Advanced Placement (AP) Biology Practice Exam

What type of genetic disorder is cystic fibrosis?

1. Autosomal dominant disorder

2. Autosomal recessive disorder

3. X-linked dominant disorder

4. Incomplete dominance disorder

The correct answer is: Autosomal recessive disorder

Cystic fibrosis is classified as an autosomal recessive disorder. This means that the condition arises when an individual has two copies of the mutant allele, one inherited from each parent. In the case of cystic fibrosis, the defective gene is located on chromosome 7, which is an autosome. Individuals who carry only one copy of the mutant gene (heterozygous) do not exhibit symptoms of the disease; they are considered carriers. The underlying mechanism involves the CFTR gene, which encodes for a protein that regulates chloride ion transport across cell membranes. Mutations in the CFTR gene disrupt this process, leading to the production of thick, sticky mucus that can obstruct various organs, particularly the lungs and pancreas. Understanding the nature of cystic fibrosis as an autosomal recessive disorder is crucial for genetics, as it informs risk assessment for offspring when both parents are carriers of the CFTR mutation.