Hemophilia is a sex-linked disorder that is caused by a recessive allele; males who receive the allele are always affected. Females are affected only 25% of the time. Which kind of inheritance must hemophilia display and why?

Question

1. Y linked recessive because males that receive the allele sometimes have hemophilia.
2. Y linked dominant because all males with the allele have hemophilia.
3. X linked recessive because all males with the allele have hemophilia, some females if they receive two recessive alleles.
4. X linked dominant because everyone with the allele has hemophilia.

Answer 1

The correct answer is 3. X linked recessive because all males with the allele have hemophilia, some females if they receive two recessive alleles.

Here's the reasoning:

• Hemophilia is a sex-linked disorder: This means it is associated with the sex chromosomes, which are X and Y. In this case, hemophilia is associated with the X chromosome.

• Recessive allele: The disorder is caused by a recessive allele, which means that in males (who have only one X chromosome), if they inherit the allele, they will exhibit the condition— hence, all males with the allele are affected.

• Female inheritance: Females have two X chromosomes, so they would need to inherit two copies of the recessive allele (one from each parent) to be affected by the disorder. Thus, females can be carriers of the hemophilia allele and only show the disease if they receive the recessive allele from both parents.

Given that hemophilia fits the definition of an X-linked recessive condition—where males are more frequently affected while females can be carriers—the other options are incorrect.

• Y linked recessive and Y linked dominant are not applicable, as hemophilia is not associated with the Y chromosome.
• X linked dominant would imply that any individual with the allele shows the condition, which is not accurate in this case, as not all females with one recessive allele are affected, they are typically carriers.