The allele idea refers to the value adopted by each gene that, in a pair, occupies the same place in the chromosomes that are homologous. Dominant, on the other hand, is an adjective that qualifies the one or the one who manages to dominate (that is, who imposes himself).
To define what a dominant allele is, we must understand what dominance is in the field of genetics. This concept alludes to the link established by the alleles that are part of the same gene when one of these alleles manages to mask the manifestation of the phenotype of the other allele. The allele that manages to prevail is the dominant allele, while the other allele is the recessive allele.
It is important to keep in mind that the dominant allele does not always play this role, in the same way that the recessive allele can become dominant in certain contexts. Thus, if the same gene has more than two alleles, one allele can be dominant with respect to the second and recessive with respect to the third.
A clear example of dominance, which has been used for a long time in science, is the shape of the seed of the pea (also known as scad and scientific name Pisum sativum), since it is one of the traits that must be inherited from one generation to another and can be smooth and rounded, two characteristics provided by the dominant allele R, or wrinkled, if the recessive allele r wins.
Although at first glance it may seem irrelevant, the study of peas by the Czech monk and naturalist Gregor Johann Mendel revolutionized at the time the field of genetic inheritance, which arose from its well-known laws. Through the observation of the reproduction of these herbaceous plants belonging to the Fabacea family, Mendel discovered many of the behavioral characteristics of genes, and thus came to notice concepts such as those of the dominant or recessive allele, something that can also be appreciated in our species.
The human being, most of the animals and certain plants are diploid organisms since they have pairs of chromosomes. Each of these chromosomes, in turn, has two versions called haploids: one developed by the father and the other by the mother. Homologous chromosomes have the same structure and genes as their corresponding homolog, although with different manifestations (alleles).
The dominant alleles are those that, either appearing in double dose or in single dose, manage to prevail in the expression of certain characteristics and, therefore, they are manifested in the specific expression of the genotype (the genetic information) known as the phenotype., for example through eye color, hair characteristics, or the type of nose and ears.
In the specific case of the eyes, there is a clear DEFINITION OF possible features, both dominant and recessive. For example, brown eyes are known to represent a dominant feature over blue, green, brown, and gray. Regarding vision, similarly, good vision is dominant above any disorder that prevents the subject from seeing normally, such as night blindness, color blindness or myopia.
Another aspect of human beings in which we can notice the presence of dominant and recessive alleles is hair color. Dark is dominant with respect to red and blonde, just as curly (or curly) dominates straight hair. We should also note that baldness is recessive, as the dominant feature is a scalp that covers the entire scalp.