Dark Budgerigar Mutation - Genetics

Genetics

The Dark mutation has an incompletely dominant relationship with its wild-type allele. That is, it shows a visible effect when present as a single factor (heterozygote) and a different effect when present as a double factor (homozygote). In the green series varieties the Dark Green has one Dark allele and one wild-type allele at the Dark locus and the Olive has two Dark alleles. In the blue series varieties the Cobalt has one Dark allele and one wild-type allele and the Mauve has two Dark alleles.

Because the Dark factor is always visibly expressed no budgerigar can be split for Dark. The heterozygotes of Dark — the Dark Greens and Cobalts — correspond to the splits of the recessive mutations.

The loci of the Dark 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 often stated to be about 14%, but several careful measurements of this COV show quite widely varying results. Early measurements by Duncker and independently by Steiner obtained values of 14% and 7.6% respectively, and T G Taylor and C Warner collected results which showed only 5 cross-overs in 140 - a COV of 3.6% . Included in these were results from T G Taylor's own experiments, in which he found no cross-overs in 86 birds bred. It is now known that the environment and other genes can influence the COV, so some variability should be expected. A reasonable average of these measurements is a COV of 8%.

Dark Green/blues have one Dark allele and one Blue allele together with one each of the corresponding wild-type alleles. The linkage between the Blue and Dark genes gives rise to two types of Dark Green/blue birds, both visually identical.

• Type I Dark Green/blues are bred by mating Mauves to Light Greens and have the two mutant alleles on the same chromatid. Geneticists call this 'coupling' rather than 'Type I'. Because of the linkage, the Dark and Blue alleles from Type I birds tend to be inherited together in their progeny. When mated to Skyblues, Type I birds produce predominantly Light Green/blue and Cobalt progeny, with Dark Green/blue Type II and Skyblues resulting rarely from a cross-over.

• Type II Dark Green/blues are bred by mating Skyblues to Olives and have the Dark and Blue mutant alleles on opposite chromatids. Geneticists call this 'repulsion' rather than 'Type II'. Because of the separation, the Dark and Blue alleles from Type II birds tend to be inherited separately in their progeny. When mated to Skyblues, Type II birds produce predominantly Dark Green/blue Type II and Skyblue progeny, with Light Green/blue and Cobalts resulting rarely from cross-overs.