Budgerigars

The locus for melanin production carries not only the wild-type gene, dil+, but also the mutations suffered by this gene throughout the generations. The wild-type gene determines production of melanin on the wings and body of a bird, to its maximum extent. This means the wings will show their characteristic black markings and the body feathers will reflect blue light.

The mutations obtained from the original gene are the ones for greywing, dil^gw, clearwing, dil^cw and dilute, dil. All four allelomorphs affect the distribution of melanin on the wings or body.

On greywings, the most visible effect is the reduction of melanin intensity on the wing markings. The production of melanin on the body is also reduced resulting on a paler body colour.

On clearwings the reduction of the wing markings intensity is even more remarkable but the production of melanin on the body is almost not disturbed.

The dilute gene produces an interesting distribution of melanin. It eliminates melanin from certain areas of the wings and body. Thus, wing markings and body colour become irregularly distributed. The intensity of colour on the areas where melanin production still occurs is similar to normal birds.

Symbols for allelomorphs (according to Mutavi)	Gene type	Effect of the allelomorphs that control melanin production

dil⁺	Wild-type	Black wing markings. Full blue light reflection.
dil^gw	Mutant	Grey wing markings. Greatly reduced blue light reflection.
dil^cw	Mutant	Lack of wing markings. Full blue light reflection.
dil	Mutant	Irregular black wing markings. Irregular blue light reflection.

Table 1 – Symbols and visual effect of the allelomorphs that occupy the "melanin production locus"

The wild-type gene is dominant to any mutant form it is paired to. This way, a bird can be visually normal and split greywing, or split clearwing or split dilute.

The allelomorphs for greywing and clearwing are incomplete dominant. A bird heterozygous for these two genes shows wing coloration typical of a greywing and body colour of a clearwing. Such a bird cannot be split for any of the two allelomorphs because both have a visible effect on the phenotype.

Greywing is dominant to dilute. A bird can be visually greywing and split dilute.

Clearwing is dominant to dilute. A bird can be visually clearwing and split dilute.

Dilute is the most recessive of the allelomorphs; a visual dilute must be homozygous for this gene.

The hierarchy among the allelomorphs is as follows: dil⁺ > dil

^gw = dil^cw > dil.

Genotype	Effect of the locus that controls melanin production

dil⁺ dil⁺	Black wing markings. Full blue light reflection.
dil⁺ dil^gw
dil⁺ dil^cw
dil⁺ dil
dil^gw dil^gw	Grey wing markings. Greatly reduced blue light reflection.
dil^gw dil	Grey wing markings. Greatly reduced blue light reflection.
dil^gw dil^cw	Grey wing markings. Full blue light reflection.
dil^cw dil^cw	Lack of wing markings. Full blue light reflection.
dil^cw dil	Lack of wing markings. Full blue light reflection.
dil dil	Irregular black wing markings. Irregular blue light reflection.

Table 2 – Visual effect of the pairs of allelomorphs that occupy the "melanin production locus"

The locus responsible for yellow pigment production carries the allelomorphs associated with this pigment.

The wild-type gene, bl⁺, determines a uniform production of yellow pigment over the wings, body and face of the bird.

The mutation, bl¹, determines that no yellow pigment production occurs. It is often called the "blue gene".

The so-called yellowface mutation (or yellow face mutant I), bl², is in fact, a gene that causes the elimination of yellow pigment. The bl² gene is incorrectly called a "yellowface gene" because, in fact, it produces exactly the same effect than the bl¹ gene. They are both "blue genes". It is only when the bl¹and the bl² allelomorphs are combined on the same bird that a visual yellowface appears. Both genes are necessary to produce this type of visual yellowface bird. Because this kind of visual yellowface does not have any true yellowface genes it is also a called creamface budgerigar.

The goldenface, bl^gf and yellowface mutant II, bl^yf2, are true "yellowface genes". They are yellow pigment production genes. The difference between the two genes is that bl^gf produces a stronger concentration of yellow pigment on the face of the bird, resulting on a richer shade of yellow while bl^yf2 produces a moderate concentration of this pigment, resulting on a slightly paler shade of yellow.

Symbols for allelomorphs (not according to Mutavi)	Gene type	Effect of the allelomorphs that control yellow pigment production

bl⁺	Wild-type	Production of yellow pigment on the body, wings and face.
bl¹	Mutant	Elimination of yellow pigment production.
bl²	Mutant	Elimination of yellow pigment production.
bl^gf	Mutant	Strong concentration of yellow pigment on the face.
bl^yf2	Mutant	Moderate concentration of yellow pigment on the face.

Table 3 – Symbols and visual effect of the allelomorphs that occupy the "yellow pigment production" locus.

The bl⁺ allelomorph is dominant over any mutant allelomorph it is paired with.

The hierarchy among the allelomorphs is: bl⁺ > bl^gf > bl^yf2. I have deliberately left out the bl¹ and bl²allelomorphs. I believe the best classification for these genes is to say they are recessive to bl⁺, incomplete dominant to the bl^gf and bl^yf2allelomorphs and also incomplete dominant between themselves.

The best explanation, I've encountered so far, of the biochemistry involving the genes that occupy this locus comes from an article by Peter Bergman of Sydney, Australia titled "gene function in Yellowface Budgerigars".

The most direct effect of genes is the production of proteins. The wild-type gene, bl⁺, possesses the instructions to build an active protein that, in turn, conducts the production of yellow pigment on the body, wings and face. The Bl⁺ gene will always produce a green phenotype, no matter the bird is homozygous or heterozygous for this allele.

The bl¹ mutation and the bl² mutation produce distinct proteins. These proteins are different from each other and different from the wild-type protein. In both cases, the "mutant" proteins have lost their active properties making them useless for yellow pigment production. Birds homozygous for bl¹ like birds homozygous for bl², can only produce the respective inactive protein. Such birds are visually blue or white.

Birds carrying both mutant genes (bl¹ and bl²) produce three kinds of protein: pure inactive bl¹ protein, pure inactive bl² protein and hybrid bl¹-bl² protein. Somehow, the hybrid protein regains some of its active properties and is capable of yellow pigment production but only on the face of the bird. This type of visual yellowface results from the association of two genes that alone have lost the capacity to produce yellow pigment. bl¹and bl², both have the same effect, total elimination of yellow pigment production. Only when combined they produce visual yellowfaces.

An homozygous goldenface bird produces pure "bl^gf" protein. This protein is very active on the facial area but incapable of yellow pigment production on the body and wings.

When the bl^gf allele is combined with the bl¹ three kinds of protein are produced: pure active "bl^gf" protein, pure inactive "bl¹" protein and hybrid "bl^gf- bl¹" protein. This hybrid protein is very active and similar in effect to the wild-type protein. The resulting phenotype is a goldenface bird with intense spillage of green over the body, resembling a green bird.

Similarly, when the bl^gf allele is combined with the bl² allele three kinds of protein are produced: pure active "bl^gf" protein, pure inactive "bl²" protein and hybrid "bl^gf-bl²" protein. This type of hybrid protein is inactive or very little active. Yellow pigment production comes mainly if not all from the pure "bl^gf" protein. Such birds produce less yellow pigment them homozygous goldenface birds resulting on a paler shade of yellow.

Birds homozygous forthe yellowface mutant II gene produce pure "bl^yf2" protein. This protein, is capable of moderate yellow pigment production on the facial area. The visual appearance is of a yellow paler than the goldenface.

Birds heterozygous for the bl^yf2 and bl^gf alleles produce pure moderately active "bl^yf2" protein, pure active "bl^gf" protein and hybrid bl^yf2-bl^gf protein. It is reasonable to assume that both types of pure proteins can produce a rich yellow colour on the facial area even if the hybrid protein turns out to be inactive. Such a bird should resemble an homozygous goldenface.

The combination of the bl^yf2 and bl¹ alleles on an heterozygous bird results on the production of pure active "bl^yf2" protein, pure inactive "bl¹" protein and a very active hybrid "bl^yf2- bl¹" protein. The "bl^yf2" protein is responsible for the production of yellow pigment over the face but such a bird resembles a green because of the spillage effect caused by the hybrid protein.

Genotype	Effect of the locus that controls yellow pigment production

bl⁺ bl⁺	Production of yellow pigment on the body, wings and face.
bl⁺ bl^gf
bl⁺ bl^yf2
bl⁺ bl¹
bl⁺ bl²
bl^gf bl^gf	Strong production of yellow pigment on the face
bl^gf bl^yf2	Strong production of yellow pigment on the face
bl^gf bl¹	Strong production of yellow pigment on the face with spillage of yellow pigment over the body
bl^gf bl²	Strong production of yellow pigment on the face
bl^yf2 bl^yf2	Moderate production of yellow pigment on the face
bl^yf2 bl¹	Moderate production of yellow pigment on the face with spillage of yellow pigment into the body
bl^yf2 bl²	Moderate production of yellow pigment on the face
bl¹ bl¹	Total elimination of yellow pigment production.
bl¹ bl²	Elimination of yellow pigment production from the body and wings (visual yellowface)
bl² bl²	Total elimination of yellow pigment production.

Table 4 – Visual effect of the pairs of allelomorphs that occupy the "yellow pigment production" locus.

Till now I have been careful not to say that a bird with normal melanin production is blue or that a bird with yellow pigment production is yellow. In order to truly understand a bird's colour we must analyse both loci simultaneously. If this task seems difficult, one should first try to understand the effect of each locus and then combine both effects. Remember, when normal production of melanin occurs, the body feathers reflect blue light. If yellow pigment is also produced, it works as a filter over the blue radiation and the bird is visually green.

Genotype			Phenotype
Locus for melanin production	Locus for yellow pigment production

dil⁺ dil⁺	bl⁺ bl⁺		Normal green
dil⁺ dil^gw	bl⁺ bl⁺
dil⁺ dil^cw	bl⁺ bl⁺
dil⁺ dil	bl⁺ bl⁺
dil^gw dil^gw	bl⁺ bl⁺		Greywing green
dil^gw dil	bl⁺ bl⁺
dil^gw dil^cw	bl⁺ bl⁺		Full bodied greywing green
dil^cw dil^cw	bl⁺ bl⁺		Clearwing (or yellow wing) green
dil^cw dil	bl⁺ bl⁺
dil dil	bl⁺ bl⁺		Yellow-green dilute

dil⁺ dil⁺	bl⁺ bl^gf		Normal green
dil⁺ dil^gw	bl⁺ bl^gf
dil⁺ dil^cw	bl⁺ bl^gf
dil⁺ dil	bl⁺ bl^gf
dil^gw dil^gw	bl⁺ bl^gf		Greywing green
dil^gw dil	bl⁺ bl^gf
dil^gw dil^cw	bl⁺ bl^gf		Full bodied greywing green
dil^cw dil^cw	bl⁺ bl^gf		Clearwing (or yellow wing) green
dil^cw dil	bl⁺ bl^gf
dil dil	bl⁺ bl^gf		Yellow-green dilute

dil⁺ dil⁺	bl⁺ bl^yf2		Normal green
dil⁺ dil^gw	bl⁺ bl^yf2
dil⁺ dil^cw	bl⁺ bl^yf2
dil⁺ dil	bl⁺ bl^yf2
dil^gw dil^gw	bl⁺ bl^yf2		Greywing green
dil^gw dil	bl⁺ bl^yf2
dil^gw dil^cw	bl⁺ bl^yf2		Full bodied greywing green
dil^cw dil^cw	bl⁺ bl^yf2		Clearwing (or yellow wing) green
dil^cw dil	bl⁺ bl^yf2
dil dil	bl⁺ bl^yf2		Yellow-green dilute

dil⁺ dil⁺	bl⁺ bl²		Normal green
dil⁺ dil^gw	bl⁺ bl²
dil⁺ dil^cw	bl⁺ bl²
dil⁺ dil	bl⁺ bl²
dil^gw dil^gw	bl⁺ bl²		Greywing green
dil^gw dil	bl⁺ bl²
dil^gw dil^cw	bl⁺ bl²		Full bodied greywing green
dil^cw dil^cw	bl⁺ bl²		Clearwing (or yellow wing) green
dil^cw dil	bl⁺ bl²
dil dil	bl⁺ bl²		Yellow-green dilute

dil⁺ dil⁺	bl⁺ bl¹		Normal green
dil⁺ dil^gw	bl⁺ bl¹
dil⁺ dil^cw	bl⁺ bl¹
dil⁺ dil	bl⁺ bl¹
dil^gw dil^gw	bl⁺ bl¹		Greywing green
dil^gw dil	bl⁺ bl¹
dil^gw dil^cw	bl⁺ bl¹		Full bodied greywing green
dil^cw dil^cw	bl⁺ bl¹		Clearwing (or yellow wing) green
dil^cw dil	bl⁺ bl¹
dil dil	bl⁺ bl¹		Yellow-green dilute

dil⁺ dil⁺	bl^gf bl^gf		Normal goldenface
dil⁺ dil^gw	bl^gf bl^gf
dil⁺ dil^cw	bl^gf bl^gf
dil⁺ dil	bl^gf bl^gf
dil^gw dil^gw	bl^gf bl^gf		Greywing goldenface
dil^gw dil	bl^gf bl^gf
dil^gw dil^cw	bl^gf bl^gf		Full bodied greywing goldenface
dil^cw dil^cw	bl^gf bl^gf		Clearwing (or white wing) goldenface
dil^cw dil	bl^gf bl^gf
dil dil	bl^gf bl^gf		White-blue dilute goldenface

dil⁺ dil⁺	bl^gf bl^yf2		Normal goldenface
dil⁺ dil^gw	bl^gf bl^yf2
dil⁺ dil^cw	bl^gf bl^yf2
dil⁺ dil	bl^gf bl^yf2
dil^gw dil^gw	bl^gf bl^yf2		Greywing goldenface
dil^gw dil	bl^gf bl^yf2
dil^gw dil^cw	bl^gf bl^yf2		Full bodied greywing goldenface
dil^cw dil^cw	bl^gf bl^yf2		Clearwing (or white wing) goldenface
dil^cw dil	bl^gf bl^yf2
dil dil	bl^gf bl^yf2		White-blue dilute goldenface

dil⁺ dil⁺	bl^gf bl²		Normal goldenface*¹
dil⁺ dil^gw	bl^gf bl²
dil⁺ dil^cw	bl^gf bl²
dil⁺ dil	bl^gf bl²
dil^gw dil^gw	bl^gf bl²		Greywing goldenface*¹
dil^gw dil	bl^gf bl²
dil^gw dil^cw	bl^gf bl²		Full bodied greywing goldenface*¹
dil^cw dil^cw	bl^gf bl²		Clearwing (or white wing) goldenface*¹
dil^cw dil	bl^gf bl²
dil dil	bl^gf bl²		White-blue dilute goldenface*¹
dil⁺ dil⁺	bl^gf bl¹		Normal goldenface, with green spillage over the body
dil⁺ dil^gw	bl^gf bl¹
dil⁺ dil^cw	bl^gf bl¹
dil⁺ dil	bl^gf bl¹
dil^gw dil^gw	bl^gf bl¹		Greywing goldenface, with green spillage over the body
dil^gw dil	bl^gf bl¹
dil^gw dil^cw	bl^gf bl¹		Full bodied greywing goldenface, with green spillage over the body
dil^cw dil^cw	bl^gf bl¹		Clearwing (or white wing) goldenface, with green spillage over the body
dil^cw dil	bl^gf bl¹
dil dil	bl^gf bl¹		White-blue dilute goldenface, with green spillage over the body

dil⁺ dil⁺	bl^yf2 bl^yf2		Normal yellowface
dil⁺ dil^gw	bl^yf2 bl^yf2
dil⁺ dil^cw	bl^yf2 bl^yf2
dil⁺ dil	bl^yf2 bl^yf2
dil^gw dil^gw	bl^yf2 bl^yf2		Greywing yellowface
dil^gw dil	bl^yf2 bl^yf2
dil^gw dil^cw	bl^yf2 bl^yf2		Full bodied greywing yellowface
dil^cw dil^cw	bl^yf2 bl^yf2		Clearwing (or white wing) yellowface
dil^cw dil	bl^yf2 bl^yf2
dil dil	bl^yf2 bl^yf2		White-blue dilute yellowface

dil⁺ dil⁺	bl^yf2 bl²		Normal yellowface*²
dil⁺ dil^gw	bl^yf2 bl²
dil⁺ dil^cw	bl^yf2 bl²
dil⁺ dil	bl^yf2 bl²
dil^gw dil^gw	bl^yf2 bl²		Greywing (blue) yellowface*²
dil^gw dil	bl^yf2 bl²
dil^gw dil^cw	bl^yf2 bl²		Full bodied greywing yellowface*²
dil^cw dil^cw	bl^yf2 bl²		Clearwing (or white wing) yellowface*²
dil^cw dil	bl^yf2 bl²
dil dil	bl^yf2 bl²		White-blue dilute yellowface*²

dil⁺ dil⁺	bl^yf2 bl¹		Normal yellowface with green spillage over the body
dil⁺ dil^gw	bl^yf2 bl¹
dil⁺ dil^cw	bl^yf2 bl¹
dil⁺ dil	bl^yf2 bl¹
dil^gw dil^gw	bl^yf2 bl¹		Greywing yellowface, with green spillage over the body
dil^gw dil	bl^yf2 bl¹
dil^gw dil^cw	bl^yf2 bl¹		Full bodied greywing yellowface, with green spillage over the body
dil^cw dil^cw	bl^yf2 bl¹		Clearwing (or white wing) yellowface, with green spillage over the body
dil^cw dil	bl^yf2 bl¹
dil dil	bl^yf2 bl¹		White-blue dilute yellowface, with green spillage over the body

dil⁺ dil⁺	bl² bl²		Normal blue
dil⁺ dil^gw	bl² bl²
dil⁺ dil^cw	bl² bl²
dil⁺ dil	bl² bl²
dil^gw dil^gw	bl² bl²		Greywing blue
dil^gw dil	bl² bl²
dil^gw dil^cw	bl² bl²		Full bodied greywing blue
dil^cw dil^cw	bl² bl²		Clearwing (or white wing) blue
dil^cw dil	bl² bl²
dil dil	bl² bl²		White-blue dilute

dil⁺ dil⁺	bl² bl¹		Normal yellowface
dil⁺ dil^gw	bl² bl¹
dil⁺ dil^cw	bl² bl¹
dil⁺ dil	bl² bl¹
dil^gw dil^gw	bl² bl¹		Greywing yellowface
dil^gw dil	bl² bl¹
dil^gw dil^cw	bl² bl¹		Full bodied greywing yellowface
dil^cw dil^cw	bl² bl¹		Clearwing (or white wing) yellowface
dil^cw dil	bl² bl¹
dil dil	bl² bl¹		White-blue dilute yellowface

dil⁺ dil⁺	bl¹ bl¹	Normal blue
dil⁺ dil^gw	bl¹ bl¹
dil⁺ dil^cw	bl¹ bl¹
dil⁺ dil	bl¹ bl¹
dil^gw dil^gw	bl¹ bl¹	Greywing blue
dil^gw dil	bl¹ bl¹
dil^gw dil^cw	bl¹ bl¹	Full bodied greywing blue
dil^cw dil^cw	bl¹ bl¹	Clearwing (or white wing) blue
dil^cw dil	bl¹ bl¹
dil dil	bl¹ bl¹	White-blue dilute

Table 5 – Possible genotypes considering both loci and the respective phenotypes.

*¹ – Paler shade of goldenface
*² – Paler shade of yellowface

From table 5, we can see the combination between the genes that occupy only two loci, are responsible for creating 150 different genotypes that correspond to 40 different phenotypes.

As I stated earlier, the expression "the green colour is dominant over the blue colour" is the result of years of confusion about the genetics involved on the origin of budgerigar's colours. The terms dominant and recessive are used only to describe the relations between genes that occupy the same locus. Employing these terms to the visual effects (phenotypes) of genes that occupy two different loci can only serve to confuse and discourage young fanciers with knowledge on biology and genetics. No colour is dominant over another!

Another common mistake is the belief that there is a gene for the green colour and a gene for the blue colour. "Green gene" and "blue gene" are simplifications one must use carefully. This erroneous concept may have become popular because of the nomenclature and gene symbols used on the book "Genetics for budgerigar breeders". Although the authors understood the effects of each gene they opted to call "green gene" to the wild-type gene that determines the production of yellow pigment, and "blue gene" to the mutation associated to the elimination of yellow pigment. However, we know now that when this "green gene" is associated with the gene for dilute or the "ino" gene (each on their respective loci) the resulting bird is yellow. The same way, if the "blue gene" is associated with the "dilute gene" the bird is white. The specific colour of a budgerigar is the result of the combined effects of two different loci.

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