Codominance vs. Incomplete dominance

[Crystal]

Hello!

I have been working on updating the Genetics section on FIC.

For those of you who are interested in learning a ton about genetics--but may not already have a strong background in the subject, Basic Genetics - from an animal science perspective is probably the most comprehensive and easy to understand guide I've come across on the internet. It is published by Cornell University and uses animals (usually mammalian coat colors) for illustration of genetic concepts.

In the process of researching for my updates to FIC, I have seen some people use "Codominance" and "Incomplete dominance" either interchangeably (or perhaps incorrectly?) so I wanted to create a PSA of sorts here. These two terms do not mean the same thing, but rather describe different phenomena. (It is important to recognize, however, that these classifications were devised by people for the sake of easier analysis and communication, and that the designation as dominant, incompletely dominant, and codominant is somewhat arbitrary, as I'll explain.)

As you probably already know, a dominant gene/allele (A) overrides a recessive gene/allele (a) such that the phenotype (or physical appearance) of an AA genotype bird is the exact same as an Aa genotype bird, but different from an aa genotype bird. An example of this is blue body in gouldians--you can not visually distinguish a bird which is normal from a bird which is split for blue, but a blue bodied gouldian is very obvious to spot.

In codominance, BOTH alleles are expressed simultaneously without any blending. If allele A codes for circles to appear on the surface of a cell, and allele B codes for squares to appear on the surface of a cell, an AB phenotype would code for both circles and squares to appear on the surface of a cell. This occurs in blood groups (A, B, AB, O) and in the coat color "roan" of horses (if you mate a horse with red hairs to a horse with white hairs, the baby horse has some red hairs and some white hairs and appears 'roan'). Here is the definition with illustrated example of codominance from the Cornell tutorial: http://www.ansci.cornell.edu/usdagen/codominance.html

One example of what I believe is codominance in finches is when interbreeding black-rumped owl finches to white-rumped owl finches. Offspring from this pairing produces young which do not have a solid color rump (as if white or black was "recessive" to the other), nor a gray rump (as if the black and white had 'blended' together as occurs with incomplete dominance), but rather the young have some solid black feathers and some solid white feathers on their rumps.

When breeding results in offspring which appear to be a blend between the two 'extremes' (i.e. if AA codes for circles, and BB codes for squares, AB would create a shape in between a circle and a square--like an octagon, maybe), this is called incomplete dominance. Here is the definition with an example from the Cornell guide: http://www.ansci.cornell.edu/usdagen/inc-dominance.html

Although I have seen several sources attribute yellow-body inheritance in gouldians to "codominance," I believe that when discussing physical appearance of the bird (i.e. at the level of the organism), the yellow-body and green body genes/alleles in lady gouldians exhibit incomplete dominance, which is why a cock bird carrying a yellow gene and a green gene (assuming he has a purple breast) appears to be a shade in between yellow and green, like a blending of the two extremes of the phenotype. If yellow body and green body were codominant (at level of the organism, or phenotypic appearance of the bird), the bird carrying a yellow gene and a green gene would have some all-yellow feathers and some all-green feathers.

Because the production of color pigments is the end result of a complex cascade which occurs on a biochemical level (some colors also being dictated by the structure of the feather itself), however, it is still possible that the green and yellow genes may show codominance at a molecular or even cellular level.

This is when the terms become arbitrary--the type of dominance inferred depends on the phenotypic level at which the observations are being made--at the level of the whole organism, the cellular level, or even the molecular level. Which is to say, that what appears to be incomplete dominance at the organism level may in fact be due to codominance at the molecular level. A good example of this is sickle cell anemia: http://www.ncbi.nlm.nih.gov/books/NBK21920/

Until we know more about the exact biochemical mechanisms that occur under the influence of each gene (and in many cases, resulting from interactions of multiple genes), ambiguity will remain.

[nixity]

Great read, Crystal

The one thing that strikes me with the Incomplete Dominance aspect of the pastel gene in Gouldians is the fact that SF birds (both cocks and hens) sometimes have rogue dark feathers often confusing people into thinking they are "pied." (Which is a misnomer, because pied in my mind infers patches of lighter or nonpigmented feathers; not patches of dark feathers)

I have noticed with this years' breedings that this trait is heritable.
SF Yellows that do not have these rogue feathers tend to produce SF offspring that also lack them, while SF Yellows with the rogue feathers tend to produce a higher proportion of offspring that exhibit rogue dark feathers.

So, as you said, there's likely more than meets the eye going on.

[Crystal]

Out of curiosity, do those rogue green feathers show up in SF pastel purple breasted the same as in white breasted?

Have you been able to determine if the rogue feathers (I'm assuming they persist from molt to molt) are inherited as linked with the pastel gene or if they could be autosomally inherited? "Dominant" or recessive?

Maybe you have something more than the garden variety pastel gene in your charm?

[monotwine]

Thanks Crystal. Great read which you made so very easy to understand.

I also have those solid green rogue feathers in SF yellow males and yellow hens. Both hens and cocks with it do pass it on to their young (with our without a partner that does not present this colouration). So far my birds retain the few solid feathers each moult. Some of their young also present a greenish tinge to their upper feathers (almost like a coating over their “shoulders”. Difficult to describe. Not a very clear solid color or particular feathers, just tinges of green to the yellow colouration. But I have only been monitoring it for 2 years. I know a breeder here trying to specifically breed this trait into his yellows to get a bird that looks like the Aust. Yellow. He takes all my best ones to add to his breeding programme. My birds that present these rogue feathers were from his original breeding too. Time will tell if it works to create the green-yellow patchwork.

[nixity]

Out of curiosity, do those rogue green feathers show up in SF pastel purple breasted the same as in white breasted?

It depends. Most of the time, PB SF Pastels are just a variation between lemon-lime or line green and the color is blended and not splotchy. I only have seen the rogue dark feathers with LB or WB SF Pastels. In LB and WB SF Pastel Blues when they pop up they're usually bluish gray feathers

Have you been able to determine if the rogue feathers (I'm assuming they persist from molt to molt) are inherited as linked with the pastel gene or if they could be autosomally inherited? "Dominant" or recessive?

Maybe you have something more than the garden variety pastel gene in your charm?

I haven't. It would take a few years of really selectively working with them to try to understand where it might be manifesting. I definitely think it is a dominant thing because every yellow baby (male and female) produced from my pair have these rogue feathers and often also have blue feathers in their rump. I will try to get some pictures next weekend to show it.
I think it must be linked to pastel only because the normal babies do not show any dark feathers but then.. that could be simply because they're already green and anything rogue would no longer be 'rogue' by proxy?