Genetics of colour and pattern in chickens


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.

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