Blue and Green BudgiesGenetics of Base Color

Summary:
Yellow-base - dominant
White-base - recessive

There are only two alleles for a budgie's base color: yellow-based and white-based. The yellow-based allele is dominant to the recessive white-based allele. Here, we will represent the yellow-based gene as "B" and the white-based gene as "b". Therefore there are three possible genetic combinations for any budgie:

As you can see, the only way that a budgie will be white-based is if both of it's genes are the recessive white-base. If the yellow-base gene accompanies the white-base gene, the budgie will be yellow-based because the yellow-base gene is dominant. Such a budgie is heterozygous and is said to be "split" for white-base, which means it carries the recessive gene, but does not show it because of the presence of the dominant gene. Breeding two yellow-based budgies who are split for white-base can result in white-based budgies. However, it is difficult to tell if a budgie is split for white-base. It is said that if there is blue in the feathers around the vent of a normal yellow-based budgie, it is split for white-base. Either knowing the varieties of the parents or selective breeding can reveal the genetics of a particular bird. Below are Punnet square examples of possible pairings.

Two homozygous yellow-based budiges
BBXBB

B B
B BB BB
B BB BB

Offspring Phenotype Results:
100% Yellow-based

Offspring Genotype Results:
100% Homozygous dominant (BB)

A homozygous yellow-based budige and a heterozygous (split for white-base) yellow-based budgie
BBXBb

B B
B BB BB
b Bb Bb

Offspring Phenotype Results:
100% Yellow-based

Offspring Genotype Results:
50% Homozygous dominant (BB)
50% Heterozygous (Bb)


Two heterozygous yellow-based budiges (both split for white-base)
BbXBb
B b
B BB Bb
b Bb bb

Offspring Phenotype Results:
75% Yellow-based
25% White-based

Offspring Genotype Results:
50% Heterozygous (Bb)
25% Homozygous dominant (BB)
25% Homozygous recessive (bb)


A homozygous yellow-based budige and a white-based budgie
BBXbb
B B
b Bb Bb
b Bb Bb

Offspring Phenotype Results:
100% Yellow-based

Offspring Genotype Results:
100% Heterozygous (Bb)


A heterozygous (split for white-base) yellow-based budige and a white-based budgie
BbXbb

B b
b Bb bb
b Bb bb

Offspring Phenotype Results:
50% Yellow-based
50% White-based

Offspring Genotype Results:
50% Heterozygous (Bb)
50% Homozygous recessive (bb)


Two white-based budiges
bbxbb
b b
b bb bb
b bb bb

Offspring Phenotype Results:
100% White-based

Offspring Genotype Results:
100% Homozygous recessive (bb)