Dr John Pilkington answers questions on Genetics

Question to Dr John Pilkington from Gary Armstrong

I know of a person who bred some birds from which he bred a blue series budgerigar, but its mask is yellow and half of its body is yellow. Its head feathers are pure white and half of its underbelly from the mask down is yellow/blue, tail lacks yellow pigmentation. In its genetic background there is no known English yellow face that has been introduced for more than 4 generations, before that is anyone's guess! Is there any chance that this bird is a true yellow face as it has a mask that is yellow or is it a halfsider?

Answer: Thanks for the question Gary. From your description of the bird in question, it sounds to me as though the bird is both a Half Sider and a Yellowface - a rare bird in deed! However, genetically, there is nothing to stop such a rare bird from appearing. (If you could send a photograph of the bird to this website that would be interesting.)Editor's note: As you can see, Gary has now sent these photos, and Dr Pilkington has added a Postscript amending his original answer in response to the receipt of these photos.

It is important to realise that all genes work independently of each other, but the net effect of all the different genes, is that they produce the Phenotype of each bird. Each individual possesses millions of genes which have a range of subtle to dramatic influence on the appearance, behaviour and function of each individual. In the case of the colour genes of interest to budgerigar breeders, the effects of most single colour influencing genes is dramatic. TThe sudden appearance of a Yellowface Half Sider is such an example.

With regard to the genetics of the Half Sider, I have previously written an article on the subject which appears on this website The Half-Sider From this article, you will see that I do not consider the Half-Sider to be a true genetic mutation, and as such, the Half-Sider will not produce further Half-Siders. I consider the Half-Sider or Quarter-Sider to be a type of "birthmark".

However, with regard to the Yellowface, this is a true genetic mutation. Unfortunately, you do not give the colour of the parents of the Yellowface Half-Sider, but since the appearance of this bird has surprised you, I will assume that neither parent was a Yellowface, nor a Half-Sider.

There are at least two different genes that produce the Yellowface; mutant Type 1 and mutant Type 2. Since the Wild-Type budgerigar, is a Normal Light Green, it is obvious that such a bird has a yellow face, and as such, it is impossible to tell if a Green series or Yellow series bird possesses a Yellowface gene, whereas, in a Blue Series or White series bird, a Yellowface genotype may produce a Yellowface phenotype. In the case of a Blue or White series bird, if it is heterozygous, possessing a single Type 1 Yellowface gene, then the phenotype will be a Yellowface (visual Yellowface), but if the bird is a homozygous (two genes) Yellowface mutant Type 1, then it will appear as a Normal Blue or White series bird without a yellow face.

By coincidence, I have previously written an article on the subject of the Yellowface which also appears on this website The Question of a Yellowface Albino It is possible for a Yellowface to appear in a nest after the Yellowface gene appears to be "lost" for the 4 generations you say is the case in the ancestry of this bird.

It is exceptionally important to understand that genes do not become diluted with the passing of each generation. It is wrong to think that an individual possesses one quarter of each of its grandparents' genes, and one eighth of each of its great-grand parents' genes, and so on. The phenotype masks most of the genes that any individual possesses - if only we could see the genotype! If an ancestor possessed a specific gene, a descendant many generations later may exhibit the same gene.

As you know, it is possible to produce a Blue budgerigar from two phenotypically Green parents; in this case, we can deduce that both parents are heterozygous, i.e., Green/Blues. Likewise, the Yellowface gene would be masked by the Green or Yellow series bird, in the same way that the phenotypically Green bird may mask the Blue gene, as in the case of the heterozygous Green/Blue.

In the case of the Yellowface mutant Type 2; this gene acts as a true dominant gene in which both heterozygous and homozygous conditions produce the same Yellowface phenotype. The Yellowface Type 2 gene produces a golden yellow mask and yellow suffusion over the whole body, whereas the Yellowface Type 1 gene produces a lemon yellow coloured mask, only some breeders claim that they can even tell if a Green or Yellow series bird possesses the Yellowface Type 2 gene since it is claimed that it produces a deeper golden yellow mask, rather than the mask colour normally seen in Green or Yellow series birds. I hope that my answer gives you food for thought.

Postscript from John Pilkington

At the time of writing the answer to Gary Armstrong's above question, I only had Gary's description to go by. I had some doubt, at the time of writing, that the description of the bird may be incorrect, hence my request for photographs.

Now that photographs of this bird have been supplied, it is clear that the markings of this bird are not those of a typical Half-Sider, because the yellow colouration is patchy rather than an area of solid yellow colour. Also, from the photographs, it looks to me that the bird in question does not have a complete yellow mask as claimed, although there appears to be a little yellow patch by the violet cheek patch on the bird's right side. (In view of the fact that I will never see this bird, I will take Gary's word that the bird has a full yellow mask.)

I have personally seen a Normal Grey with similar yellow patches on both body and the nape of the neck not unlike the Skyblue bird in the photograph. Such markings are obviously unusual and are a kind of mottling, but of a different type to those described in other articles on the Mottled budgerigar. In the case of the Skyblue budgerigar in the photograph, and the Grey budgerigar that I have described, certain feathers appear yellow because of the presence of yellow pigment in the cortex of the feather; for some reason only a few feathers are affected.

In many different species "spotting" or "mottling" occurs, and in domesticated animals, breeds are developed deliberately to establish such markings - Dalmatian dogs being the classic example.

In the case of the yellow mottled Skyblue in the photograph, it seems to demonstrate that a blue bird has some genetic capability to produce yellow pigment, and as such, it suggests that the blue gene acts as a suppressor to the production of yellow pigment.

The fact that certain feathers in an otherwise blue bird are yellow, suggests that in the very early embryonic stages of development one cell divided (mitosis) and the genetic information for the blue gene on one chromosome, was not correctly replicated. In view of the fact that the Blue gene is recessive, and for a budgerigar to appear blue, both chromosomes of the pair must possess the same Blue gene; any alteration will thus prevent the feathers appearing blue in colour. The original modified cell will give rise to a few descendant cells which migrate from the source during development and will thus produce patches of yellow on an otherwise blue bird. The fact that the yellow markings are one-sided, indicates that the origins of the yellow colouration are developmental (although genetically based), and in view of the the explanation I have given, I think it is exceptionally unlikely that the yellow mottling is heritable. If the genetic aberration had occurred during sperm or egg production (meiosis) then the condition would be heritable (the origins of spotted breeds)

In the case of Mottled budgerigars described in other articles on this website, the mottling is caused by the absence of melanin in the feathers and this is obviously a totally separate gene system to the Green/ Blue gene system.

Editors' note: I regret that Dr Pilkington is unable to answer any further questions.

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