Final answer:
Red hair in humans caused by a mutant allele of the MC1R gene is a recessive trait, characterized as a loss of function allele that fails to produce the normal amount or type of melanin required for darker hair pigment.
Step-by-step explanation:
The red hair allele of the MC1R gene, which causes red hair in humans, can be characterized based on the description given. Since red hair is a recessive trait, the allele can be described as a loss of function allele. The activity of the MC1R gene product is necessary for the production of the darker eumelanin pigment. A mutant allele that is recessive, like that which causes red hair, typically results in a reduced function or a complete loss of function of the gene product. This means that in the case of the MC1R gene, the red hair allele does not produce the normal amount or type of melanin, leading to red pigment in the hair.
In contrast, a haplosufficient allele provides enough gene product to exhibit the dominant phenotype with just one copy of the allele, and a haploinsufficient allele does not produce enough product for a normal phenotype with one copy, exhibiting a dominant phenotype. A gain of function allele would involve an increased or new function for the gene product, which is not the case with the red hair allele. Therefore, the correct characterization for the red hair allele of MC1R would be E) it is likely a loss of function allele.