Gene interaction between allelic genes
According to Mendel, when crossing two different individuals (pure races) for the same character, the resulting hybrids manifested in their phenotype the dominant character (that of one of their parents). This type of dominance is known as complete dominance .
But this is not always true, since it is possible that this dominant character is not expressed in F1, but rather that it does so in a diluted way (intermediate inheritance) or that another different character appears in which the two alleles are expressed (codominance).
Review: Types of genetic inheritance (4th ESO).
Codominance occurs when each of the two alleles of a gene from a heterozygous individual manifest independently. That is, in the phenotype of the hybrid, the two codominant alleles are manifested simultaneously, they both have the same intensity. By crossing these individuals, a 1: 2: 1 phenotype (AA: 2Aa: aa) is obtained.
For example, blood group AB or Short-horn cattle , whose hair can be red, white and roan (half red and half white).