USA Biology Olympiad (USABO) Practice Exam

According to a sex-linked pedigree, what is the likelihood that a child of individuals 1 and 2 will have the disease?

1. 0% if it is a boy; 50% if it is a girl

2. 0% if it is a girl; 50% if it is a boy

3. 100% for either gender

4. 25% regardless of gender

The correct answer is: 0% if it is a girl; 50% if it is a boy

To understand the likelihood of a child inheriting a sex-linked disease from individuals 1 and 2 in a pedigree, it is important to recognize that sex-linked traits, particularly those located on the X chromosome, have different inheritance patterns for males and females. If one parent carries the disease allele on one of their X chromosomes, then their daughters (who inherit one X chromosome from each parent) will have a 50% chance of inheriting the affected X from the affected parent. If the mother is unaffected, the daughter must inherit an unaffected X from her, giving her a 50% chance of being a carrier, but not affected. In contrast, if a son inherits the affected X chromosome from his mother, he will express the disease because he has only one X chromosome and one Y chromosome. Therefore, in this scenario, the likelihood of a child being affected by the disease depends on their gender. The son has a 50% probability of having the disease if he inherits the affected X from his mother, while daughters have a 0% chance of being affected (assuming the father does not carry the disease gene). This explains why the correct answer reflects the different probabilities for a boy and a girl: 0% if it is a