An allele is an alternate form of a gene that is found in a specific position on a specific chromosome . These DNA encodings determine the different traits that can be transmitted from parent to child through sexual reproduction . The process by which alleles are transmitted was discovered by Gregor Mendel and was formulated in what is known as Mendel’s law of segregation.. In people, traits such as eye color can be caused by the interaction of a single pair of alleles. But in the more complex traits, such as height, they are generally caused by the interactions of series of alleles. Some alleles are dominant over other alleles, as in the case of heterozygous pairs .

What are alleles?

Alleles are the pairs of genes that occupy a specific place on a chromosome. The word is derived from allelomorph , which means that it can arise in unequal forms .

  • Allele genes, homozygous and heterozygous
  • Types
  • Where allele genes are located
  • Examples

Allele genes, homozygous and heterozygous

The word allele is derived from the word allelomorph , the meaning of which is that it can be displayed in different ways. In the field of biology proper, the word allele is used to designate each gene that is located in the same place where the homologous chromosomes are . In other words, alleles are the different forms that a gene can take, each with its own sequences. When these genes manage to manifest themselves, they determine some of the characteristics of the individual depending on their properties. For example, the color of the skin, eyes, blood type, etc. is manifested. When cellular meiosis occurs , the chromosomes fight the field and during the confrontation, the allele will be the value that is given to the gene and it will depend on whether or not it manages to dominate the transmission of information . When we talk about homozygous allele genes , we mean the genetic makeup of a specific trait that a diploid organism has . Each of the homozygous alleles is inherited in a particular way from its parent, so that when the alleles of that gene are the same, it will be homozygous . The heterozygous allele speaks to us in two different waysthat have a particular gene, are individuals who have different pairs of alleles that determine a characteristic .


  • Dominant alleles : the dominant word is used to qualify what is able to dominate or impose itself. It is also important to know that the dominant allele does not always play this (dominant) role, in the same way that the recessive one can become a dominant gene in certain cases. In this way, if the same gene has more than two alleles, one can be dominant with respect to the second and recessive with respect to the third. The dominant alleles are those that are responsible for determining the characteristics that can be observed in the offspring, that is, those that determine the phenotype . In addition, they are responsible for determining the characteristics that are manifested at the genetic leveland that they are inherited from one of the parents.
  • Recessive alleles : they are those that have the least possibilities of expressing or determining physical or genetic characteristics in the individual. They manifest only in the presence of another recessive allele . Despite this situation, even if a recessive allele does not appear in the phenotype or genotype of the offspring, it may be present. When both parents have recessive alleles, their offspring show the characteristic of that gene. Recessive alleles are represented by the lowercase letter r and when both are recessive they are represented by the symbol rr .
  • Multiple alleles: multiple alleles are the differences that a certain gene can have , they are capable of having more than two options to characterize a gene. They are responsible for the different characteristics that we can observe in living beings . For example, the type of blood, the color of hair and skin, or the color of flower petals.
  • Codominant alleles : they occur when there are two different alleles that are in the genotype and both are expressed . This expression results in a phenotype, which means the characteristics of both alleles. The expression of a gene in an organism that is heterozygous is then equivalent to that of its peer and will be related to some character.

Where allele genes are located

Alleles are located in the same position as homologous chromosomes.


In Chaetodipus intermedius mice , skin color is controlled primarily by a single gene. The gene encodes the protein that makes the pigment dark. Some of these mice have dark fur, and some have light fur. The dark coat allele is dominant and the light coat allele is recessive .

For example, in the flower mouth dragon , Antirrhinum majus , a cross between a flowering plant white homozygote C and a flowering plant red homozygous C, will produce offspring with flowers roses . This type of relationship between alleles, with a heterozygous phenotype intermediate between two homozygous phenotypes, is called incomplete dominance.

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