Question

1. cystic fibrosis is an autosomal recessive disorder in humans. what is the probability that an offspring will have cystic fibrosis if both parents are carriers of cystic fibrosis?

options: 75%, 25%, 100%, 50%

Explanation:

Step1: Define carrier genotype

Autosomal recessive disorder: Let dominant allele = $A$, recessive (disease) allele = $a$. Carriers have genotype $Aa$.

Step2: Set up Punnett square

Cross $Aa \times Aa$:

$$\begin{array}{c|cc} & A & a \\ \hline A & AA & Aa \\ a & Aa & aa \\ \end{array}$$

Step3: Calculate disease probability

Only $aa$ has cystic fibrosis. 1 out of 4 genotypes is $aa$.
Probability = $\frac{1}{4} = 25\%$

Answer:

C. 25%