A female married to a normal male but her father had vitamin D resistant rickets. So what are the chances that her childrens will be colour blind: March 2013
**Core Concept**
The question is testing the understanding of X-linked recessive inheritance patterns, specifically the relationship between vitamin D resistant rickets (caused by mutations in the PTH1R gene) and color blindness (caused by mutations in the RBMY gene). Vitamin D resistant rickets is a classic example of an X-linked dominant disorder, while color blindness is an X-linked recessive disorder.
**Why the Correct Answer is Right**
The correct answer is 50% because the female is a carrier of the mutated PTH1R gene on her X chromosome. When she passes this X chromosome to her children, there's a 50% chance that a son will inherit the mutated gene and express the disease (vitamin D resistant rickets), but sons cannot be carriers. Since color blindness is an X-linked recessive disorder, there's a 50% chance that a son will inherit the mutated RBMY gene, making him color blind, but daughters can only be carriers.
**Why Each Wrong Option is Incorrect**
**Option A: 0%** - This is incorrect because the female can pass the mutated gene to her children, and there's a possibility of them inheriting the disease.
**Option C: 75%** - This is incorrect because it's not possible for all children to inherit the mutated gene, as the inheritance pattern is X-linked.
**Option D: 100%** - This is incorrect because the female is not guaranteed to pass the mutated gene to all her children, and there's a possibility of them not inheriting the disease.
**Clinical Pearl / High-Yield Fact**
In X-linked recessive disorders, the risk of transmission to sons is higher than to daughters, and daughters are typically carriers. This is because sons have only one X chromosome, and if they inherit the mutated gene, they will express the disease. Daughters, on the other hand, have two X chromosomes and are usually carriers, but they may not express the disease themselves.
**✓ Correct Answer: B. 50%**