The term allele derives from allelomorph: that it can arise with unequal forms. In the field of biology, an allele is called each gene that, in a pair, is located in the same place on the same chromosomes. Alleles are the different forms that a gene can take, each with its own sequences. When they appear, they determine certain characteristics according to their properties. Blood type and eye color, for example, are expressed through alleles.
Mammalian animals, like 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 in the same place on the chromosome. The allele is the value assigned to the gene throughout this confrontation and it depends on whether or not it is able 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 way as being put into 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 alleles can be dominant (if the mother and father have it, it will always be manifested on the chromosome of their descendant and see 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 on the resulting chromosome). This link between alleles is known as dominance: one manages to hide the phenotype (the way the genotype expresses itself according to the environment) of the other allele located in the same position on the chromosome. The heredity depends on these dominance relations.
The monk and naturist Gregor Johann Mendel, born in the present-day 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 the organisms that living beings carry out through it when they procreate. Mendel's laws are considered the basis of current genetics, however from their publication in 1865 until their revival 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 by the namewild 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.
