Essential Genetics for the Terrified
by Chris Rutt

Part 3: Incomplete Dominance

In Part 2 we looked at one of the simplest forms of inheritance, known as dominance, where the presence of one gene from a parent completely suppresses the appearance (scientifically often called the "expression") of another gene, which is known as a recessive. This is, as we will see, not the only way in which two genes can interact with each other. Let us now consider the result of mating Olive Green Lovebirds to Light Green partners.

In this case we do not breed ANY birds identical to either parent, so clearly the Dominance principal we have already looked at is not at work here. Instead the offspring appear darker than the Light Green parent, but lighter than the Olive. While this may appear to be a blending of the two, it can still be explained by the mechanism we have already described, with each parent passing one half of its own colour make up to its offspring. Let us do our experiment with the sacks again.

Instead of the sack of Marine marbles, we will substitute a sack of dark green ones. Drawing one marble from each sack to represent the pairs found in the F1 (first generation of offspring), each of which will have one light green and one dark green (heterozygote), and the appearance (remember the word "phenotype"?) will be what we call "Jade", or a mid green. So, while the dark green gene does suppress the light green, it does not completely assert itself in the way that green did over Marine, and is therefore known as an INCOMPLETE dominant.
Looking at the result of mating two of these mid green (Jade) birds we get an F2 generation consisting of:

Hopefully you will now have understood these principles sufficiently to be saying, "That cannot be right. There is not a "dark GREEN" gene, because we can use Olive Birds to produce Slates (sometimes called Mauves, but effectively very dark Marines) and Cobalts (Dark Blues)." Well done!

The gene we are dealing with in this example is in fact a gene for "Darkness", and is passed on independently from the gene for "Green-ness". I will deal with this in the next part of this series.

Don't forget, if you are finding any aspect difficult, please write to me so that your problem can be dealt with in a follow up piece at the end of the series.

Part 4: Multiple Genes (1)

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