Yellowface II Budgerigar Mutation - Genetics

Genetics

The genetics of the several Yellowface mutations and their relation to the Blue mutation are not yet fully and definitively understood.

Much confusion and misunderstanding have arisen because the popular names given to these mutations are misleading. These mutations do not generate a yellow face, as the names might suggest. Rather the action of all these mutations is to reduce the yellow pigmentation, either entirely or to some degree, with respect to the wild-type Light Green. Had these mutations been named 'Yellow-less' rather than 'Blue' or 'Yellowface' their action might have been more easily understood from the outset. But the traditional names are engrained and are retained here.

The prevailing view is that the Yellowface II mutation, together with the Yellowface I and Blue mutations, are members of an allelic series situated at the Blue locus. Although some breeders still dissent from this view it is the one followed here.

On its own, the Yellowface II is a simple autosomal recessive with respect to the wild-type. Visibly, its action appears to be similar to that of the Blue mutation. The heterozygote or Light Green/yellowface II with one Yellowface II allele and one wild-type allele is visibly indistinguishable from a Light Green, and the homozygote with two Yellowface II alleles is like a Skyblue with a variable green suffusion on the breast.

The SF Yellowface II Skyblue variety, described in Appearance above, is a composite of the Blue and Yellowface II mutations, having one allele of each.

The loci of the Dark budgerigar mutation and the Blue allelic series are situated on the same autosome, so the Dark mutation is linked to the Blue allelic series (see genetic linkage). The cross-over value (COV) or recombination frequency between the Dark and Blue loci is commonly stated to be about 14%, but some experiments have found much smaller values (see Genetics in the Dark budgerigar mutation).

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