Final answer:
Dominant refers to an allele that masks the expression of a recessive allele in an organism's phenotype. This occurs in both homozygous and heterozygous genotypes, with the dominant trait being expressed when paired with a recessive trait. Epistasis can also play a role in the dominance of certain genetic traits.
Step-by-step explanation:
The term dominant in genetics refers to an allele or a genetic variant that masks the expression of the recessive allele in the phenotype of an individual. This dominance occurs when the dominant allele is paired with a recessive allele in a heterozygote (represented as 'Aa'), where the trait associated with the dominant allele ('A') is expressed, overshadowing the other trait.
For instance, regarding the blood type example, the allele for blood type A is dominant over the allele for blood type O. As such, a person with one allele for blood type A and one for blood type O will exhibit blood type A in their phenotype because the presence of the A allele is dominant over the O allele. However, to have the recessive blood type O phenotype, an individual must have two O alleles ('oo'), as this is the only way for the recessive trait to be phenotypically expressed.
It's important to note that the dominance of a trait is a result of the interactions at the molecular level where some genes can mask or interfere with the expression of others—a phenomenon known as epistasis. Moreover, traits can be dominant or recessive, leading to discontinuous variation where traits show noticeable differences between them.