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 man whose brother has Lesch–Nyhan syndrome will be affected?

Verified step by step guidance

1. Identify the inheritance pattern: Lesch–Nyhan syndrome is an X-linked recessive disorder.
2. Understand the family relationship: The man in question has a brother with the syndrome, indicating that their mother is a carrier of the disorder.
3. Determine the genetic status of the man: Since the disorder is X-linked recessive, and the man does not have the syndrome, he must have inherited a normal X chromosome from his mother.
4. Consider the genetic contribution to the son: The man will pass on his Y chromosome to his son, while the mother of the son will contribute one of her X chromosomes.
5. Calculate the probability: Since the mother of the son is not specified to be a carrier, assume she is not a carrier, resulting in a 0% probability of the son being 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 chromosome. Females, with two X chromosomes, can be carriers without showing symptoms unless both X chromosomes are affected.

Carrier Status and Family Genetics

In this scenario, the man's brother has Lesch–Nyhan syndrome, indicating that he has the affected X chromosome. The man himself does not have the syndrome, but he could be a carrier if he inherited a normal X chromosome from his mother. Understanding the carrier status of the parents is crucial for determining the probability of the 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 case, it can help visualize the probability of the man passing on either his normal X chromosome or the Y chromosome to his son. By analyzing the genetic contributions from both parents, one can calculate the likelihood of the son being affected by Lesch–Nyhan syndrome.