True statement about inheritance of an X linked recessive trait is
**Core Concept**
X-linked recessive traits are inherited in a pattern where the gene responsible for the trait is located on the X chromosome. In males, who have only one X chromosome, a single copy of the mutated gene is sufficient to cause the trait, as they do not have a second X chromosome to compensate. In females, who have two X chromosomes, the presence of one mutated gene and one normal gene typically results in a carrier state, but the trait may be expressed if the normal gene is mutated as well.
**Why the Correct Answer is Right**
The inheritance pattern of X-linked recessive traits follows the Lyon hypothesis, where one X chromosome is inactivated in females to prevent X-chromosome incompatibility. This inactivation is random, and the probability of inactivation of the X chromosome carrying the mutated gene is equal to the probability of inactivation of the normal X chromosome. However, if a female inherits two mutated X chromosomes, she will express the X-linked recessive trait due to the lack of a normal gene to compensate. Females with one mutated X chromosome are typically carriers, as they have a normal X chromosome to compensate for the mutated gene.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because X-linked recessive traits are not typically expressed in females who are carriers, unless they inherit two mutated X chromosomes.
**Option B:** This option is incorrect because the Lyon hypothesis explains the random inactivation of X chromosomes in females, but it does not explain the inheritance pattern of X-linked recessive traits.
**Option C:** This option is incorrect because X-linked dominant traits are inherited in a pattern where a single copy of the mutated gene causes the trait, but the trait is typically expressed in both males and females.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that X-linked recessive traits are more common in males, as they only have one X chromosome and a single copy of the mutated gene is sufficient to cause the trait. This makes X-linked recessive traits a significant concern in families with a history of affected males.
**Correct Answer:** C.