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Dominant Inheritance

When a bird has a dominant gene it can always be seen in their physical appearance, e.g. a spangle or a violet.

A bird can have the dominant factor on a single gene it will always show in the bird's appearance, it will not be affected by the other gene in the pair and it does not matter which one of the pair genes has the dominant gene. Such a bird is known as a single-factor bird, e.g. a single-factor spangle.

A bird can have the dominant factor on both copies of the same gene. Such a bird is a double-factor. Double factor birds may appear identical to single-factor birds (e.g. grey factor) or they appear very different (e.g. double-factor spangles appear all yellow or all white).

When breeding, each bird in the pairing just passes one of the two copies of each gene to the offspring; the one that gets passed is randomly selected. This all means that the offspring still have two copies of each gene, one from each parent.

To see the effect of pairing two birds we use expectation tables:

I am going to use the following symbols:

G	gene carrying the grey factor
g	normal gene (no grey factor)

(There are millions other genes, but we will consider just one pair at a time.)

So the following combinations are possible of these two genes:

gg	no grey factor
Gg	single-factor grey
gG	single-factor grey
GG	double-factor grey

So a bird has millions of genes, if just one of the two genes that can carry the grey factor has it, then it will appear grey (grey-green, grey (-blue), grey yellow, grey white, etc.)

Do you remember that only one of each pair of genes is passed on by each parent? We can use this to calculate what to expect from a pairing.

single-factor grey x single-factor grey
(Gg x Gg)

can produce:

	GG	double-factor grey
	Gg	single-factor grey
	gG	single-factor grey
	gg	normal

This was produced by simply one gene from each bird until all possible combinations have been produced. (In this case, I simply took the first gene of the first bird (G) and matched it to the first gene of the second bird (also G) to give GG (the first line of the result) and then matched it with the second gene of the second bird (g) to get Gg (second line). I then repeated the process using the second gene of the first bird to get gG (g + G) and gg (g + g).)

Or more simply, watch this animated diagram:

Another way of finding the possible gene pairs is by using a table:

Dominant Genes

The arrows show which columns and rows each gene appears within (so the G of bird along the top row appears in the two cells on the left and the g of other bird appears in the bottom two cells - hence the bottom-left cell had the G from bird 1 and g from bird 2).

You may find tables easier to make and understand once you know how inheritance works.

Percentages

The percentage of each type of offspring can be calculated simply by looking at the fractions of each type. Since in the above pairing there are 4 possibilities of combining the two sets of genes and each one has an equal chance of occurring:

1 in 4 will be GG = 25% GG

1 in 4 will be Gg = 25% Gg

1 in 4 will be gG = 25% gG

1 in 4 will be gg = 25% gg

But, the gG and Gg birds are identical (no difference in their appearances or when they breed) so

25% Gg + 25% gG = 50% Gg

giving:

25% GG (double-factor grey)

50% Gg (single-factor grey)

25% gg (normal)

In a single pairing, you are will not always (or even often) get these exact percentages, but over many thousands chicks produced from such pairings these percentages are very accurate.

Simulating Pairings using Dice

You can simulate a pairing like this with two dice. Each dice represents both genes from one of the parents, say that G is represented by even numbers and g by odd. Shake the dice and see what you get (e.g. 6 + 3 = G + g = Gg (single-factor grey)).

Other Dominant Genes & Incomplete Dominance

I have used the Grey factor in my examples because it is very common and most aviaries will have at least one or two budgies carrying it, but there are many others, see below:

Green

Dark

Violet

Grey

Spangle

Dominant pied

Most of these factors are actually incompletely dominant genes, which means that a single-factor bird is different in appearance to a double factor bird.

Dark is an incomplete-dominant factor; in a green series bird a single dark-factor produces a Dark Green bird whilst a double-factor produces an Olive.

Spangle is an incomplete-dominant factor that produces a very different bird when both genes carry it compared with a single-factor Spangle.

Green, Grey and Dominant Pied are completely dominant, it is impossible to tell if a bird carrying any of these genes has them in single- or double-factor simply by looking at it.