The first child of a normal couple was diagnosed to have cystic fibrosis. What is the percentage of the chances for the second child to be affected?
## **Core Concept**
Cystic fibrosis is an autosomal recessive genetic disorder caused by mutations in the **CFTR (cystic fibrosis transmembrane conductance regulator) gene**. This disorder affects the respiratory, digestive, and reproductive systems by disrupting the normal functioning of epithelial cells. For a child to be affected, they must inherit two defective copies of the gene (one from each parent).
## **Why the Correct Answer is Right**
The fact that the first child has cystic fibrosis means both parents are **carriers** of the disease, each having one normal and one mutated allele of the CFTR gene. The genotype of the parents can be represented as **Aa**, where **A** is the normal allele and **a** is the mutated allele. The probability that the second child will inherit two defective copies (one from each parent) and thus be affected is calculated using a Punnett square. For two carrier parents, the chance of each child being affected (aa) is **25%**.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a 0% chance, which would only be true if one of the parents was not a carrier or if the disorder were dominant.
- **Option B:** This option suggests a 50% chance, which would be the risk of a child being a carrier if one parent is affected and the other is not a carrier, or in certain other genetic scenarios not applicable here.
- **Option D:** This option suggests a 75% chance, which does not align with the autosomal recessive inheritance pattern of cystic fibrosis.
## **Clinical Pearl / High-Yield Fact**
In autosomal recessive diseases like cystic fibrosis, **carrier parents have a 25% chance with each pregnancy of having an affected child**, a 50% chance of having a child who is a carrier like them, and a 25% chance of having a child who is neither affected nor a carrier.
## **Correct Answer:** . 25%