Lesch–Nyhan syndrome (OMIM 300322) is a rare X-linked recessive disorder that produces severe mental retardation, spastic cerebral palsy, and self-mutilation.

What is the probability that the first son of a woman whose brother has Lesch–Nyhan syndrome will be affected?

Key Concepts

X-linked Recessive Inheritance

Lesch–Nyhan syndrome is an X-linked recessive disorder, meaning the gene responsible for the condition is located on the X chromosome. Males, having one X and one Y chromosome, are more likely to express the disorder if they inherit the affected X. Females, with two X chromosomes, can be carriers if one X carries the mutation, but they typically do not express the disorder unless both X chromosomes are affected.

Carrier Status

In X-linked recessive disorders, females can be carriers if they have one affected X chromosome and one normal X chromosome. In this case, the woman’s brother has Lesch–Nyhan syndrome, indicating that their mother is likely a carrier. This means there is a 50% chance that the woman herself is a carrier, which is crucial for determining the probability of her son being affected.

Punnett Square Analysis

A Punnett square is a tool used to predict the genetic outcomes of a cross between two individuals. In this scenario, if the woman is a carrier, the Punnett square can help determine the probability of her son inheriting the affected X chromosome. The analysis will show that there is a 50% chance of the son being affected if the mother is a carrier and a 0% chance if she is not.