The Parblue Puzzle

Part 3 — Multiple Alleles

You may recall that towards the end of Part 1 of this article, before embarking on a brief review of recessive, dominant, and co-dominant genes in Part 2, I referred to the fact the parblue genes in each species appeared to form part of a multiple allelic series together with the blue gene and its wild-type counterpart. We are now in a position to have a closer look at this type of genetic interaction.

But first let me try to clear up this business of genes and alleles. You will have noticed that I have used these two words fairly indiscriminately, and you will find this to be true of most genetic literature. To all intents and purposes they mean the same thing, though the very use of the word allele implies the existence of a mutant, defective, or alternative form of any particular gene. Consequently, all the genes responsible for any of the colour varieties of parrots are alleles. The breeder‘s word factor also expresses the same idea. (Although you will encounter the full original term, allelemorph, in Genetics for Budgerigar Breeders, this has fallen out of use and will seldom be seen in modern genetic literature.)

In these circumstances, to say that such and such genes are alleles, or even that one gene is an allele of another, is perhaps a little vague and ambiguous. What we really mean is that the alleles in question have a definite relationship one with the other; that they are both mutant, defective, or alternative forms of the same wild-type gene. It is better to always specify that such genes, or alleles, form part of a multiple allelic series.

Multiple allelic series

A multiple allelic series arises whenever a wild-type gene mutates more than once and so has more than two alternative forms. The most simple series comprises a total of three alleles; the original wild-type and two different mutant types. If we assume this basic model as being true for a series of alleles containing a blue gene and a parblue gene, we can examine this idea as before. Previously we looked only at the dominant wild-type B, and the recessive b. Now we can slot in the parblue allele, labelled b^p in the correct genetic tradition, between the other two so that we have:

These three alleles of the same gene are able to pair in six different ways as shown in the following table, which is presented in a generalised (non species-specific) way:

Genetic type	Description	Appearance	Remarks
BB	Normal, pure, or wild-type Green	Green	Identical alleles - true breeding
Bbp	Green/Parblue	Green	Dissimilar alleles - genetic hybrid
Bb	Green/Blue	Green	Dissimilar alleles - genetic hybrid
bpbp	DF Parblue	?	Identical alleles - true breeding
bpb	SF Parblue or Parblue/Blue	?	Dissimilar alleles - genetic hybrid
bb	Blue (true)	Blue	Identical alleles - true breeding

Although there are six genetic types of bird in the table, I have made the assumption that the wild-type allele is fully dominant to both the parblue and blue alleles and also that these latter two alleles are co-dominant with each other. This is certainly so in the budgerigar and there is every indication that, at least in some instances, the same can be said of the blue series of genes in other parrots.

Let us take a look at the consequences of this assumption. At its most simple, each allele in a multiple allelic series is completely recessive to any allele above it in the series and completely dominant to any below it in the series. But, in the present case, we are supposing that the parblue allele is only partially dominant (co-dominant) to the blue allele. This means that:

• The two different genetic types of Parblue, b^pb^p and b^pb, will also be visually different
  
  
• and shows that the proposition of a multiple allelic series including only one parblue gene, co-dominant with the blue, is capable of explaining the occurrence of two distinct Parblue varieties in any species. It is probably the most simple genetic model capable of this explanation and is the theory, as we shall see later, accepted to be true for each of the Parblue varieties in the budgerigar.
  
  

On a more generalised note, the names given to the different parblue and blue forms, and their appearance, will vary from species to species. Some of the more interesting observations to be brought out from consideration of the above table are:

• The six possible genetic types of bird can be mated in 21(!) different combinations.
  
  
• This time there are three genetic types with identical alleles which will breed true when mated like to like: BB, b^pb^p, and bb.
  
  
• A Green can be split for parblue (Bb^p) or it can be split for blue (Bb), but it cannot be split for both.
  
  
• There are two types of Parblue, the DF Parblue (b^pb^p) will be true breeding and the SF Parblue (b^pb) will be split blue.
  
  
  

Complications

If only this were the whole story. Unfortunately, matters are more complicated as you will recall from my comments towards the end of Part 1 concerning an article by George Smith on the Ringneck Parakeet. In this instance, although the inheritance pattern appears to be the same, the way the parblue allele acts is different to that in the budgerigar. As if that were not bad enough, there are indications in other commonly bred species that their parblue inheritance may be different again and that we may have to look for different genetic models. In this search we may have to look at more complex multiple allelic series, or even the possibility of more than one independently inherited mutant gene affecting the biochemical pathway which produces psittacin pigment(s).

The clues leading to the solution of these problems will be found not only in the available breeding results but, also, in a study of the history of the Parblue varieties and the sequence of their appearance. Unfortunately, as always, accurate and reliable information is never easy to come by within aviculture.

In concluding Part 2 of this article I referred to the fact that where a gene is dominant or co-dominant to the wild-type, the single-factor form will be the first to appear and is always likely to be the more common. Where there are three or more alleles, as in a multiple allelic series, and these have an hierarchical structure of relative dominance or co-dominance, the situation is not quite so straightforward.

The implication with regard to Parblue varieties is that if a parblue gene comes into existence, by mutation of the wild-type gene, in a situation where there is a pre-existing blue gene it is the SF Parblue (Parblue/blue) form which is likely to be seen first. (There must also be the possibility of a parblue gene coming into existence by a further mutation, or partial back mutation, of such an existing blue gene.) On the other hand if there is no blue gene then, like any other recessive variety, a visual Parblue will not occur until two parents who both possess the parblue gene both pass it on to one or more of their young. The form seen in this instance will be the DF Parblue.

In part 4 of this article I will attempt to summarise what is known about the Parblue varieties which occur in various commonly bred parrot species.

End of Part 3

Copyright: Clive Hesford, 1992 and 1998

Forward to Part 4

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