Final answer:
Overdominance occurs when a heterozygote genotype has a selective advantage, possibly due to a deletion mutation causing a beneficial phenotype. Deletion mutations can create new dynamics in allele expression, leading to an observed overdominance.
Step-by-step explanation:
Overdominance is a genetic phenomenon where the heterozygote genotype has a selective advantage over both the homozygote dominant and homozygote recessive genotypes. A deletion mutation can contribute to overdominance if it causes a frame-shift or removes a critical portion of the DNA for one allele, resulting in a variant that, when combined with the normal allele in heterozygotes, results in a superior phenotype. This might occur if the altered allele due to deletion, while detrimental in a homozygote state, increases the survival or reproductive success of heterozygotes. For example, in conditions like sickle cell anemia, individuals with one normal allele and one sickle allele (heterozygotes) have some resistance to malaria, a benefit not shared by homozygous individuals with two copies of the normal allele.
Option B describes the impact of deletion on heterozygosity and does not directly relate to overdominance. Options A and D about allele frequencies do not describe the process leading to overdominance.
The correct answer is not explicitly stated, but the closest is Option C, as overdominance is a circumstance in which the heterozygote advantage could be considered an enhancement of the functional expression of both alleles, considering that the heterozygote's phenotype is superior. However, 'enhancement' may not be the correct term as the deletion may not enhance the expression of the alleles but rather create a new dynamic between them.