In humans, hemophilia A (OMIM 306700) is an X-linked recessive disorder that affects the gene for factor VIII protein, which is essential for blood clotting. The dominant and recessive alleles for the factor VIII gene are represented by H and h. Albinism is an autosomal recessive condition that results from mutation of the gene producing tyrosinase, an enzyme in the melanin synthesis pathway. A and a represent the tyrosinase alleles. A healthy woman named Clara (II-2), whose father (I-1) has hemophilia and whose brother (II-1) has albinism, is married to a healthy man named Charles (II-3), whose parents are healthy. Charles's brother (II-5) has hemophilia, and his sister (II-4) has albinism. The pedigree is shown below.

If Clara and Charles's first child has albinism, what is the chance the second child has albinism? Explain why this probability is higher than the probability you calculated in part (b).

Key Concepts

X-linked Recessive Inheritance

X-linked recessive inheritance refers to genetic conditions that are associated with genes located on the X chromosome. In this mode of inheritance, males (XY) are more likely to express the disorder because they have only one X chromosome. Females (XX) can be carriers if they have one affected X chromosome and one normal X chromosome, but they typically do not express the disorder unless both X chromosomes are affected.

Autosomal Recessive Inheritance

Autosomal recessive inheritance involves genes located on non-sex chromosomes (autosomes). For an individual to express an autosomal recessive condition, they must inherit two copies of the mutated allele, one from each parent. Carriers, who have one normal and one mutated allele, do not show symptoms but can pass the mutated allele to their offspring, making it crucial to consider both parents' genotypes when predicting the likelihood of the condition in children.

Probability in Genetic Crosses

Probability in genetic crosses is used to predict the likelihood of offspring inheriting specific traits based on parental genotypes. When calculating probabilities, the outcomes of previous births can influence future probabilities, especially in cases of recessive traits. For example, if Clara and Charles's first child has albinism, it suggests that both parents are likely carriers of the recessive allele, thus increasing the probability that subsequent children may also inherit the condition.