Some members of a family have a recessive sex-linked disorder. Which of the following statements about the family would be TRUE?
a. All males would have the disorder.
b. All females would be carriers.
c. Only males would have the disorder.
d. Only females would be carriers
The Correct Answer and Explanation is:
The correct answer is c. Only males would have the disorder.
Explanation:
A recessive sex-linked disorder is one that is caused by a gene located on the X chromosome. Since males have one X and one Y chromosome (XY), they are more likely to express X-linked recessive disorders if they inherit a defective allele on their single X chromosome. In contrast, females have two X chromosomes (XX), so they need two copies of the defective allele (one on each X chromosome) to express the disorder.
Here’s why only males would have the disorder:
- Males (XY) inherit one X chromosome from their mother and one Y chromosome from their father. If the X chromosome from the mother carries the recessive allele for the disorder, the male will express the disorder because he does not have a second X chromosome to potentially provide a normal copy of the gene. This is why males with an X-linked recessive disorder do not need a second copy of the allele to show the condition.
- Females (XX), on the other hand, inherit one X chromosome from each parent. If one of these X chromosomes carries the recessive allele for the disorder, the female will be a carrier (not affected) as long as the other X chromosome has the normal allele. To actually express the disorder, females need two copies of the recessive allele—one on each X chromosome, which is rarer. Therefore, females are typically carriers (heterozygous for the gene) unless both X chromosomes carry the defect.
Breakdown of the answer choices:
- a. All males would have the disorder: This is incorrect because only males who inherit the affected X chromosome from their mother will have the disorder, not all males.
- b. All females would be carriers: This is incorrect because females may or may not inherit the defective allele. They can be carriers or completely unaffected.
- c. Only males would have the disorder: This is correct because males are the ones who will express the disorder when they inherit the recessive allele on their X chromosome.
- d. Only females would be carriers: This is incorrect because males cannot be carriers of a sex-linked disorder. They either have the disorder (if they inherit the defective X) or they don’t (if they inherit a normal X).
Thus, only males are at risk of expressing a recessive sex-linked disorder due to their single X chromosome.