Question

genetic disorders and mendelian genetics
some genetic disorders are inherited through specific allele combinations. sickle cell anemia is an example of a recessive disorder. individuals who carry one recessive allele do not show symptoms but can pass the allele to their offspring.
apply your knowledge: if both parents are carriers of a recessive disorder, the probability that their child will have the disorder is ________

Explanation:

Step1: Define carrier genotype

Carriers have genotype $Aa$ (where $a$ = recessive disease allele, $A$ = normal dominant allele)

Step2: Set up Punnett square

Cross $Aa \times Aa$. The possible offspring genotypes are:

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

Step3: Calculate disease probability

Only $aa$ shows the disorder. There is 1 $aa$ out of 4 total genotypes.
Probability = $\frac{1}{4}$

Answer:

$\frac{1}{4}$ (or 25%)