Incomplete Dominance

Incomplete dominance shows itself when there are three possible phenotypes, representing each of the two homozygous and one heterozygous genotypes. In a straight Dominant - Recessive gene interaction there are only two possible phenotypes. So, as a recap, usually dominance works like this:

AA - homozygous dominant gene, appears as dominant phenotype
Aa - heterozygous, also appears as dominant phenotype
aa - homozygous recessive gene, appears like recessive phenotype

Incomplete dominance works like this:

AA - homozygous dominant gene, appears as dominant phenotype
Aa - heterozygous, appears in-between the two homozygous types
aa - homozygous recessive gene, appears like recessive phenotype

Now, if these terms make no sense to you, please go back to Budgie Genetics and read up on the basics first. 

For now, using the spangle variety, lets see how this type of inheritance works in real life.

Spangle as an Example of Incomplete Dominance

We start with the three possible genotypes:

sp/sp - Wild Type, or not a Spangle
Sp/sp - single factor Spangle
Sp/Sp - double factor Spangle

If spangle inheritance was a straight forward dominant - recessive, then the last two genotypes would both look the same. However, with  genes that are incompletely dominant they are unable to fully express itself as a single factor so it creates an intermediate form. With this variety the three phenotypes are shown to the right:

- the yellow bird in the top right photo is a double factor spangle
- its sister next to her is single factor spangle
- and the picture below them is a normal (non-spangle)

The Dark Factor

The most common place to see incomplete dominance in action is with the gene that determines what shade of colour your budgie is; light, medium or dark.

These are the three shades that all budgie colours come in. With the blues we call them skyblue (light), cobalt (medium) and mauve (dark). With the greens we call them light green (light), dark green (medium) and olive (dark). For more information on the body colours see Budgie Body Colours.

The gene that determines shade is called the 'dark' gene and has the symbol 'D'. The wild type budgie has the non-dark gene ('d') and is light green.

So the three genotypes and phenotypes for this gene are:

- d/d no dark gene = light shade
- D/d one dark gene = medium shade
- D/D two dark genes = dark shade

Now lets do a punnett square to see how this works in the real world. If you do not know what a punnett square is please return to Budgie Genetics read about them before is only a couple of paragraphs and won't take long...!

For an example: cobalt hen x cobalt cock:

Both birds have the same phenotype (cobalt) and genotype bl1/bl1 D/d, (bl1/bl1 = blue, D/d = medium shade)

So the punnett square looks like this:

- 25% bl1/bl1 d/d = skyblue
- 50% bl1/bl1 D/d = cobalt
- 25% bl1/bl1 D/D = mauve

Without a knowledge of genetics you would probably expect to get 100% cobalts, but with a little knowledge you can see there is more to it.

Now we have a basic understanding of incomplete dominance, you should be able to work out expectations for a few more types of matings. Here is a list of the common budgie genes that are incompletely dominant.

- dark = D
- spangle = Sp
- dominant pied = Pb
- violet = V
- greywing = dilgw
- clearwing = dilcw

Now lets look at the next type of inheritance.

Return from Incomplete Dominance to Budgie Genetics

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