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. <span>Step 1: Understand the inheritance pattern. Lesch–Nyhan syndrome is an X-linked recessive disorder, which means the gene causing the disorder is located on the X chromosome.</span>
2. <span>Step 2: Determine the mother's genotype. Since the disorder is X-linked recessive and the first son is affected, the mother must be a carrier (heterozygous) for the disorder. This means she has one normal allele (X^N) and one affected allele (Xⁿ).</span>
3. <span>Step 3: Consider the father's genotype. Since the disorder is X-linked, and males have only one X chromosome, the father's genotype does not affect the probability of the sons being affected. He contributes a Y chromosome to his sons.</span>
4. <span>Step 4: Calculate the probability for the second son. The mother can pass either her X^N or Xⁿ chromosome to her son. Since the father contributes a Y chromosome, the probability that the second son inherits the Xⁿ chromosome from the mother (and thus is affected) is 50%.</span>
5. <span>Step 5: Conclude the probability. Since each son is an independent event, the probability that the second son is affected remains 50%, regardless of the first son's condition.</span>

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 mutated X chromosome. Females, with two X chromosomes, can be carriers if they have one mutated X, but typically do not express the disorder unless both X chromosomes are affected.

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 inheritance.

Carrier Status

Carrier status refers to individuals who possess one copy of a mutated gene but do not exhibit symptoms of the associated disorder. In X-linked recessive conditions, a mother who is a carrier has one normal X chromosome and one mutated X chromosome. This status is crucial for determining the risk of passing the disorder to her offspring, particularly sons, who will inherit their X chromosome from her.