Cystic fibrosis is an autosomal recessive genetic disease caused by a mutation in the gene on chromosome 7. If both parents are asymptomatic carriers (), what is the risk that their first child will be affected with cystic fibrosis?
Answer and explanation
Correct answer: C. 25%
- Step 1: In an autosomal recessive inheritance pattern, an individual must inherit two mutant alleles () to express the clinical phenotype.
- Step 2: Both parents are carriers (). By Mendelian monohybrid segregation, the offspring genotypes are , , and .
- Step 3: Only the homozygous recessive genotype () develops cystic fibrosis, representing a probability of .
Answer
( in ).
Why not the others: represents the carrier probability (); represents the probability of a phenotypically unaffected child ( or ); would require at least one homozygous dominant parent.
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