USA Biology Olympiad (USABO) Practice Exam

What genetic information must a genetic counselor obtain to assure a couple that none of their children will inherit a particular recessive disease?

1. Both parents must have identical mutations

2. Both parents have mutations in different genes causing the same phenotype

3. One parent is a carrier, and the other is not

4. Neither parent has any mutations

The correct answer is: Both parents have mutations in different genes causing the same phenotype

The key to assuring a couple that none of their children will inherit a particular recessive disease lies in understanding the genetics of recessive inheritance. For a child to inherit a recessive disease, they must inherit two copies of the mutated gene—one from each parent. If both parents have mutations in different genes causing the same phenotype, it means that each parent carries a mutation that affects a different gene. Consequently, even if they each pass on one of their mutations to a child, that child would receive one normal copy from one parent and one mutated copy from a different gene from the other parent. In this scenario, the child would not exhibit the disease because they do not inherit two copies of the same recessive disease mutation. This rationale highlights why having both parents carrying mutations in different genes associated with the same recessive condition is pivotal. It ensures that offspring do not have the necessary double dose of the same recessive allele, thus preventing the manifestation of the disease. Other scenarios, such as having identical mutations or having one parent as just a carrier while the other is unaffected, do not guarantee protection for their children from inheriting the disease.