Genetics
The Blue mutation is recessive to its wild-type allele, so a bird possessing a single Blue 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 full production of the yellow psittacin pigment. Among the budgerigar fancy such a bird is said to be a Light Green split blue, usually written Light Green/blue. In a bird which has two Blue alleles (the homozygote), the lack of the wild-type allele means the yellow pigment can no longer be produced, and so the body colour is blue — the Skyblue.
The locus of the Blue gene is situated on one of the autosomal chromosomes. The Yellowface Blue I mutation, the Yellowface II mutation form an autosomal co-dominant series of alleles with the Blue mutation .
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).
Read more about this topic: Blue Budgerigar Mutation