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


If the first son of the woman described in (a) is affected, what is the probability that her second son is affected?

Verified step by step guidance

1. Understand that Lesch–Nyhan syndrome is an X-linked recessive disorder, meaning the gene responsible for the condition is located on the X chromosome, and males (XY) are more likely to be affected because they have only one X chromosome.
2. Recognize that for an X-linked recessive disorder, a female carrier (heterozygous) has a 50% chance of passing the affected X chromosome to her offspring. Sons who inherit the affected X chromosome will express the disorder because they do not have a second X chromosome to compensate.
3. Since the first son is affected, this confirms that the mother is a carrier of the disorder. However, the inheritance of the X chromosome by each child is independent of previous children.
4. For the second son, the probability of inheriting the affected X chromosome from the mother remains 50%, as each child’s inheritance is independent of their siblings.
5. Conclude that the probability of the second son being affected is 50%, based on the principles of independent assortment and the inheritance pattern of X-linked recessive disorders.

Key Concepts

X-linked Recessive Inheritance

X-linked recessive inheritance refers to genetic conditions that are associated with genes located on the X chromosome. Males, having one X and one Y chromosome, are more likely to express these disorders if they inherit a single affected X chromosome. In contrast, females have two X chromosomes, so they would need to inherit two affected X chromosomes to express the disorder, making them typically carriers if they have one affected X.

Probability in Genetics

Probability in genetics involves calculating the likelihood of inheriting specific traits based on parental genotypes. In the case of X-linked disorders, the probability of a son being affected can be determined by considering the mother's genotype and whether she is a carrier or affected. This involves using Punnett squares or basic probability rules to assess the chances of offspring inheriting the affected allele.

Punnett Square

A Punnett square is a diagram used to predict the genetic makeup of offspring from a particular cross. It helps visualize the possible combinations of alleles from the parents. For X-linked traits, the Punnett square can illustrate the inheritance patterns of the X chromosome from the mother to her sons, allowing for the calculation of probabilities regarding whether the second son will be affected by the disorder.