This is a bit lengthy, but bear with me - and I hope I'm not repeating someone else's post which I have yet to come across.
It seems like there's heaps of folk here that are right up there on the different budgie mutations and I'm learning heaps on the subtle differences between the different varieties - thanks to all who've shared so much of their knowledge, I am truly amazed at what others have stored in their heads.
I've read heaps of posts now but there is one thing that keeps cropping up - the half truth about the colour 'Green'. I keep reading that Green is dominant to Blue and I do apologise that it does niggle me to some extent. Daz will undoubtedly know where I am going with this since I've seen his attempts at pointing it out. Not sure if I will be just typing gobbledygook here, but I thought I'd try.
To burst the green colour bubble, genetically speaking, there is actually no 'Green' gene in budgies and hence, budgies do not produce green pigments. Yipes - some of you must think I'm a fruit loop. Now to burst the blue bubble - there are no 'blue' pigments in budgies as well. Aarrgghhh - now you really think I belong in the nut house.
okay. Let me try and show my screws aren't loose.
Parrots produce yellow from the pigment psittacofulvins (this pigment is also responsible for reds and orange in other parrots). The other pigment found in budgies is melanin which produces the blacks, browns and greys in the shell markings - the grey in the 'grey wings' is different to the grey in the 'grey body colour' - but I won't go there.
So if budgies only produce these two pigments, then how do we get the green colours??? Feather structure is the simple answer. The sub-cellular structure of the body feathers are such that they only reflect the 'blue' wavelength of white light. So, this combination of reflected blue light together with the yellow pigment in the feather gives us the phenotypic (visual) 'Green' budgie. [Take one drop yellow food colouring and one drop blue food colouring and mix together - what colour do you get?]
Now, the yellow pigment in parrots is not produced through diet (ie, it cannot eat foods high in carotenoids and become yellow as a result, unlike some other bird species). This pigment is produced genetically. So, if the gene that is producing the yellow protein (this is a simplified statement) has a fault and is 'shut off' or becomes 'non-functional', no psittacofulvins are produced (but keep in mind that all genes are in complimentary pairs and hence both copies have to be 'shut off' before there is truly no yellow pigment produced). So if both 'yellow' genes are 'off', without the yellow pigment, but with the feathers still reflecting blue light, we see 'blue' budgies.
So what's all this about Green being dominant to Blue? As explained above, we need to keep in mind that as long as one 'yellow' gene is operating, there will be enough yellow pigments to combine with the blue reflected light to give green. So, you need BOTH copies of the 'yellow' gene to be 'shut down', before you only see a blue bird. This is why it appears that 'Green' is dominant to Blue. Am I making sense here?
How do we get the yellow birds then? Similar but longer story - but in brief, shut down the genes that code for the sub-cellular structures that reflect blue light and you only see the yellow pigments.
And that's why when you cross a yellow bird with a blue bird, you can get white birds - because white birds do not have the functional genes to produce yellow pigments nor blue reflected light. Daz pointed this out in one of the posts.
In more scientific jargon, the above can be explained in just one sentence (taken and summarised from the website given below):
"The feathers of the Budgerigar contain only black and yellow pigments, yet Budgies come in white, yellow, blue and green. Why? Absence of pigment makes them white; cortical pigments make them yellow; core melanins combined with a reflective cloudy zone make them blue. Cortical yellow pigment combined with structural blue produces green."
What about all the other colours then? Well, that's all nicely explained in various parts of this forum but in brief, all budgie colours are really a different shade of blue or grey that is affected by genes controlling intensity (word used loosely) of colour or where the colour is (or is not) laid down in the feather. It's almost as simple as that.
I hope I haven't confused the issue or for that matter, taken away the mystery of colour in this fascinating hobby. It's just that I've seen some incomplete information spreading across the internet and thought I might throw in new information that is now available. To be fair, I have also noticed web sites where the so called 'Green' gene has already been dropped from the list of genetic codes.
For a more twisted explanation, see this article from the Birder's World web site.