Coral Glow Genetics

[paulh]

no, co dominance displays both phenotypes in their entirety. For example if a normal looks normal and the super fire looks like a BlkEL. The fire would have parts that look normal and parts that look BlkEL. no mixing, no inbetween. A true co-dom would look like a paradox of some kind.

The closest we come in ball pythons is scaleless/scaleless head. Normals are normal, Scaleless are scaleless and the scaleless head show scaleless and normal parts. If you inspect them closely though, scaleless heads also have smaller scales sometimes which is a mix between the two, so we really can't call it co-dom truly. but its the closest we got.

A true codominant need not look like a paradox of some kind to the eye if the spotting effect is cellular. Example: Burmese, Tonkinese and Siamese cats. The Tonkinese is the heterozygote. The Burmese gene and Siamese gene produce different amounts of melanin in the color cells. The Tonkinese has less melanin than the Burmese cat and more melanin than the Siamese cat, producing an intermediate color.

This is a confusing subject because I know of three definitions for codominant vs incomplete dominant:

1. Each of the three possible gene pairs has its own phenotype. Codominant and incomplete dominant are synonyms.

2. Each of the three possible gene pairs has its own phenotype. In incomplete dominance, the heterozygote's phenotype is roughly intermediate between the two homozygote phenotypes. In codominance, the heterozygote's phenotype is a mixture of the two homozygote phenotypes, and both can be distinguished in the heterozygote. Example: four o'clock flowers (ID) and human blood types A, B, and AB (CD).

3. Each of the three possible gene pairs has its own phenotype. In incomplete dominance, one allele produces a functional product and the other produces a nonfunctional product. The difference between the three phenotypes is caused by the amount of product produced. In codominance, both alleles produce a functional product, and the heterozygote's phenotype is produced by the mixture of products. Example: four o'clock flowers (ID) and human blood types A, B, and AB (CD).

Anybody care to add another definition?

Definition number 3 is out because we generally don't know whether a given gene produces a functional product.

Definition number 2 is hard to teach because we have to teach three terms--incomplete dominance, codominance and overdominance (the heterozygote's phenotype is outside the range bounded by the two homozygotes' phenotypes). Overdominance is rare, but we have to include it for completeness. By the way, overdominance has other definitions.

Definition number 1 is the simplest and easiest to teach and use for beginning genetics students. IMO, most herpers would be classified among beginning genetics students.

There is no one to one correspondence between the above definitions. Is the Burmese/Tonkinese/Siamese cat example incomplete dominance (def 2) or codominance (def 3)? And one pair of alleles can be classified different ways using different tests.