Final answer:
Mexican hairless dogs, or Xoloitzcuintlis, exhibit hairlessness only when they have a homozygous genotype with two copies of the recessive allele. Dominant alleles mask recessive ones, requiring homozygosity for certain recessive traits to be expressed.
Step-by-step explanation:
The Mexican hairless dog, known as the Xoloitzcuintli, exhibits its hairless trait when it has a homozygous genotype. This means that the dog has inherited two copies of the same gene variant associated with hairlessness. According to Mendelian genetics, for a trait to be manifested in a homozygous recessive organism, both alleles must be recessive. As with many other animal and plant characteristics, the dominant allele typically masks the presence of the recessive allele in a heterozygous pairing. Therefore, the phenotype will only reflect the recessive allele if there is no dominant allele present, which is the case for hairlessness in these dogs, requiring two copies of the recessive allele to express the trait.
In the example of the Labrador retrievers, genotypes such as ee_ result in a yellow lab due to the mechanism known as epistasis, where one gene's expression suppresses that of another gene. Similarly, a recessive trait, like hairlessness in the Xoloitzcuintli, will only show up when both alleles are recessive, affirmed by our understanding of the Law of Dominance. This principle is also reflected in the coat color of rabbits where multiple alleles lead to a hierarchy of dominance, further indicating the requirement of a homozygous recessive genotype for the expression of certain traits. It's a fundamental concept in genetics that explains why homozygosity is necessary for the expression of some inherited characteristics.