What is the chance of an offspring being affected with an affected mother and normal father, in an X-linked recessive condition?
## Core Concept
X-linked recessive conditions are genetic disorders caused by mutations in genes on the X chromosome. These conditions are more common in males, who have only one X chromosome, as they do not have another X chromosome to compensate for the mutated gene. Females, on the other hand, would need to inherit two copies of the mutated gene (one from each parent) to express the condition.
## Why the Correct Answer is Right
In the case of an X-linked recessive condition, an affected mother must be homozygous for the mutated gene (or at least have one X chromosome with the mutation, but since she's affected, we consider the scenario where she can pass on the affected X chromosome). Let's denote the mutated gene as "Xr" and the normal gene as "X". The affected mother would be XrXr, and the normal father would be XY. All of their sons will inherit the Y chromosome from the father and an Xr chromosome from the mother, making them XrY (affected). All of their daughters will inherit an Xr chromosome from the mother and an X chromosome from the father, making them XrX (carriers). Therefore, the chance of an offspring being affected is 50% for sons.
## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a 0% chance, which would be incorrect because there is a chance that the sons will be affected.
- **Option B:** This option suggests a 25% chance, which is incorrect because the chance is not diluted to this extent given the genotype of the parents.
- **Option C:** While this option seems to align with a possible interpretation of Mendelian genetics for certain conditions, it does not accurately reflect the 50% chance of sons being affected in this scenario.
## Clinical Pearl / High-Yield Fact
A key point to remember is that in X-linked recessive conditions, all daughters of an affected male will be carriers, and all sons of an affected male will be normal (since they receive the Y chromosome from their father). For an affected female (who must be homozygous for the condition), there's a 50% chance each son will be affected and each daughter will be a carrier.
**Correct Answer: C. 50%**