by Ken Yorke
This article deals with some of the budgerigar genetic theory and the assumptions made in the creation of the software program BUDGEN. This article comes substantially from an optional reading section in the BUDGEN users manual, but is capable of standing in its own right as a text on general bird genetics described in very simple terms. This section does not profess to be a complete work on detailed genetic theory nor does it attempt to be. People with no knowledge of genetics should read this article and use it as a springboard for more detailed reading on the subject should it be of interest.
A CRASH COURSE IN GENETICS
All of the colours and physical characteristics of the budgerigar are controlled by genetic instructions. While some of these physical characteristics can be modified by environmental forces such as diet, health, injury, climate etc the basic characteristics of any budgerigar are determined by a set of genetic instructions it receives from each of its parents at conception. These instructions that the budgerigar receives remain basically unaltered in its body for its entire life and are partially inherited by its own offspring.
These instructions are known as genes (sometimes simply called factors). Connected strings of these genes are called chromosomes (Just like a string of beads, where the beads are genes and a string of beads is a chromosome). These genes, as do the chromosomes, occur in matching pairs. (Think of two essentially similar strings of beads laid side by side with the beads of the same color directly beside each other.) Different literary sources at different times have stated that the budgerigar has somewhere between 13 and 60 pairs of chromosomes in each cell. Irrespective of the exact number, all of the colour and marking factors (such as Violet, Opaline, Pied etc) that budgerigar breeders are most interested in are dispersed around these pairs of chromosomes. In general, most of the common varieties of budgerigar owe their appearance to individual factors. That is, the presence of a particular factor in either single or double dose is enough to change its appearance.
Since there are probably more varieties of budgerigar than there are numbers of chromosomes within the budgerigar then its seems logical that some of these chromosomes are home to more than just one type of factor. This has in fact been proven. Some of the menus in BUDGEN have grouped these linked factors together. The GREEN, BLUE & YELLOWFACE MENU also incorporates the Dark Factor since they share a common chromosome. The GREYWING, CLEARWING, DILUTE MENU as well as the SEXLINKED MENUS also reflect respective common chromosomes. There are likely to be other linked factors as well which as yet have not been identified and BUDGEN obviously doesn't reflect these and therefore treats them as unique separate factors.
All budgerigars have the same number of essentially identical chromosomes. A Greywing budgerigar for instance has nothing extra nor anything less in the way of factors (genes) than say a Pied budgerigar. The difference between the two genetically is in the way certain of their genes in specific places on specific chromosomes have undergone some kind of chemical change. This change results in what is known as a mutation (think of it as a particular random bead changing colour ). With the exception of the Sexlinked factors, all factors must occur in pairs. Any individual pair of genes can be configured in any of three possible states. 2 original non-mutant (Normal wild type) genes or 2 mutant genes or 1 mutant gene with 1 non-mutant gene. Birds which have 2 non-mutant genes will appear the same as the normal wild light green budgerigar as far as that particular characteristic is concerned. Birds with 2 mutant genes will usually show the new mutant characteristic (possibly a change in colour or wing pattern or just about anything). Birds with one mutant gene and one non-mutant gene have conflicting instructions and their physical appearance depends on which instruction is the stronger.
If the mutant gene is stronger than the non-mutant then this mutation is said to be Dominant. If the mutant gene is weaker than the non-mutant gene then this mutation is Recessive. Such a bird is said to be Split for that mutant characteristic. If the mutant and non-mutant genes are of approximately the same "strength" then a compromise is reached and this is known as Semi-Dominant.
Thinking back to the string of beads analogy, if a black coloured bead represents a normal wild characteristic, then a mutation of that bead might be white coloured bead. Therefore at that point on a pair of strings you could have either two black beads, two white beads or one black and one white. Extending the analogy a bit further, if the black beads has an inbuilt instruction to make say, a bird black and a white bead the instruction to make a bird white, then the following is possible:- two black beads create a black bird, two white beads create a white bird. One white and one black bead could have three possibilities:- 1) If White is recessive then you will have a black bird split for white, 2) If White is Dominant you will have a White bird (also known as single factor white), 3) If white is semidominant you get a compromise mixture of black and white, ie a Grey bird .
I'm sure that any professional geneticist would have a heart attack at some of the over-simplifications made in the above explanations but this is designed to give only a very simple grounding in genetics.
GREEN, BLUE , YELLOWFACE & DARK FACTORS
Normal Light Green is the natural state of the original wild budgerigar and any change in colour etc is due to a mutation. The Blue factor is a recessive mutation compared to the wild non-blue factor. The Dark factor is semi-dominant compared to the wild type non-dark factor. The Dark factor controls the shade of colour. Much controversy and confusion surround the Yellowface factor/s. Even now the true story may not be completely known. It is possible that two, three or even four different yellowface mutations exist, all of them with similar breeding rules and subtly different visual appearances. BUDGEN uses the theory that three different YellowFace mutations exist.
1) Australian Yellowface ( also believed to be called Goldenface)
2) Mutant 1 Yellowface (also called Type 1 or Creamface or Lemonface or English)
3) Mutant 2 Yellowface (also called Type 2 or English Type 2)
In Australia, a common misconception is to use the terms Type 1&2 to describe the single and double factor Australian Yellowface. This is wrong.
The use of the term "Mutant" should be encouraged to describe the Yellowface varieties and not the term "Type" as "Type" has another meaning when discussing genetics as will be explained below.
All three Yellowface varieties are believed to be mutations of the same factor as that which causes green or blue colouring. Such cases of more than one mutation occurring at the same gene location are known as multiple allelomorphs (alleles). The Dark and Blue (therefore also Yellowface) factors are known to share the same chromosome. All factors which share common chromosomes should be looked at as linked groups of factors rather than as individual factors.
Unfortunately Mother Nature isn't perfect and sometimes a pair of chromosomes can break in half and while they usually repair themselves it is common for them to swap places before rejoining. This process is known as genetic crossover. This crossover is known to occur between the Blue (and Yellowface) and Dark factors on average in 1 in every 7 cells. BUDGEN incorporates this 1 in 7 crossover value in its calculations. In most cases this crossover has no effect on results of any matings, however there are exceptions. This crossover causes two different genetics types of Dark Green/Blue known as Type I and Type II.
Diagram (a) shows these two types a little more clearly.
Similarly Dark Green/Yellowface Type I and Type II exist in all three yellow face mutations as does Yellowface(sf) Cobalt Type I and Type II.
GREYWING, CLEARWING, DILUTE FACTORS
Mutations happen randomly and once a particular gene has mutated there is no reason why it cannot mutate yet again sometime in the future. One particular gene is known to have mutated at least three times (possibly more) and resulted in the Greywing factor, the Clearwing factor and the Dilute factor. Including the original non-mutant gene this makes four possible factors that could reside at that location on the chromosome. This is another example of a multiple allele. In a pair of genes this results in 10 possible combinations instead of the usual 3.
The Greywing, Clearwing and Dilute factors are all recessive to the non-mutant gene. Greywing factor is semi-dominant over the Clearwing factor and is dominant over the Dilute factor. The Clearwing factor is dominant over the Dilute factor. Dilutes cannot be split for Clearwing or Greywing.
Sexlinked factors as applied to current known varieties of budgerigars are essentially a special case of Recessive factors. Amongst the numbers of pairs of chromosomes in each cell is one particular pair of chromosomes whose task amongst other things is to control the gender of the bird. In the cock bird this pair of chromosomes follow the same rules as all the other chromosomes. Geneticists have labelled these particular chromosomes "X chromosomes". In the hen however, the two chromosomes which make up the pair are not quite the same length. The pair are made up of an X chromosome (similar to what a cock bird has) and one shorter chromosome labelled the "Y chromosome". Cocks cannot have a Y chromosome as its very presence means the bird must be a hen.
The varieties of budgerigar that we call sex-linked are known to have factors which lie at various positions on the long leg of the X chromosome. Looking at these factors, the cock can have the usual combinations of factors in his pair of genes but the hen doesn't have a pair of genes at this location , only one single gene on the X chromosome, due to the short Y chromosome. Therefore there can be no conflict of genetic instructions in the hen, which in turn means that whatever factor the hen has, it will be expressed. For this reason hens cannot be split for sexlinked factors. All the current sexlinked varieties act as simple recessives in the cock bird with one partial exception (i.e. Texas Clearbody).
Crossovers between sexlinked factors are known to occur in the cock bird. The crossover value used by BUDGEN between the Opaline and Cinnamonwing factors is approximately 1 in 3. It is probable that the Cinnamonwing and Ino factors lie very close together and BUDGEN uses a crossover value between Cinnamonwing and Ino of approximately 1 in 30. Crossovers involving Slate are also known to occur but experimentation is still ongoing in determining the crossover values, although they are believed to be very small.
BUDGEN assumes that the Lacewing variety is a simple combination of the Cinnamonwing and Ino factors. If this is the case then the Lacewing was probably first produced as a result of a crossover between the Cinnamonwing and Ino genes.
The Texas Clearbody factor is a multiple allele of the Ino factor. The Texas Clearbody is sexlinked recessive to normal but sexlinked dominant over Ino.
For many years the true genetic picture regarding the Crested factor/s has been a mystery and is still not completely resolved, although the current "Determiner and Initiator Theory" as put forward by Dr J.E. Fox comes close.
BUDGEN is based on this Determiner and Initiator theory. For more detailed information on this theory it is recommended you consult the "Handbook of the Crested Budgerigar Club".
The following factors are simple dominants:-
· Australian Grey
· Australian Dominant Pied
· Continental Clearflighted Pied (Dutch Pied )
With true dominants the "single factor" (i.e. one mutant and one non-mutant gene) are visually identical to the "double factor" (two mutant genes) birds.
The following factors are simple recessives:-
· Danish Recessive Pied (Harlequin)
· Fallow (Australian/German, English, Scottish)
The recessives must have two mutant genes in order to visually show the characteristic.
The following factors are simple semi-dominants:-
With true semi-dominants the single factor and double factor birds are visually different and both are obviously different from normal.
(It can be argued that some double factor Aust Pieds do look different to single factor Aust Pieds and therefore under that situation Aust Pied could be classed as semi-dominant)