A short primer for budgie breeders.

A gene is a stretch of DNA that codes for a trait. Each gene sits at a fixed location called a locus. The alternative versions of a gene are called alleles — for example, the budgie base-colour locus has two alleles: B (green, dominant) and b (blue, recessive).

Budgies inherit one allele from each parent at every autosomal locus, giving them a genotype (the actual allele pair, e.g. Bb) and a phenotype (what we see — in this case, green, because B dominates b).

A dominant allele expresses its trait with only one copy ( Aa looks the same as AA). A recessive allele needs two copies (aa) to be visible.

Incomplete dominance is dose-dependent: heterozygous birds look between the two homozygous extremes. The budgerigar dark factor is the classic example:

• dd Light green (or sky blue)
• Dd Dark green (or cobalt)
• DD Olive (or mauve)

Each dose deepens the body colour. Violet, spangle and crest behave similarly.

Birds, unlike mammals, use the ZW system. Cocks are ZZ; hens are ZW. Recessive mutations on the Z chromosome — Ino, Cinnamon and Opaline — therefore behave differently between sexes.

• A cock needs two recessive Z alleles to express (e.g. ii = lutino).
• A hen needs only one, because her single Z has no partner ( i/W = lutino).
• A hen cannot be “split” for a sex-linked recessive — she either shows it or she doesn’t.

This is why pairing a normal cock split-for-ino to a lutino hen produces lutino daughters and split-ino sons.

A split bird is heterozygous for a recessive mutation: it carries the allele but doesn’t show it. Splits are written with a slash: Light green / blue means a green bird carrying one blue allele.

You can only confirm a split by test breeding: pairing the suspected carrier with a visual bird of that mutation and looking for visual chicks.

A Punnett square is just a grid of parent A’s possible gametes × parent B’s possible gametes. Each cell is a possible offspring genotype, and each cell is equally likely. With multiple loci, multiply each locus probability together (loci segregate independently).

The breeding calculator on this site does this automatically across all 13 modelled loci, including the ZW sex split.