The term allele derives from allelomorph: that can arise with unequal forms. In the field of biology, each gene that, in a pair, is located in the same place on the same chromosomes, is called an allele. Alleles are the different forms a gene can take, each with its own sequences. When appearing, they determine certain characteristics according to their properties. Blood type and eye color, for example, are expressed through alleles.

Mammalian animals, such as humans, usually have two sets of chromosomes, one from the mother and the other from the father. They are, therefore, diploid entities. The different pairs of alleles are found at the same place on the chromosome. The allele is the value that is assigned to the gene throughout said confrontation and it depends on whether or not it manages to establish its dominance and marks how the copies of the gene that were procreated will spread. It should be noted that the copy, or correctly the set of copies, of the gene that is procreated is not always in the same circulation, since it can also be different.

Taking into account the power of the allele just mentioned, it is not singular that we can determine a hierarchy and that is why we say that the alleles can be dominant (if the mother and the father have it, it will always be manifested in the chromosome of its descendant and sees in it only with one copy of the producers) or regressive (they have to be provided by the parents when reproduction occurs and two copies of a gene are necessary for its expression in the resulting chromosome). This link between the alleles is known as dominance: one manages to hide the phenotype (the way the genotype is expressed according to the environment) of the other allele located in the same position on the chromosome. Genetic inheritance depends on these dominance relationships.

The monk and naturist Gregor Johann Mendel, born in what is now the Czech Republic in 1822, was especially interested in genetic inheritance, to the point of having specified laws that establish the essential set of rules about the broadcasting of the traits of animals. organisms that through it living beings carry out when they procreate. Mendel’s laws are considered the basis of current genetics, however from its publication in 1865 until its resurgence in 1900 they were unknown. Mendel still argues that the common thing is that each gene presents more than one allelic form, so that we find the normal allele (also known as the wild or wild allele) in a much higher proportion than the surplus, and that the equivalents, that is, those that occur in the female, can appear in different degrees of abundance and are called polymorphisms.