Inheritance patterns

The study of inheritance is usually pursued under two main categories; quantitative inheritance, and qualitative inheritance. As colour breeders, we are almost exclusively concerned with qualitative inheritance in which significant and recognisable changes, caused by single mutant genes passed down the generations, show up in discernable patterns in lineage and pedigree charts.

It is this type of inheritance which exercised the mind of Gregor Mendel. A mind which might well have recognised the mathematical basis which underlies the patterns produced by qualitative inheritance. There are those who believe that in embarking on his plant breeding experiments Mendel was seeking not to discover but, instead, to confirm what he had already deduced on the grounds of reason and mathematics alone.

Humans have been breeding and manipulating the characteristics of other animals since the dawn of civilisation. This was in the past based purely on the observation that like produces like, and that by constantly pairing those animals which exhibited the most desirable features there was a steady improvement (from the human viewpoint) in the quality of stock. Without any theoretical knowledge, these livestock breeders were exploiting the mechanisms which form the basis of quantitative inheritance. Unconsciously they were gathering together and consolidating in their animals many genes which each contributed, however slightly. toward the characteristics which brought improved economic or aesthetic value.

And, in going beyond economic improvemnt towards aesthetics, breeders were sometimes selecting for characteristics such as coat colour where qualitative inheritance comes into play: alleles at a single locus bringing about major alternative phenotypic effects. It is hard to imagine that some of the obvious patterns of inheritance went unnoticed by the more perceptive of these practical breeders. Had scholars, even before Mendel, stooped to consult such breeders the mechanisms of inheritance might have been unravelled that much sooner. Perhaps the forces of religous bigotry, which even now sometimes seek to deny the laws of nature, were even more inhibiting than we imagine.

Two broad divisions are resonsible for the differences we see in patterns of inheritance. One of these is the relationship between the characteristics produced by alternative alleles (or different forms) of the same gene, which is expressed in term of relative dominance. The other is produced by a fundamental difference in the genetic endowment of the two sexes resulting in some characteristics inheriting in a sex-linked pattern. It is the first of these we shall consider at the present time.

Dominance and Recessiveness

Colour varieties or morphs are often said to be caused by either a dominant gene or a recessive gene. This is an oversimplification. It is not the gene, or more explicitly an allele of that gene, which is dominant or recessive; rather, it is the effects or characteristics produced by that gene or allele, its expression, which are inherited in a dominant or recessive fashion.

Similarly, it is important to realise that the terms dominant and recessive are relative, not absolute, terms. Neither a gene nor the characteristics it may produce are inherently dominant or recessive; that relationship of relative dominance exists between the characteristics produced by one form of a gene, or allele, and an alternative form, or allele, of the same gene.

As can be seen from the above, the language of genetics can be very confusing and the more we go into it the more careful must be our choice of words. We need to keep separate our understanding of the word gene from the effects or characteristics that gene might produce; its expression. I shall try to make this distinction clear where it might affect our understanding. It is entirely possible for a gene or allele to control or produce multiple effects and it is also possible that one effect, or set of effects, is inherited in a dominant manner whilst another is inherited recessively.

Generally speaking, when we refer to a colour variety as being either dominant or recessive we describe its relationship to the natural or wild form of the species; this being variously known as the wild-type, the normal, or the ‘green’ (where this is appropriate). A colour variety is produced by a mutant allele, or colour factor, which replaces either one or both of the wild-type alleles present in the bird.

When a variety is described as dominant you should be able to assume that it reproduces in a dominant manner with normal or wild-type birds and, conversely that a variety described as recessive reproduces in a recessive manner with normal or wild-type birds. There is a lot more to it than this, but for the moment we will concentrate on these two simple modes of inheritance.

Recessive inheritance

At this stage we will look at the situation in which only two alleles, or different forms, of the same gene are involved. [Later it will be necessary to consider what happens when there are several alleles which might occupy any particular gene locus.] 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.

At a more technical level, different terminology is used by geneticists to describe these two different states at each gene locus of a matching pair. The double-factor condition, where both alleles of a pair are identical, is known as homozygous; whilst the single-factor condition, where the two alleles are different, is known as heterozygous. These are terms which you will increasingly see as breeders become more aware of genetic theory.

Because a dominant factor always shows itself visually breeding these varieties is quite straightforward. Breeders rarely bother to deliberately produce double-factor birds, but they are likely to occur whenever two visuals are paired and may in some circustances be quite useful breeding stock since all the young they produce will be visuals. Most likely this will only come to light once they have already become part of a breeding team; it will rarely be worthwhile test-mating every such visual to identify those which are double-factor.

However once a bird has been identified from its breeding performance as being double-factor, a note should be made in that bird’s Individual Record for future reference. Where a certain dominant colour factor is an essential ingredient in a very desirable composite variety, such a bird could help to maximise production of that variety.

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 simply as dominant when, in fact, they should be recognised as incomplete-dominants involving the wild-type gene.
  
  

There are those who would describe both these conditions simply as co-dominant; and technically this is quite true. However, I find it useful to distinguish between the two conditions and where the wild-type gene is involved to refer to the mutant allele, or factor, as being incompletely-dominant to the wild-type. Thus when a colour factor is described in these pages as an incomplete-dominant it is immediately apparent, without further explanation, that it bears this relationship with the normal or wildtype.

The overall term co-dominance wil be reserved for those instances where this relationship exists between alleles other than the wildtype. This only occurs where a gene has mutated more than once to form a multiple-allelic series and there are at least three alleles which may be present at the relevant gene loci. And in such cases, to be considered in the next section, it is always necessary to spell out which two alleles are so related.

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. [Ordinary co-dominance between alleles below the wildtype in a series will be dealt with at a later stage.] 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.

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 incompletely- or 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.

Multiple alleles

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 (a situation which is quite common across parrot species), 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 although every effort should be made to unify the system as much as possible. 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.
  
  
  

In the budgerigar three parblue varieties have traditionally been recognised; the Goldenface or Australian Yellowface, the Yellowface Mutant 11, and the Yellowface Mutant 1 or Creamface (Aust). However, this last one is now thought to be the unusual product of two blue alleles which have mutated in different ways (See article Gene function in Yellowface budgerigars by Peter Bergman in these pages.) The single-factor, or heterozygous, forms of the first two are sometimes referred to as Seagreens.

Parblue varieties have also been established in a number of other species (even where no Blues are known) and a wide range of names have been used. Attempts are being made to standardise on the name Aqua, but others you will come across include Turquoise, Marine, Seagreen, and even Pastel Blue(!).

© Clive Hesford: March 2001

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