LeschNyhan syndrome is a rare Xlinked recessive disorder tha

Lesch-Nyhan syndrome is a rare X-linked recessive disorder that produces a range of severe developmental phenotypes.

A] What is the probability that the first son of a woman whose brother has Lesch-Nyhan will be affected?

B] If the first son in [A] is affected, what is the probability that her second son will be affected?

C] What is the probability that the first son of a man whose brother has Lesch-Nyhan will be affected?

Solution

(A) We shall assume the woman is a carrier as the disease is X-linked recessive.

Then the probability of the first son being affected= 0.5

or the woman can be a non-carrier, as well. Then, the probability will be= 0

(B) 0.5

(C) Since the disease is X-linked recessive, men will always be affected if they inherit the diseased copy of X chromosome from the mother. Unaffected males are free from the disease and are not carriers, as well. On the other hand, two copies of diseased X chromosome are needed for a woman to be affected. If there is only one diseased X chromosome copy present, then the woman will be carrier.

Probability, if the woman is unaffected and non-carrier = 0

Probability, if the woman is a carrier= 0.5

Probability, if the woman is affected= 1