The Dilute budgerigar mutation is one of approximately 30 mutations affecting the colour of budgerigars. It is one of the constituent mutations of several recognised varieties: the Light, Dark, Olive, Grey and Suffused Yellows and the Grey and Suffused Whites.
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Appearance
The Dilute mutation changes the body colour of the wild-type Light Green to yellow, with a variable amount of light green suffusion. The suffusion is deepest on the rump and around the vent. The spots and markings on the wing, head and neck, which are black in the wild-type, are pale grey. The cheek patches are pale lavender and the long tail feathers are pale bluey-grey. The eyes remain normal, with white irides when adult. There is considerable variation in the intensity of the green suffusion, but the best exhibition birds of the 1930s showed very little and also had very faint wing markings. These are the Dilute Light Greens, usually known as Light Yellows.
The green suffusion becomes progressively darker when single and double factors of the Dark mutation are present. These are the Dilute Dark Greens and Dilute Olives, usually known as Dark and Olive Yellows. When the suffusion is particularly heavy the bird is known as a Suffused Yellow.
In the blue series the absence of the yellow pigment turns the body colour to white, although this is usually suffused with blue, often quite heavily. When the suffusion is light, Dilute Skyblues are known as Whites; when it is heavy they are known as Suffused Whites. Dilute Cobalts and Mauves are usually known as Suffused Whites.
Genetics
The Dilute mutation is recessive to its wild-type allele, so a bird possessing a single Dilute allele (the heterozygote) is identical in appearance to the wild-type Light Green. That is, the presence of a single wild-type allele is sufficient to permit the production of the normal number of melanin granules. Among the budgerigar fancy such a bird is said to be a Light Green split dilute, usually written Light Green/dilute, although Light Green/yellow has been used.
In a bird which has two Dilute alleles (the homozygote) the number of melanin granules is greatly reduced, to around 5% of the normal amount. This results in a much reduced intensity of the black markings, and less absorption of light which passes through the cloudy layer in the medulla of barbs. As this absorption of light is a necessary part of the process which generates the blue colouration the intensity of blue is also greatly reduced.
The Dilute mutation is one of a series of multiple alleles at the same locus, called dil+ in the wild-type. The others are the Clearwing (dilcw) and Greywing (dilgw) mutations. The Dilute allele (dild) is recessive to all other alleles at this locus, so for the Dilute character to be expressed in the phenotype the genotype must be homozygous for this allele.