The Parblue Puzzle

Part 2 — Simple Genetics

To understand the relationship between the parblue genes and the blue gene we need to remind ourselves of the ideas of recessiveness, dominance, and co-dominance, before going on to combine these into one concept. For those who have difficulty with genetics I will attempt to make the treatment as straightforward as possible and avoid those off-putting technical words wherever there are everyday or breeders’ alternatives. Please try to bear with me. My attempts to bring out the relevant facts might just make something click into place for you.

Recessive inheritance

First we look at the situation in which only two alleles, or different forms, of the same gene are involved. This is the case where we consider only the normal (wild-type) Green of a species and the mutant Blue variety of the same species. The Blues are invariably recessive varieties and are known technically as autosomal recessives, although aviculturalists rarely find it necessary to use this expression and such varieties are usually known simply as ‘recessives’.

It is normal genetic practice to represent a mutant allele with the initial letter of the name given to the variety it gives rise to. In this case the letter b for Blue; here in lower case to signify that this is the recessive allele. The dominant wild-type allele is represented by upper-case, or capital, B.

So we have two alleles of the same gene:

Any individual has two copies, and only two, of any one gene or its alleles. One copy comes from the father and the other copy from the mother; producing a matched pair. The two alleles introduced above are able to pair in three combinations; BB, Bb, and bb. The results are more clear if set out in tabular form.

Although the genetic hybrid Bb carries both genes, it is the dominant B allele which determines the appearance of the bird. The recessive allele b is said to be hidden or carried and the bird is described as being Green split blue or Normal split blue. (You will frequently see the alternatives Green split for blue or, by our American cousins, Green split to blue, used in both speech and print. Both are valid but best avoided.) Very often this is simplified in print as Green/blue or Normal/blue.

Some interesting points emerge if we consider this table:

• Altogether, there are six possible ways of mating these three genetic types of bird:

• Pure Green (BB) mated to pure Green (BB) can only produce pure Green (BB).
  
  
• Blue (bb) mated to Blue (bb) can only produce Blue(bb).
  
  
• Green/blue (Bb)mated to Green/blue (Bb), genetically the hybrid cross, produces all three genetic types BB, Bb, and bb, in the classic 1:2:1 ratio. Alternatively, this can be expressed in percentage terms as 25%BB, 50%Bb, and 25%bb.
  
  (An important investigative cross-pairing for the geneticist this is, generally, an undesirable mating for the practical breeder since it is impossible to tell which Greens might be split for the recessive character.)
  
  
• Because B is dominant to b, the three genetic types produce only two visual types in the ratio 3:1 (75% visual Green, 25% Blue). In the hybrid, or split, the recessive b is hidden by the dominant B.
  
  
  

From the point of view of the practical colour breeder of the Blue, or any other recessive variety, the most useful matings are:

• Green x visual recessive, where all the offspring are known, or guaranteed, splits.
  
  
• Green/recessive x visual recessive, where the offspring are either visual recessives or known splits in roughly equal numbers.
  
  
• And, where a recessive variety is well established and sound vigorous stock is available, the mating of visual recessive to visual recessive will produce young which are all visual recessives.
  
  
  

Dominant inheritance

In the above example the wild-type gene was dominant and the mutant blue allele was recessive. Sometimes the situation is reversed and it is the mutant allele which is dominant. An example of this is the grey factor, common in the budgerigar and well established in the Ringneck, where the grey allele G is dominant to the wild-type g. Again we can look at the three genetic types in a simple table format, not only to see how the basic inheritance pattern is exactly the same, but also to bring out the different terminology used:

This table introduces a couple of terms which breeders use when discussing dominant varieties; single-factor (SF), and double-factor (DF). Birds of these two types look alike but will give different breeding results. The single-factor Grey Green bird is, in effect, split for normal Green and will produce a proportion of normal Greens if mated either to another single-factor Grey Green or to a normal Green. A double-factor bird will not produce normal Greens whatever it is mated to.

It is just as important for the colour breeder to know whether a dominant colour variety is single-factor (SF), possessing only one mutant gene, or double-factor (DF), where both alleles of a pair are of the mutant type, as it is to know whether an apparently normal bird is split for a recessive factor.

In the budgerigar, a number of different genes produce a general grey green or olive colouration in yellow ground birds: the dark (only when DF - see later), the violet, the grey, and the slate. Any of these genes might be discovered in other parrots and breeders should not jump to what might seem obvious conclusions. If a parrot of this appearance occurs it should not automatically be thought of, or described, as an Olive. The true nature of such a gene may not be apparent until a blue gene also occurs in that species.

Incomplete- or co-dominant inheritance

At this point we are ready to consider the concept of co-dominance, which is also known variously as partial-, semi-, or incomplete-dominance. Co-dominance applies when neither of two alleles is able to exert control over the other in the genetic hybrid and another, third, visual form is produced which is usually intermediate in appearance.

Broadly speaking, there are two situations in which we come across co-dominant alleles:

• the first is that where there is an hierarchical structure of dominance within a series of mutant alleles which are all recessive to the wild-type (a multiple-allelic series). The most simple structure of dominance sees each mutant allele being recessive to the allele immediately above it in the series and dominant to the allele immediately below it in the series. More commonly, however, the situation is not so clearcut and co-dominance exists between some or all of these alleles. This is the type of situation found with the parblue alleles and also the greywing, clearwing, dilute series in the budgerigar.
  
  
• the second situation is found where a mutant allele is dominant, but not completely so, to the wild-type and these two alleles produce three visual types. Good examples of this are the dark and spangle genes in the budgerigar. Very frequently in avicultural literature such genes are described as dominant when, in fact, they should be recognised as co-dominants involving the wild-type gene.
  
  Where the wild-type gene is involved it is useful to refer to the mutant allele, or factor, as being partially- or incompletely-dominant to the wild-type. My preference would be for the term partially-dominant since it is more readily understood by the layman, but the expression incomplete-dominant seems to be the most widely used in avicultural writing and I will abide by this convention.

To illustrate incomplete-dominance (that form of co-dominance involving the wild-type gene) we will consider the dark gene, which is starting to appear in a number of species besides the budgerigar. The letter used to represent the dark allele will be D, upper case in this instance because the mutant form is (incompletely or partially) dominant to its wild-type allele d.

• A point illustrated by the above example is that a gene, or allele, may not always be given the same name as the colour variety, or varieties, that it produces. Besides the dark gene, we have the very versatile ino gene which commonly produces either the Lutino or Albino varieties depending upon whether or not yellow ground colour is present. Topically, since we are discussing the parblue genes, such varietal names as the Creamino, Ivorino, Rosino, etc., have also been coined where these genes produce the appropriate ground colour in some parrot species. Whether these latter names are desirable or not is another matter.

Returning to the table, although many of the observations already made are still valid, there is one vital difference. This time the hybrid cross mating, Dd x Dd, besides producing the three genetic types in a 1:2:1 ratio, will also produce three visual types in the same ratio. When genes are co-dominant they both express themselves to some degree and there is no hidden gene. The breeding potential of colour varieties due to co-dominant genes is a direct reflection of their appearance.

Another point of some practical interest is that the single-factor form of a dominant, or co-dominant, gene will appear first and is always likely to be the more common. In the budgerigar it is the SF Spangle which has found favour as a result of its unusual markings, brought about by partial suppression of melanin by its one spangle gene. The DF (or Clear) Spangle, in which melanin production in the feathers is almost completely inhibited by the combined action of its two spangle genes, is treated as little more than a curiosity.

Now that we have briefly looked at the genetics of the blue gene and the concept of incomplete, or co-dominance, we are in a position to consider the situation when a third form (or mutation) of any particular gene occurs. This is the subject of Part 3 of this article.

End of Part 2

Copyright: Clive Hesford, 1992 and 1998

Forward to Part 3

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