any son) has DMD then the woman must be a carrier and there is a #50%# chance that any other son would have DMD.

Benjamin Alvin · Answer

undefined The probability that the first son of a woman whose brother is affected will be affected is 50% if the affected trait is autosomal dominant and 25% if it's autosomal recessive.

If the first son was affected, the probability that the second son will be affected depends on the mode of inheritance of the trait and whether the trait is dependent on the mother's genetics. If the trait is autosomal dominant, the probability remains the same for the second son (50%). If the trait is autosomal recessive and both parents are carriers, the probability would be 25% for each subsequent son. If the trait is X-linked recessive, the probability would also depend on the mother's status as a carrier.