What is codominance?

When studying Genetics, it is common to resolve issues involving dominance and recessivity. Dominance concerns those alleles that are expressed even in a single dose, and recessive alleles are those that are only expressed in pairs. However, not all alleles behave this way, there are cases, for example, of codominance.

→ What is codominance?

Codominance can be defined as a situation in which heterozygous organisms express both alleles of a gene at the same time.. In these situations, relationships of dominance and recessivity are not perceived, as both alleles are fully expressed.

Another characteristic of codominance concerns the phenotype. In situations of dominance and recessivity, we observe two phenotypes, as the heterozygote has the phenotype of the dominant allele. However, in the case of codominance, the phenotype of the heterozygote is different from that observed in homozygotes.

→ ABO system, an example of codominance

An important example of codominance in the human species is the ABO system

. This system determines four types of blood: A, B, AB and O. These blood types are determined by three different alleles: I^THE, I^B Hey. As it has three alleles determining this characteristic, in addition to codominance, the ABO system is a case of multiple alleles.

Among alleles I^THE and I^B, there is codominance, and each one of them is dominant over the i allele.Therefore, when an individual is I^THEI^B, the phenotype is blood AB, as the two alleles are expressed. However, when the combination I^THEi, we have blood A, and when combination I occurs^Bi, we have a B blood. Therefore, we have:

Phenotype	Genotype
THE	I^THEI^THEor I^THEi
B	I^BI^B or I^Bi
AB	I^THEI^B
O	ii

Curiosity:Another well-known case of codominance is observed in Shorthorn cattle. Individuals with red coats are AA homozygotes, and those with white coats are BB homozygotes. AB heterozygotes have white and red hairs that are distributed alternately.

By Ma. Vanessa Sardinha dos Santos

Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/biologia/o-que-e-codominancia.htm