Cystic fibrosis is inherited as an autosomal recessive condition. A normal couple has one daughter affected with the disease. They are now planning to have another child. What is the chance of her sibling being affected by the disease
## Core Concept
Cystic fibrosis is an autosomal recessive disorder caused by mutations in the **CFTR (cystic fibrosis transmembrane conductance regulator) gene**. This means that a person must inherit two defective copies of the gene (one from each parent) to express the disease. Carriers, who have one normal and one defective copy of the gene, typically do not show symptoms but can pass the defective gene to their offspring.
## Why the Correct Answer is Right
Given that the couple has a daughter affected with cystic fibrosis, we know both parents are carriers of the disease. Carriers have a **50% chance of passing the mutated gene to each offspring**. For a child to be affected, they must inherit a mutated gene from both parents. The probability of this occurring is calculated using a Punnett square, which shows that there is a **25% chance (or 1/4) that any given child will be affected**, a **50% chance that a child will be a carrier**, and a **25% chance that a child will be neither affected nor a carrier**.
## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a 0% chance, which would only be true if one of the parents were affected (and thus could only contribute a mutated gene) or if both parents were not carriers. Since we know the parents have an affected child and are otherwise normal, they must both be carriers.
- **Option B:** This option suggests a 50% chance, which is the risk of a child being a carrier, not affected.
- **Option D:** This option suggests a 75% chance, which does not align with the genetic risk calculation for an autosomal recessive condition.
## Clinical Pearl / High-Yield Fact
In autosomal recessive diseases like cystic fibrosis, **the risk of two carrier parents having an affected child is 25% for each pregnancy**. It's also crucial to remember that **carriers of cystic fibrosis are usually asymptomatic** but can have a child with the disease if the other parent is also a carrier or affected.
## Correct Answer: .