Final answer:
The recurrence risk for parents with a child affected by an autosomal recessive disorder to have another affected child is 25%, as the parents are likely carriers and there's a 1 in 4 chance for both to pass on the recessive allele.
Step-by-step explanation:
When a set of parents has a child affected by an autosomal recessive disorder, the most likely recurrence risk for them to have another child affected by the same disorder is 25%. This is because the parents are most likely carriers of the disease, meaning each parent has one normal allele and one mutated allele for the genetic disorder. Such disorders require a child to inherit two mutated alleles to exhibit symptoms, one from each parent. A classic Punnett square illustrates this scenario, showing that there is a 1 in 4, or 25%, chance that both parents will pass on the recessive allele leading to the child being affected by the disorder.
Examples of autosomal recessive genetic illnesses include cystic fibrosis, sickle-cell anemia, Tay-Sachs disease, and phenylketonuria. Each of these requires two copies of the mutated gene for the disease to manifest in an individual. It is important to note that the 25% recurrence risk applies when both parents are carriers, not when one is a carrier and the other has two unaffected alleles. In the latter scenario, there is a 0 percent probability of inheriting the disease, but a 50 percent chance of the child being a carrier.