Question

if sophie has a brother or sister from the same parents, what are the chances that sibling would have cystic fibrosis? 50% 75% 100% 25% 0%

Explanation:

Step1: Cystic fibrosis is autosomal - recessive.

Let the normal allele be \(A\) and the mutant allele be \(a\). Parents of an affected individual (Sophie) are both carriers (\(Aa\)).

Step2: Use Punnett - square analysis.

When \(Aa\times Aa\), the possible genotypes of offspring are \(AA:Aa:aa = 1:2:1\).

Step3: Determine the probability of having the disease.

The genotype for having cystic fibrosis is \(aa\). The probability of getting \(aa\) from \(Aa\times Aa\) cross is \(\frac{1}{4}= 25\%\).

Answer:

D. 25%