What is the factor that seems to disappear due to a dominant allele?

Answer 1

An allele is one particular form of a gene. A large population of living things typically have several different allele for any particular gene.

For example, one important gene in humans determines blood type compatibility. That gene comes in 3 different alleles -- A, B, and O.

Most plants and animals are diploid -- they have 2 of each gene, one inherited from each parent.

For example, any one human has one of six possible genotypes for that gene: AA, BB, OO, AB, AO, BO.

A recessive allele seems to disappear when paired with a dominant allele. If something has a dominant and recessive allele, the dominant will overshadow the recessive, but the recessive will still be there (just not showing). For example, the O allele is recessive when paired with the A allele, which is dominant, and so humans with the AO genotype as well as the AA genotype have "type A blood".

Only humans with the OO genotype show "type O blood".

According to the Wikipedia "allele" article, some people once thought that all genes had only one "normal" allele, which was both common and dominant, and all other versions of that gene (all other alleles) were rare and recessive.

However, most genes have many different "normal" alleles, whose frequencies vary from one population to another.

With some genes, the most common allele is recessive.

Answer 2

a recessive allele