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The Yellowface I budgerigar mutation is one of approximately 30 mutations affecting the colour of budgerigars.
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Appearance
The Yellowface I Skyblue variety is the same in appearance as a normal Skyblue except that the forehead and mask, which is white in the normal Skyblue, is replaced by bright yellow, the short tail feathers show yellow instead of the normal white, and the undulations on the wings are often faint yellow. The yellow on the mask tends to leak down onto the breast to a small degree, giving it a green tinge. In juvenile plumage the yellow is considerably fainter and does not appear on the forehead, which is barred in the usual way, nor does it leak onto the breast to as great an extent.
The Yellowface I Cobalt and Yellowface I Mauve varieties have similar yellow markings.
The appearance of birds with other combinations of the Yellowface I mutation is discussed under Genetics below.
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 I mutation, together with the Yellowface II 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 I is a simple autosomal recessive with respect to the wild-type. Visibly, its action appears to be identical to that of the Blue mutation. The heterozygote or Light Green/yellowface I with one Yellowface I allele and one wild-type allele is visibly indistinguishable from a Light Green, and the homozygote with two Yellowface I alleles is visibly indistinguishable from a Skyblue. Due to this similarity in the action of the Blue and Yellowface I mutations, Bergman and Onsman have adopted the convention that these mutations be named Blue I and Blue II.
The Yellowface I Skyblue variety, described in Appearance above, is a composite of the Blue and Yellowface I mutations, having one allele of each. When two Yellowface I Skyblues are paired together, half the progency will be Yellowface I Skyblues and half will be normal Skyblues in appearance. But half of these apparent Skyblues will, in fact, be double factor Yellowface I's.
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).