Genetics of dilution
The relationship between Black Eyed Self’s and Suffused yellows/whites (Dilute’s)
By Nigel Tonkin, Peter Glassenbury and John Mulley
Normal – dominant to Greywing, Clearwing and Dilute
Greywing – recessive to Normal, co-dominant with Clearwing, dominant to Dilute
Clearwing – recessive to Normal, co-dominant with Greywing, dominant to Dilute
Dilute – recessive to Normal, Greywing and Clearwing.
The Black Eyed Self of today is the Dilute selected by breeders over many generations to remove body colour suffusion for the purpose of enhancing the ground colour. Suffused yellow and white is the terminology used for Dilutes in these colours which carry the same mutation as the Black Eyed Self’s but these birds are unselected for colour modifiers, so they retain their original wild-type suffusion. These suffused birds are derived from the English imports of two decades ago; having been separated from the Australian Black Eyed Self’s for many, many generations, probably for at least a century.
Where possible, the term Dilute is used in this paper rather than Suffused as it better serves the birds in question. Due to relatively recent intercrossing in Australia between the Black Eyed Self as described in our Standard and the ‘suffused’ Dilutes bred out of the imported English lines, Dilutes now exist in all shades of suffusion between the two extremes. The aim of this intercross is to improve the feather and body structure of the Black Eyed Self’s, Thus, many of the Black Eyed Self birds benched in that class now carry the unwanted by-product of that mating, that is, more suffusion than desirable. We saw a dilute with minimal suffusion benched at a recent club show in the AOSV class when it was of equivalent colour and suffusion as other birds presented in the Black Eyed Self Class. That bird could not be wrong classed from either class because both classes take birds derived from this same original mutation.
There are four alleles or genetic variants of major effect at the gene or genetic locus responsible for the phenotypes (visual) Normal, Greywing, Clearwing and Dilute.
A Normal (Light Green for example) has black wing markings and full intensity body colour (full melanin pigment if you like). Greywing, Clearwing and Dilute display reduction of melanin pigment in either their wings or body or both. Dilutes have extreme reduction of pigment in both the wing and the body. Clearwings have a reduction of wing melanin (ideally to zero) and ideally no reduction in body colour. Greywings (Australian) have an intermediate degree of reduction in the wing markings and yet approach ideally a similar body colour to the Normal and the Clearwing.
The Normal allele is the wild type and is dominant to the three other mutant alleles.
Conversely, the Dilute allele is recessive to the Normal, Greywing and Clearwing alleles.
The Greywing, Clearwing and Dilute mutant alleles are all recessive to Normal (not visible when combined with a wildtype allele). A normal appearing bird (full body coloured) can therefore be split (or carry in hidden form) the alleles for Greywing or Clearwing or Dilute.
Greywing and Clearwing are dominant to Dilute. Thus a visual Greywing or a visual Clearwing can be split for Dilute.
Greywing and Clearwing are co-dominant with each other, which means if a pure breeding Clearwing is paired to a pure breeding Greywing certain varietal characteristics of both are ‘displayed’ on the progeny. The phenotype or visual appearance of these progeny is full body colour from the Clearwing and the intermediate (halfway between normal and clearwing) grey wing markings from the Greywing. These are generally referred to as full body coloured Greywing’s.
Most budgerigar varieties (“varieties” as distinct from the four budgerigar colours – green, blue, English yellowface, Australian yellowface) that we exhibit in separate classes result from a mutation at various wildtype genes. These mutations include Danish Recessive Pied, Dominant Pied, Fallow, Opaline and Cinnamonwing.
All of these are mutations of different genes. In contrast, the three mutations giving rise to Greywing, Clearwing and Dilute can be confusing since they are three different mutations of the same gene at the same genetic locus (same gene position on one of the budgie chromosomes). The resulting phenotypes are sometimes referred to as multiple allelomorphs, indicating that there are visible and discrete variations of different mutations of the same gene.
These three mutations at the one gene locus are expressed in the form of five different phenotypes or morphs (Normal, Greywing, full body coloured Greywing, Clearwing and Dilute). Not mentioned here are suffused since they are simply Dilutes unselected to remove the colour modifiers that account for their suffusion, in contrast to the Black Eyed Self’s which are Dilutes where colour modifiers have been selected against and ideally colour other than ground colour has been eliminated.
The Dilute in its purest form we exhibit as Black Eyed Self’s, although we have seen the colour of that variety deteriorate in recent years through intercrossing with ‘suffused’ Dilutes with superior feather and body size. The Dilute colouration in its original native form is part of the mixture of birds cobbled together as suffused yellows/whites (see page 39 of The Standard). The Greywing-Clearwing-Dilute series of mutations arise from variations of the same gene. The Black Eyed Self and the birds encompassed within The Standard as suffused yellow/white primarily arise from the same mutation, Dilute. Their phenotype diverges due to selection over a number of generations of genetic body colour modifiers which are part of the normal genetic background. Genetic modifiers can be described as an unknown number of genes of small effect at different loci which modify the appearance of the mutation at the primary gene, in this case the Dilute. Historically this is clearly documented with the appropriate references given on page 6 of The Standard. Any suggestion that Dilutes (or suffused yellows and whites) and Black Eyed Self’s are different mutations can only arise from a fundamental lack of understanding of the difference between a primary mutation of large effect and genetic modifiers of small effect which are ever present as normal genetic variation in the background genome (other genes). Like mutations of large effect, modifiers originally arise by mutation as well.
Confusion surrounding the Dilute phenotype has been recently exacerbated by creation of a so-called standard for “suffused yellows/whites”. These birds include Black Eyed Self’s with grey factor (now excluded from the Black Eyed Self class, but often erroneously going undetected when benched in that class. This is not necessarily the judges’ fault as light variables can impact in this area, and the two can be very difficult to distinguish unless one breeds them and is very familiar with them. But this is another story.). These grey factor ‘Dilutes’ are generally referred to as Grey Yellows and Grey Whites.
The “Suffused variety” included in The Standard (Australian 2003) includes Suffused greens/blues. This is the original Dilute mutation prior to breeding programs aimed at eliminating suffusion and leading to the Black Eyed Self’s, originally happening about a century ago, as outlined on page 6 of The Standard. The reason why Black Eyed Self’s now tend to show a degree of suffusion is because the Dilute greens and blues popping out of the Normals and originating from the imported English birds are often of very good quality so have been used as outcrosses to the Black Eyed Self’s in an attempt to increase their size and feather. One of the authors has gone down that track. This is an alternative route to improvement as opposed to the more conventional approach of using Cinnamonwing Dark Greens as outcrosses. Another of the authors continues down this track.
For the Greywing and Clearwing variations, this is how it works: Greywing and Clearwing are co-dominant alleles of the same gene. The Greywing allele paired with itself, or in combination with the Clearwing or the Dilute allele, produces more pigment in the wings, causing the greyish coloured wing markings. The Clearwing allele paired with itself, or paired with the Dilute allele, produces less pigment in the wings, very light markings and brighter body colour. When a budgie has a Greywing allele paired with a Clearwing allele the compound heterozygote (an individual having two different alleles of the same gene) is a full-body-coloured Greywing. The Greywing allele makes up for the lack of wing pigmentation conferred by the Clearwing gene, and the Clearwing gene makes up for any lack of body feather pigmentation conferred by the Greywing allele.
So when a budgie is a homozygous Greywing (two doses of the Greywing allele) or is heterozygous with one Greywing allele and one recessive Dilute allele, the budgie has the grey wing markings and less than full, or approaching full, body colour. It is never diluted to the extent of the Dilute yellows and whites. When a budgie is a homozygous Clearwing (two doses of the Clearwing allele) or has the Clearwing allele in combination with the recessive Dilute allele (heterozygous for these two alleles), the budgie has very light wing markings (ideally none) and a full body colour. When a budgie has both the Greywing allele and Clearwing allele, it is a full-body-colour Greywing with grey wing markings and full body colour. When a budgie has two of the recessive Dilute alleles it shows the traits of Dilute with washed out markings and colour all over. In its purest form as the standard of perfection, this approaches the standard for Black Eyed Self’s. In its somewhat more marked form there has been an era and unfortunately there still is in some places a habit of exhibiting these as Greywings, where the wing markings on the Dilute are heavier than usual, due to modifiers affecting that trait. The only solution to this problem is for judges to wrong class these birds, or at least take them to the end of the line. The recently introduced standard for Suffused Yellow and Whites (Dilutes) describes the phenotype close to its original wildtype mutant colour, prior to selection to reduce colour modifiers responsible for the suffusion. It also includes the other ‘rejects’ from the Black Eyed Self class, the Grey Yellows and Grey Whites.
As you can see there are only five discrete phenotypic possibilities (ignoring the continuum comprising the suffused’s colouration and the continuum of wing markings as sometimes evident in Clearwing classes) but many more possible genetic combinations (or genotypes). It is fundamental to remember that the Greywing and Clearwing alleles are co-dominant. The ‘normal’ Greywing has grey wing markings and is not necessarily full body coloured, but at least approaches full body colour. The ‘normal’ Clearwing has very light markings and no dilution of body colour. When the Greywing and the Clearwing alleles are both present, we get the full-body-colour Greywing, which has the grey markings of the Greywing mutation and the body colour of the Clearwing mutation. Other than the co-dominant relationship between the Greywing and Clearwing alleles, all other combinations work in a dominant-recessive relationship. The dominant Normal allele will prevail when partnered with any of the other recessive alleles. The Greywing allele prevails when partnered with the Dilute allele. The Clearwing allele prevails when partnered with the Dilute allele. Only when both alleles are Dilute does the Dilute phenotype show up since Dilute is recessive to all of the other alleles. The Dilute allele is recessive to the Normal, Greywing and Clearwing alleles. Any continuity between the above varieties is due to faults introduced by the breeder. These are discouraged by judging each variety to The Standard that is designed to keep the phenotypes discrete.
The concept of multiple alleles at the one gene locus is not unusual in budgerigar genetics. Ino (Lutino and Albino) is an allele of Texas Clearbody, both being recessive mutations away from Normal, with Texas Clearbody dominant to Ino. Hence, it is not unusual to buy a Texas Clearbody cock to find that when mated to one’s own hens (Normal or Clearbody) it produces Clearbody and Ino hens by virtue of the Clearbody cock being split Ino.
Similarly, green, blue, “English” yellowfaced blue and “Australian” yellowfaced blue are four alleles of a “colour” gene. All of the above has a direct bearing on the debate surrounding inclusion of the Suffused yellows/whites (Dilutes) as a separate class in the national show. Genetic modifiers (other genes affecting the same phenotype but with small effects) cause departures from the standard of perfection for each of the Greywing, Clearwing and Black Eyed Self varieties with the result that overlapping phenotypes are seen in birds benched with faults in their markings and colour. The judges’ role is to seriously penalise these departures from The Standard, on the basis of the discrete descriptions for each of these varieties as given in The Standard.
Other sets of modifiers affect all of the varieties, apart from the Greywing-Clearwing-Dilute (Black Eyed Self) series as described above. Whilst there is no argument in our minds regarding the derivation of the Black Eyed Self through the selection of modifiers, some also argue that Greywings and Clearwings were derived similarly, by directional selection for modifiers from the same original Dilute mutation. That would seem unlikely since such a gradual transformation has not been documented. Given the historical accounts which suggest a sudden origin of Greywings and a sudden origin of Clearwings, on pages 8 and 9 of The Standard, that is more consistent with new mutations of large effect creating these varieties rather than a gradual modification of the phenotype through directional selection of genetic modifiers.
About the authors:
Nigel Tonkin imported from England a Grey Yellow cock which led to the appearance of a number of strong Grey Yellows during the 1990s (Dilutes with the grey factor, or Dilute Grey Greens).
Peter Glassenbury has for many years bred the classic form of the Black Eyed Yellow and has recorded a number of wins with that variety over the years at national level.
John Mulley is a professional geneticist who has won a national with the Black Eyed Self and has had considerable experience using the Dilutes bred out of English lines as outcrosses to improve the genuine Black Eyed Self. He breeds Black Eyed Selfs both with and without the grey factor.
Heterozygous – having two alternative genetic variants (alleles) at a given gene
Phenotype – visual characteristic
Genotype– alleles carried which determine phenotype, subject to dominance or recessivity
Allele – an alternative form of gene
Melanin – a dark brown or black pigment