Ino budgerigar mutation

Appearance

In the green series the Ino is known as the Lutino, with pure yellow contour feathers, white or pale yellow flight feathers and tail feathers and silvery-white cheek patches. In some lights the body can show a very pale green sheen.

In the blue series the Ino is known as the Albino, and is pure white throughout. The cheek patches are almost the same colour as the body, but slightly more silvery. In some lights the body can show a very pale blue sheen.

The eyes of both the Lutino and Albino are red at all ages with white irides when adult, the beak is orange and the feet and legs are pink. The cere of an adult Ino cock is greyish-purple rather than blue.

The World Budgerigar Organisation has established precise standards for some budgerigar body colours using the Pantone Codes, as shown to the right for the Lutino.

The Ino mutation also induces changes in the nestling. The down is white rather than grey and appears only sparsely, never growing down the centre of the back. As the feathers appear, those down the spine and along the ventral centre line are late to develop.

The Ino gene masks the effect of virtually all other mutations, including Opaline, Dark, Dominant Grey, Dilute, and Clearwing. These genes, when present in an Ino in either heterozygous or homozygous form, cause no change in the appearance of the Ino. But the Ino gene does not entirely mask Cinnamon. A Cinnamon Ino, usually called a Lacewing, has pale brown or fawn spots, tail and wing markings. These markings are quite clear, but considerably fainter than the markings of a normal Cinnamon.

The Dark-eyed Clear has a similar body colour to the Ino, but has solid reddish-purple eyes without a white iris.

Cinnamon Dilute German Fallows, NSL Inos and Inos are all very similar and difficult to distinguish from each other, but the first two are so uncommon difficulties arise rarely in practice.

Genetics

The Ino mutation is a sex-linked recessive at the ino locus on the Z chromosome. The wild-type genetic symbol is ino⁺ and the ino mutant allele has the symbol ino. Its effect is to inhibit the production of the melanin pigment which is normally present in all feather barbs in either the medullary or cortical cells or both. The presence of black melanin pigment in the cortex of the barbs is necessary for the production of the black markings and in the medulla of barbs for the production of the blue colouration (which combines with the yellow pigment in birds of the green series to produce the green colouration), so this mutation removes all black and blue colourations resulting in a white bird in the blue series and a yellow bird in the green series.

Because the Ino mutation totally inhibits the production of normal melanin pigment it prevents the visible expression of all the other mutations which depend on the presence of melanin to show their effect. This is called epistasis, and Ino is phenotypically epistatic over many other mutations, including Dark, Grey, Opaline, and the Dilute series. It is not epistatic over the Blue mutation, so there are two forms of the albino budgerigar, one in the green series called the Lutino and one in the blue series called the Albino. Both these varieties may be masking many other hypostatic mutations, so the genotype of an Albino or Lutino with respect to these mutations cannot be determined visually. Nor is the Ino mutation epistatic over the Cinnamon mutation — see below.

In birds, the cock has two Z chromosomes and the hen has one Z and one W chromosome. So in hens whichever allele is present on the single Z chromosome is fully expressed in the phenotype. Hens cannot be split for Ino (or any other sex-linked mutation). In cocks, because Ino is recessive, the Ino allele must be present on both Z chromosomes (homozygous) to be expressed in the phenotype. Cocks which are heterozygous for Ino are identical to the corresponding Normal. Such birds are said to be split for Ino, usually written '/ino'.

The table on the right shows the appearance of all possible genetic combinations involving just the Ino mutation.

The Ino mutation does not mask the Cinnamon mutation, these two genes being neither fully epistatic nor hypostatic to each other. When combined in doubly homozygous form (cin-ino/cin-ino in cocks or cin-ino/Y in hens) the Lacewing phenotype is produced. The Cinnamon markings are clearly visible, although considerably fainter than in a normal Cinnamon. For many years the Lacewing was thought by many to be a separate mutation but it was demonstrated in 1979 that it was simply a Cinnamon Ino when a Lacewing was deliberately produced by combining separate Cinnamon and Ino genes. Once brought together, these two genes are almost always inherited together due to the close linkage between them, giving the impression of being a single gene.

The Ino mutation is a member of a series of multiple alleles at the ino locus. Only one other member is known -- the Sex-linked Clearbody mutation. For details of the genetic relationship, see the Genetics section in the Sex-linked Clearbody article.

The Ino gene is linked to other genes located on the Z chromosome, i.e. to the genes of other sex-linked mutations. In addition to the Cinnamon mutation, these mutations include Opaline and Slate. The cross-over or recombination values between Ino and these linked genes has not been measured accurately, but some approximate measurements of the cross-over values have been made:

Cocks split for both Cinnamon and Ino have one Cinnamon allele and one Ino allele together with one each of the corresponding wild-type alleles. The linkage between the Cinnamon and Ino genes gives rise to two types of split cinnamon-ino cocks, both visually identical.

Hens cannot be split for any sex-linked gene, so only cocks exist in Type I and Type II form.