Although you can look over a field of Icelandic sheep and see an apparently infinite range of colours, the genetics which determine which colour a sheep will be are quite straightforward.
There are only two colours, black or moorit (brown). Black is the dominant colour, so a black sheep may carry two black genes, or one black and one moorit. A moorit sheep will be carrying two recessive moorit genes. But to complicate this, a year of living outside can fade the outer fibres of a black sheep to a rich chocolate brown, and moorit can fade to coffee.
These two colours can altered or concealed by the pattern genes. There are six possible, each sheep carries two.
White – the dominant pattern, if a sheep carries the white pattern gene, it will disguise either the black or moorit colour and appear white.
Mouflon – this pattern gives the sheep a white tummy, throat, legs and eyebrows.
Badgerface – the opposite to mouflon.
Grey – Again the colour may be black or moorit, the grey pattern gene adds white thel fibres to this.
The mouflon, badgerface and grey patterns are equally dominant, and if a sheep carried either of these both would be expressed, so you could have a black or moorit grey mouflon or badgerface.
Solid – this pattern is recessive to the others, so for a sheep to be solid coloured, it would have to carry two solid genes.
Grey Mouflon – although it doesn’t seem to exist in this country, there is apparently a single grey mouflon gene which is dominant over the other patterns (except white). A sheep with this gene has white eye rings.
There is an additional “spotting” gene, which if it is expressed, shows as random patches of white on any colour or pattern. And finally, the fleece of some sheep is coloured golden-brown, usually around the head and legs, with phaeochromatin, which is unrelated to the other pattern or colour genes.