Question

what are the odds that two carriers for cystic fibrosis, an autosomal recessive disorder, would have an offspring with the cystic fibrosis disorder? 0/4 1/4 1/2 3/4

Explanation:

Step1: Define alleles

Let the normal allele be $A$ and the mutant allele be $a$. Carriers have the genotype $Aa$.

Step2: Set up Punnett - square

When two carriers ($Aa\times Aa$) have offspring, the possible gametes from each parent are $A$ and $a$. The Punnett - square has 4 possible combinations: $AA$, $Aa$, $aA$, and $aa$.

Step3: Determine affected genotype

The genotype for an individual with cystic fibrosis (autosomal recessive) is $aa$. Out of the 4 possible genotypes from the Punnett - square ($AA:Aa:aA:aa = 1:2:1$), the proportion of $aa$ is $\frac{1}{4}$.

Answer:

1/4