In X-linked recessive inheritance, why does the disease gene often appear to skip generations?

Options

A. x-linked diseases are only expressed in males
B. All X-linked diseases display incomplete penetrance
C. Disease is primarily transmitted through unaffected carrier females
D. Males with an affected gene may transmit but not show the disease

Answer

### Step-by-Step Solution

1. **Understanding X-linked Recessive Inheritance**:
- X-linked recessive inheritance refers to genetic disorders that are caused by mutations on the X chromosome. In humans, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

2. **Impact on Males and Females**:
- Since males have only one X chromosome, if that chromosome carries a recessive mutation, they will express the disorder. Females, on the other hand, have two X chromosomes. If one X carries the mutation and the other does not, the female may be a carrier but typically will not express the disorder.

3. **Crisscross Inheritance**:
- In X-linked recessive inheritance, the pattern of inheritance is often described as "crisscross." This means that the trait can be passed from affected fathers to their daughters (who become carriers) and then from those daughters to their sons.

4. **Skipping Generations**:
- The disease gene appears to skip generations because affected males cannot pass the disorder to their sons (since they pass on their Y chromosome). However, they can pass the affected X chromosome to their daughters, who become carriers. These carrier daughters may then have unaffected sons who inherit the affected X chromosome, thus expressing the disorder.

5. **Conclusion**:
- As a result, the disease can seem to skip generations because it is often carried by females who do not express the disorder. The disorder may only appear in males when they inherit the affected X chromosome from their carrier mothers.