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 genetic disorder caused by mutations in the CFTR gene, leading to abnormal chloride transport in epithelial cells. This results in thickened mucus and respiratory complications. The inheritance pattern follows the laws of Mendelian genetics.
**Why the Correct Answer is Right**
In autosomal recessive conditions, an individual must inherit two defective alleles (one from each parent) to express the disease. Since the parents are normal, they must be carriers of the mutated gene, with one normal and one mutated allele. The affected daughter has inherited two mutated alleles. For the next child to be affected, they must inherit the mutated allele from both parents. The probability of a normal carrier parent passing on the mutated allele is 50%. Since the other parent is also a carrier, the probability of inheriting the mutated allele from both parents is 25% (50% x 50%).
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it assumes a 50% chance of the sibling being affected, which is the probability of inheriting the mutated allele from one parent, not both.
**Option B:** This option is incorrect because it implies a 100% chance of the sibling being affected, which is not possible since the parents are carriers, not affected individuals.
**Option C:** This option is incorrect because it suggests a 25% chance of the sibling being a carrier, which is not the question being asked.
**Clinical Pearl / High-Yield Fact**
In autosomal recessive conditions, carriers are typically asymptomatic but can pass on the mutated gene to their offspring. Understanding the genetic inheritance pattern is crucial for genetic counseling and family planning.
**Correct Answer:** C. 25% chance of the sibling being affected by the disease.