Difficulty: Hard

Average Score: 33%

Which of the following best explains how a child can inherit a genetic disorder from parents who exhibit no symptoms of the disorder?

Rationale

The disorder is caused by a recessive allele, and each parent carries only a single copy of the allele.

In cases of genetic disorders caused by recessive alleles, both parents can be carriers without exhibiting symptoms themselves. Each parent possesses one normal allele and one recessive allele, meaning the child can inherit two recessive alleles, resulting in the expression of the disorder.

A (The disorder is caused by a recessive allele, and each parent carries only a single copy of the allele.) CORRECT

This statement accurately explains how a child can inherit a genetic disorder when both parents are asymptomatic. Each parent carries one copy of the recessive allele but does not express the disorder since it requires two copies for phenotypic manifestation. Consequently, there is a 25% chance the child inherits both recessive alleles, leading to the disorder's expression.

B (The disorder is caused by a dominant allele, and each parent carries at least one copy of the allele.) YOUR ANSWER

This choice is incorrect because if the disorder were caused by a dominant allele, at least one parent would exhibit symptoms of the disorder. Dominant disorders require only one copy of the allele to manifest, meaning asymptomatic parents cannot both carry a dominant allele without showing the disorder themselves.

C (The disorder is caused by multiple genes, and both parents acquired mutations in those genes before the child was born.) YOUR ANSWER

While some disorders are polygenic, this choice does not explain how asymptomatic parents could have a child with a genetic disorder. If both parents had mutations leading to a disorder, they would likely show some symptoms or have affected children, making this explanation inadequate for recessive disorders.

D (The disorder is caused by a toxin in the environment, and the parents were exposed to the toxin after the child was born.) YOUR ANSWER

This option is incorrect because it suggests that environmental factors cause the disorder rather than genetic inheritance. If the disorder arises from environmental toxins, it cannot be inherited from asymptomatic parents; such exposure would influence the child only after birth.

Conclusion

Understanding how recessive genetic disorders can manifest in children born to asymptomatic parents is essential in genetics. In such cases, both parents are typically carriers of a recessive allele, allowing for the possibility of their child inheriting the disorder despite the parents showing no symptoms. This genetic mechanism emphasizes the importance of carrier screening and genetic counseling in families with a history of hereditary conditions.

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