For a normal husband and wife the first child was diagnosed to have cystic fibrosis. What is the percentage of chances for the second child be affected –

**Core Concept:** Cystic Fibrosis (CF) is an autosomal recessive genetic disorder affecting the exocrine glands, primarily the lungs, pancreas, and sweat glands. The disease is caused by mutations in the CFTR gene, which encodes the cystic fibrosis transmembrane conductance regulator protein. In a couple with a single child affected with cystic fibrosis, the probability of having an unaffected child in the next pregnancy depends on the specific mutations present in the parents.

**Why the Correct Answer is Right:** In this case, the couple has a child with cystic fibrosis, which indicates that both parents carry the mutated CFTR gene (heterozygous carriers). Cystic fibrosis is an autosomal recessive disorder, meaning that for a child to be affected, both parents must be carriers of the mutated gene and the child must inherit two mutated copies of the gene (one from each parent). In the first pregnancy, the probability of having an affected child is 25% (1 in 4) because the child could inherit one normal gene from each parent.

**Why Each Wrong Option is Incorrect:**

A. 50% chance is incorrect because cystic fibrosis is an autosomal recessive disorder, meaning that only one affected child in two pregnancies is sufficient to conclude that both parents are carriers. In this scenario, both parents are carriers, so the chance of having another affected child is 25%.

B. This option does not address the inheritance pattern of cystic fibrosis and focuses on the probability of having another child with CF.

C. 25% chance is incorrect because it only applies when both parents are carriers, which is not the case for this couple.

D. 12.5% chance is incorrect as it does not account for the fact that the couple has a child with cystic fibrosis, indicating they are carriers.

**Clinical Pearl:** In couples with a child affected with cystic fibrosis, the probability of having another affected child is 25%. This is because cystic fibrosis is an autosomal recessive disorder, and the child with CF has received one mutated allele from each parent. In this scenario, both parents are carriers, which means the probability of having another affected child is 25% in a subsequent pregnancy.