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When two copies of a mutant allele are necessary to cause symptoms, it indicates that the condition follows a recessive inheritance pattern. In recessive genetic diseases, individuals must inherit two alleles for the disease to manifest, one from each parent. This is contrasted with dominant genetic diseases, where only one copy of the mutant allele is sufficient for the individual to express the disorder.

The significance of this is that carriers, who have only one copy of the mutant allele (heterozygotes), do not show symptoms or health issues related to the condition. This means the condition can be passed on through generations without individuals being aware they carry the allele. The concept also explains why such diseases can reappear in families after several generations if two carriers have children.

In the context of other options, sex-linked disorders typically refer to genes located on sex chromosomes and show different inheritance patterns based on whether they are linked to the X or Y chromosomes. Autosomal disorders do refer to those that occur on non-sex chromosomes and can be recessive or dominant, but the key point remains that the necessity of having two copies specifically denotes a recessive trait. Dominant diseases, on the other hand, would present symptoms with just one copy of the mutant allele, which does not apply