The Genetics of Colour in the Budgerigar and other Parrots
An original article by Peter Bergman of Sydney, Australia
Gene function in Yellowface Budgerigars
Part 1 - When is a Yellowface not a Yellowface?
Yellowface budgerigars have probably had more written about them than any other variety. Opinions differ about what Yellowfaces really are and debates about Yellowface genetics have at times become quite heated.
The prevailing view today is that the three Yellowface varieties are brought about by alleles of the genes for Blue and Green. It makes sense that this should be so because it allows the same genetic infrastructure which produces the yellow pigment in the Green bird to produce the yellow pigment in the Yellowface varieties as well.
If the Yellowface genes were not alleles of Blue and Green then it would require a restructuring of the yellow producing apparatus which would be more complicated to evolve than the simple mutation of a pre-existing gene.
Not all breeders agree that the Yellowface genes are alleles of Blue. Some breeders claim that they have paired Green birds to Blues and produced Greens, Yellowfaces, and Blues from the one pair. If the Yellowface genes are alleles of Blue this result should not be possible. However,it should be pointed out that in the instances I am aware of where it had been claimed that a Green to Blue pairing yielded Greens, Blues, and Yellowfaces in the nest, the Yellowfaces involved were Goldenfaces. That in itself suggests the Green birds in the pairings may not have been true Greens at all but heavily pigmented Goldenfaces.
Patterns of inheritance
Most articles about Yellowfaces have focused on the genetics. Genetics is fine as far as it goes, but genetics is only about patterns of inheritance. To understand why the Yellowfaces look the way they do one has to step back and take a broader perspective and attempt to unravel what is going on in terms of the way genes are known to function.
The Yellowface varieties are the budgerigar equivalent of the Parblue varieties in other species of parrot. However, uniquely in budgerigars, the double factor birds contain less yellow than single factor birds. This paradox has caused much puzzlement over the years and is the source of the debate about whether the Yellowface alleles are yellow producing genes or yellow reducing genes.
Blue budgerigars first appeared in the early 1880s. These Blues were established and their descendants distributed around the world. It took several decades for Blues to become widely available, but within a decade of their becoming a common variety all three Yellowface varieties appeared in quick succession in the mid 1930s. The nearly simultaneous appearance of all three Yellowface genes suggests that they lay hidden as splits in the Green population for a long time.
The most curious of the Yellowface varieties is the Mutant 1 Yellowface in which the double factor bird looks exactly like an ordinary Blue. This simple fact begs the question, how can we be sure that the common ordinary Blues arent really the double factor Mutant 1 Yellowfaces and that those Blues we think of as double factor Mutant 1 Yellowfaces arent in reality the true Blues? The way I see it there is an important detail the hobby has overlooked, and that is:
There is no rule that says mutant genes have to appear in any particular order.
We only assume that the common Blues are true Blues because they happened to have appeared and become established as the true Blues first. The Mutant 1 Yellowface gene appeared later but it could just as easily have happened the other way round. How can we be sure the hobby hasnt got it backwards and that the double factor Mutant 1 Yellowfaces arent really the true Blues?
If that sounds farfetched then have a look at the situation in Peach-faced Lovebirds. Breeders have established two types of Parblue but true Blues have yet to make an appearance.
Imagine a different World
Imagine a world in which our double-factor Mutant 1 Yellowfaces had become established as the true Blues first and those birds we currently think of as Blues appeared later and were dubbed double-factor Mutant 1 Yellowfaces..
Our greenish single-factor Goldenfaces would be thought of as Goldenface-Mutant 1 Yellowface composites while those birds we currently think of as Goldenface-Mutant 1 Yellowface composites would be regarded as single-factor Goldenfaces. Single-factors would have the same distribution of yellow as the double-factors but would be somewhat paler in their depth of yellow.
The Goldenface and Mutant 2 Yellowface alleles would be seen to function in a straight forward manner like the Parblue varieties in other species of parrot. A single Yellowface gene would put yellow pigment into the bird and a second dose would simply produce a deeper yellow. No longer would fanciers have reason to regard the Yellowface alleles as suppressor genes.
Our whole perception of Yellowfaces has been moulded by a random event in history. Simply changing the order in which the Blue and Mutant 1 Yellowface alleles appeared would cause a 180 degree reversal of how Yellowface budgerigars are perceived. Unfortunately for Yellowface breeders, the wrong allele appeared first.
All of this might be good enough to correct the situation in Mutant 2 Yellowfaces and Goldenfaces but we are still left with the question of why does the double factor Mutant 1 Yellowface look blue?. This is a very difficult question to answer in terms of the way genes are known to work. Perhaps we have been asking the wrong question. This brings me to the second detail the hobby has overlooked:
There is no rule that says there cannot be more than one allele for blue.
As it now stands we are expected to believe there is one Blue allele and three Yellowface alleles. What is wrong with there being two different Blue alleles and only two Yellowface alleles?
What defines a variety?
In the double factor bird we see the action of a gene in its purest form. The double factor bird defines the variety. Therefore if the double factor Mutant 1 Yellowface looks Blue then a Blue is what it is. It is a Blue allele not a Yellowface allele.
This is how I have come to regard the Yellowface and Blue series. For my own purposes I have designated the common allele as Blue Mutant 1 (b1), and I have redesignated the traditional Mutant 1 Yellowface allele as Blue Mutant 2 (b2). The birds are still all the same, I have merely shifted my perspective a bit. Instead of having to answer the difficult question why does the double factor Mutant 1 Yellowface look Blue? the question has become why is it when you combine two different kinds of Blue you get a Yellowface budgerigar?. With a bit of knowledge of biochemistry this problem isnt too difficult to solve. To understand how this can happen we have to understand how genes work in the first place.
Genes are like blueprints
Genes are sets of instructions for making proteins. Genes are like the blue-prints, the proteins actually carry out all their functions. If genes appear to behave in a peculiar manner such as two different kinds of Blue producing a Yellowface, then we have to see if it is explainable in terms of the kinds of proteins the genes could be making.
Most of the proteins genes code for are enzymes. Enzymes are the proteins which perform the chemical reactions in the body. In a Green bird the gene for Green (designated B) codes for an enzyme which is responsible for the production of yellow pigment. The b1 allele is a mutant form of the B gene therefore it produces a slightly different version of the enzyme. This (b1) enzyme is defective so no yellow pigment is produced and we have a Blue bird, but it is important to note that the enzyme is still there. Mutant 2 Blues have no yellow pigment because the b2 allele likewise produces a defective enzyme. Because the b1 and b2 alleles are different each version of the enzyme has a different defect.
The question now becomes how can two kinds of defective enzyme work together to make yellow pigment?.
To find the answer to this question we have to look at protein structure. Proteins are made as chains of amino acids and are reconfigured into the final product afterwards. Different kinds of enzymes have different structures. Some kinds of enzymes are made of only a single protein chain. Others are made of two copies of the protein chain. Still others are made of four copies of the protein chain.
If the enzymes produced by this series of alleles consisted of only a single protein chain then there would be no Mutant 1 Yellowfaces. Birds with the genetic constitution b1b2 would look like ordinary (white-faced) Blues. What the Mutant 1 Yellowface tells us is that the enzyme which produces yellow pigment is made of more than one protein chain. It might be two protein chains it might be four. For the purposes of this discussion it is reasonable to assume the enzyme is made of only two protein chains until proven otherwise. Based on the evidence at hand:
However since the protein chains are assembled into the enzymes randomly Mutant 1 Yellowfaces also produce (b1/b2) hybrid enzyme. It is the (b1/b2) hybrid enzyme that produces the yellow pigment.
Each blue allele makes an equal contribution to the hybrid enzyme. The Mutant 1 Yellowface is really only a by-product of the interaction of two different kinds of Blue. There is no Mutant 1 Yellowface allele or more precisely it was incorrectly named in the first place. Neither of the two blue alleles is dominant or recessive to the other. They stand at the bottom of the allelic dominance hierarchy on an equal footing B bg by2 b1, b2. This type of inheritance pattern has sometimes been called allelic complementation.
The Mutant 1 Yellowface is a composite variety along much the same lines as the full body coloured Greywing. The full body coloured Greywing has one greywing allele and one clearwing allele, but there is no full body coloured greywing allele.
Why not Creamface?
Here in Australia the Mutant 1 Yellowface is commonly referred to as the Creamface. The term is both descriptive and does not mislead fanciers into thinking there is an actual Yellowface gene involved. I will adopt the term Creamface for much of the rest of this discussion. By knowing how to read the clues, the Creamface has already given us much information:
The reasoning behind the third point is as follows:
In order for the hybrid (b1/b2) enzyme to function at all, the (b1) and (b2) protein chains have to be able to at least partly compensate for each others defects. This can happen if the defect in the (b1) protein chain is in a different location to the defect in the (b2) protein chain. The (b1/b2) hybrid enzyme works, but doesnt work particularly well, which is the main reason the yellow is so pale. After all, each hybrid enzyme molecule still contains two defects. Since the defects evidently occur in different locations along the protein chain it automatically follows that the mutations occurred in different locations along the original gene for Green.
Point 3 will figure more prominently in Part 2 of this discussion.
Copyright: Peter Bergman (Sydney, Australia)
I would be pleased to have your comments on what I feel is a very important article