Advanced Genetics

The topics discussed in this chapter are not necessary in order to understand the breeding or production of cornsnake morphs. The information is presented for those who are curious and want to learn more about genetics topics.

Shades of codominance

When two alleles are codominant to each other, the appearance of the heterozygotes is different from both of the homozygotes. There are many synonyms for codominant, such as incomplete dominant, semidominant, partially dominant, mostly dominant, etc. What all of these terms have in common is that they specify a relationship where the different pairings of two genes can create three distinct phenotypes, as opposed to a dominant/recessive relationship where only two phenotypes exist. For all practical purposes, as far as this distinction is concerned, the above terms mean the same thing.

On a more technical level, the third phenotype is sometimes used to further sub-categorize the relationship. It can be a blending of the two looks where both alleles are partially or fully expressed, a mixing where certain parts of the animal fully express one allele and other parts of the animal fully express the other, and many other situations.

When subdividing codominant relationships, the term "incomplete dominant" is used to specify cases where a blending occurs, while "codominant" specifies that both genes are fully expressed in their own separate areas.

(BB)

(WW)

For example, let's take two alleles called black and white. When a specimen is homozygous for the black allele, it is solid black. When it is homozygous for the white allele, it is solid white.

Back Page 21 Next