The term allele derives from allelomorph: which can appear in different forms. In the field of biology, each gene is called an allele that, in a pair, is located at the same site on homologous chromosomes.
Alleles are the different forms that a gene can take, each with its own sequences. When manifesting, they determine certain characteristics according to their properties. The blood group 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. It is, therefore, diploid organisms. The various pairs of alleles are at the same site on the chromosome.
Genes, in the context of cellular meiosis, dispute their position on the chromosomes. The allele is the value that is assigned to the gene throughout this confrontation and it depends on whether or not it manages to establish its domain, and it marks how the transmission of the copies of the gene that was procreated will be. It should be noted that the copy, or the set of copies, of the gene that is procreated is not always in the form of an identical transmission, since it can also be different.
Taking into account the potency of the aforementioned allele, it is not unusual that we can establish a classification, and that is why we say that alleles can be dominant (if the mother and father have it, it will always be expressed in the chromosome of their descendant and it is seen in it only with a copy of the procreators) or recessive (they must be provided by the parents when procreation occurs and two copies of a gene are necessary for its expression on the resulting chromosome).
This link between the alleles is known as dominance: one manages to mask the phenotype (the form of expression of the genotype –genetic information– 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 naturalist Gregor Johann Mendel, born in the present Czech Republic in the year 1822, was especially interested in genetic inheritance, to the point of having defined laws that establish the basic set of rules about the transmission of the traits of the organisms that through it living beings carry out when they procreate. The laws of Mendel are considered the basis of the current genetic, although since its publication in 1865 until its revival in 1900 were ignored.
Precisely, in Mendel’s extensive and in-depth research, a DEFINITION OF alleles is found that is quite graphic and accessible to people outside of genetics. It focuses on the reproduction of peas, and points out that the gene in charge of regulating the color of its seed has two alleles: one to determine the proportion of yellow, and another for that of green.
Mendel also ensures that it is normal for each gene to present 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 rest, and that the corresponding ones, that is, those that occur in the female, can appear in different degrees of abundance and are called polymorphisms.
From this it follows that for each pair of alleles (a1 and a2) we can find three different classes of combinations in diploids, which are the following: (a1, a1); (a2, a2); (a1, a2).