We humans like to use selective breeding to play around with various charcteristics of
domestic animals – just look at the huge variety of dog breeds. To a certain extent
we breed for behavior and productivity – cats that miaou to people, sheep that have twin
lambs, beef cattle that grow lots of muscle, and chickens that lay lots of eggs. But we’re
also fascinated by the way animals look and we like to play with shapes and colours. If
you go to a poultry show, you won’t see many plain brown hens like the hybrids that
are used to produce almost all of the eggs you buy in the supermarket, and you won’t
even see many show birds that look much like their original ancestors, although the Welsummers
and brown leghorns do resemble them a little. Rather what you will see is a huge range of
chickens in different shapes and colours, and with different patterns of colour. This
video introduces the genetics of feather colour and pattern.
Make sure you understand the basic genetics of chickens first – you can check out the
playlist of my earlier videos about genetics – I’ll put a link here and in the description
below this video. All of the colours that we see in chickens’
feathers are caused by the interaction of two pigments – eumelanin and pheomelanin.
You can think of eumelanin as a kind of dark brown or black, and pheomelanin as red. Animals
and humans have these two pigments too. Pheomelanin is that pink or reddish colour
that makes our lips look redder then our cheeks. And it’s what gives the red colour to the
hair of red-heads. It’s pheomelanin that makes the golden colour of a ginger cat’s
fur. And we can see the red pheomelanin in the red breast of chickens with the colouring
sometimes called ‘duckwing’, like the brown leghorn or Welsummer.
Eumelanin is related chemically to pheomelanin but it’s a slightly different molecule and
its colour is dark brown or black. It’s eumelanin that determines our eye colour,
it makes hair colour brown or black, and it’s the black colour in fur or feathers. Apart
from just looks, eumelanin also has several functions – when our skin is exposed to
the sun, our melanocytes produce extra melanin, which makes our skin darken, and helps protect
the skin tissue from damage from the ultraviolet radiation. In birds, feathers with eumelanin
are stronger and more resistant to abrasion, because the melanin granules fill the space
between the keratin strands that form the feathers.
We talked a bit about the dilution effect of the Blue gene in my video about the genetics
of blue coloured chickens. The Blue gene is actually an inhibitor gene, which inhibits
the production of eumelanin, so that the black colour is diluted to grey. The Blue genes
only work, to make blue, or splash, if the feather would otherwise have the black eumelanin
colour. The Blue gene doesn’t work on Pheomelanin, only on Eumelanin.
The lavender gene (lav) is also a diluter gene, but it can act on both eumelanin and
pheomelanin. The melanocyte cells in the feathers still produce pigments, but the lavender gene
inhibits the transfer of the pigment granules to the feather structure. When the feather
would otherwise have been black, a double dose of the recessive lavender gene dilutes
the black eumelanin colour to “lavender” (which is also a kind of grey). If the original
pigment is pheomelanin, the lavender gene dilutes the basic buff brown to a pale straw
or cream colour. Other genes can also have an effect on the
black eumelanin in different ways that make patterns, either on each feather or on the
bird as a whole - these are called restrictor genes. A well-known restrictor gene is the
Colombian gene, denoted as Co. This restricts the black eumelanin pigment to the hackle
and tail feathers. When all the rest of the bird is white, the resulting “Colombian”
pattern is what we see in a light Sussex, like my Lighsa – the only places that show
the black eumelanin colour are her tail and her hackles.
As I said, some restrictor genes work not on the chicken as a whole but on each feather.
The Mottled gene (mo) restricts the eumelanin from being expressed on the tip of each feather,
so each feather has a white tip. If the bird would otherwise have been black, then each
feather is black with a white tip, so the overall effect is mottled like my mottled
Houdan, Fling. The mottled gene is an autosomal recessive gene, so every chicken that looks
mottled has two copies of the gene, and mottling breeds true.
The Barring gene (B) causes the black eumelanin pigment to be restricted to stripes across
the feather. The most well known chicken with barred feathers is the barred Plymouth Rock,
although my cuckoo leghorns have the barring gene. The Barring gene has its effect by turning
the pigment depositing off and on again as the feather grows out from the chicken’s
skin. Leghorns are fast feathering chickens, so the stripes or bars on each feather are
blurrier than the crisp stripes on a barred rock feather, although there are also other
factors. The Barring gene is particularly interesting because it’s carried on the
“Z” sex chromosome, so homozygous males have two copies of the Bar gene and so look
lighter than the barred females. Of course females can only have one copy of the Barred
gene because they only have one Z chromosome. (Check out my video about the genetics of
chicken sex determination if you’re not sure about that.) And when you know that the
Barring gene is a restrictor gene that turns off the depositing of eumelanin, you understand
that of course two copies of it lead more white on the feathers of the roosters.
And then there’s lacing, when each feather has one colour in the centre, outlined by
a rim of black around the edge, but that’s a bit more complicated so that had better
wait for the next video.