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Discussion of Poultry Genetics

Genetics of eye colour:

The genetic basis of eye colour has not been extensively studied as have other aspects of phenotype. However, some things are accurately known. First of all, the wild-type eye is characterized by the Light Brown Leghorn. Eye colour is a result of pigmentation of a number of structures within the eye (iris, retina, uveal tract, ciliary).

The bay-colour eye (various shades of reddish brown) is due to carotenoid pigments and the blood supply of the iris. Brown eyes are increasingly melanized with the darkest eye colour due to the fibromelanotic gene characterized by heavy eumelanin deposits throughout the eye. Little is known about pearl eye and Smyth has speculated that it has the same eumelanin distribution as the bay but without the carotenoids.

Eye colour is modified by a number of genes that are known to be associated with shank and plumage colour. The sex-linked dermal melanin genes, id+ and idM enhance dermal shank and eye pigmentation. The inhibitor of shank dermal melanin, Id, also inhibits eye pigmentation. Smyth hypothesized that the idM gene together with extended black, E, is responsible for dark brown eyes. idM also darkens the eye on the e+ background.

A dominant sex-linked inhibitor of eye pigmentation is known, Br. This trait is not useful for developing sexable day-old chicks because chickens do not get their final eye colour until they reach sexual maturity.

In the absence of other melanin inhibitors, the E-locus alleles, E (extended black) and ER, birchen, result in a brownish eye with the E allele making the darker eye. Sex-linked barring, B, and eumelanin inhibitors at the E-locus, like eWh have an effect on eye colour. Recessive white seems to have no effect on eye colour and dominant white, I, has a strong ability to inhibit eye pigmentation. The genetics of pearl are not known, however, it is known that the white skin gene, W, is not the genetic basis of pearl eye, since Cornish have yellow skin and can also have pearl eye.

Genetics of chick down colour:

Virtually everyone who hatches their own baby chicks wants to know what the chick down colour tells them about the genes of that individual chick. This section is an effort to give some guidance in this direction.

Some Basic Chick Down colour Genetics......

 
Chick Down Genotype
Phenotypic Remarks
E, Extended Black basically black down. Variation includes gray and cream bellies. The cream can invade the head and face. Often Australorps have cream in their faces and heads although the adults are standard black colour. Sex-linked barring puts the cream head spot on the black down. Recessive mottling (mo) makes these chicks look like penguins
e^Wh, Dominant Wheaten Without columbian (Co) Wheaten down is a light cream for both sex-linked silver and gold. One cannot distinguish silver from gold with any accuracy on dominant wheaten. Heterozygotes can have varying amounts of striping. Some New Hamps have light reddish stripes on their backs at hatch. This may be due to heterozygousity or some other modifiers in these lines. With Co wheaten downs are easily sexable. There is a high degree of difference between silver and gold wheaten down when columbian is present. This is why commercial white-tailed reds have dominant wheaten and columbian.
e+, Wild-type The dark eye stripe is characteristic of wild-type. The wild-type is often referred to as the 'chipmunk' look. The buff colour of the face and back stripes are affected by sex-linked silver and gold. You can easily tell Silver Duckwing chicks from Light Brown Leghorns, but you tend to have trouble in crosses, involving Ss heterozygotes, so there are probably modifiers that affect the gold colour of the pure line chicks.
eb, Brown Can range from a solid dark mink brown to light brown with stripes. The most accurate typing for eb is that they have brown heads with no stripes on the head like ebc.
ebc, Buttercup more yellow than e+. This dilution may be due to ebc or Db that seems to be in all the crosses and pictures involving this allele. Adult females are like eb females and do not have salmon breasts.
ey, Recessive Wheaten Sometimes pictured as being yellow but more brown than e^Wh. Both are said to be cream in colour. Recessive wheatens are often called dark wheatens because the adult females have more stippling on their backs than dominant wheaten females usually have.
eb,Co Chicks having sex-linked silver in addition to the above genes are cream coloured with varying amounts of gray on their backs. Their backs can be nearly black. Sex-linked gold chicks show buff on their flanks and faces and cream bellies with the same varying amount of gray on their backs. The less gray the more buff on the backs of sex-linked gold chicks.
eb, Co, Db Mostly light (yellow) body with brown head and back stripe. Buff breeds may have wheaten, Co, and Db.

Some additional genes that affect chick down colour are: Dominant and recessive white with extended black at the E-locus gives yellow chick down. If black spots leak through it is usually because the chick is heterozygous for dominant white. Dark prown, Db, makes the black down of extended black to be a reddish brown. Blue or grey chicks can be extended black and Bl (blue) heterozygotes. These chicks can also look black. Chicks that are homozygous for recessive lavender are blue / grey.

A fun exercise:

It is fun to consider the genetic make up of some popular breeds.  From what we know at this point about poultry genetics, we can make some judgments about the genes that birds have to have in order to look the way they do. 

Let’s consider Rhode Island Red as an example. With respect to feather colour, the Rhode Island Red (RIR) has gold (on the sex chromosome), s+ (sometimes called G), and the mahogany (red enhancing) gene, Mh (see the tables below).  A Rhode Island Red without any black in her hackle or tail may have a black suppressing gene such as dominant white, I, and Columbian, Co, serves as an additional black suppressing influence.  She will have no black extending genes, such as nigrum, which would change her to a black bird.  Rhode Island Reds come in both single and rose comb types so she may have at least one copy of the rose comb gene.  The single comb RIRs have neither the pea comb nor rose comb genes.  RIRs have yellow skin, shanks and feet, which requires two copies each of the w, Id, e+ genes.  RIRs can have either one or two genes (hetero- or homozygous) for eggshell colour giving the brown egg.

White Leghorns are white chickens that lay white eggs.  The white could be achieved in a number of ways.  The task at hand is to effectively inhibit both black and red.  We construct a Leghorn by having two dominant white genes, I, which are black inhibitors, but the dominant white gene allows some black flecks through (see the table at the end) and needs help.  Sex-linked barring, B, acts as a black inhibitor in white birds and is used as such in Leghorns.  The red inhibitors in Leghorns are the silver gene, S and birchen gene, ER, or nigrum, E.  To get the single comb, the Leghorn has (r, r) and (p, p) for comb genes and she needs (o, o) for eggshell colour genes for the white eggs.  The Leghorn has yellow skin, shanks and feet which requires two copies each of the w, Id, e+ genes.

Genotypes of common interest:

Below is a table of partial genotypes for breeds of common interest. There may be other genotypes and so I do not claim that any given breed must be exactly the genotype specified. If a gene does not appear in the table, it is intended that wild-type be assumed although sometimes wild-type genes are listed for emphasis.

Partial Genotypes of Breeds of Common Interest

 
Breed Autosomal Genes Sex-Linked Genes
Comments
Australorp, Black
W/W, E/E, co+/co+, db+/db+, (Ml/Ml), i+/i+,(Pg)
S, Id
Evidence of unknown black enhancers and the pattern gene have been observed in Black Australorps.
Silver Spangled Hamburg
ER/ER, Co/Co, Db/Db, Ml/Ml, pg+/pg+
S, id+
The combination of dark brown and melanotic may be responsible for the white undercolour.
Silver Laced Wyandotte
eb/eb, Co/Co, db+/db+, Ml/Ml, Pg/Pg
S,Id
 Yellow legged blacks are usually based on the eb E-locus allele.
New Hampshire
eWh/eWh, Co/Co, Mh/Mh, w/w
s+, Id
 With polygenes for red ear lobe and brown eggshell colour. A primary difference between New Hampshire and Rhode Island Red is the wheaten allele at the E locus in the New Hamps.
Rhode Island Red
ey/ey, Co/Co, Mh/Mh, w/w, Db?
s+, Id
 With polygenes for red ear lobe and brown eggshell colour.
Barred Plymouth Rock
E/E, Co/Co, w/w
K, S, B, Id
 With polygenes for red ear lobe and brown eggshell colour. The slow feathering gene, K, is believed to aid in obtaining a cleaner barring. Barred Rocks have yellow shanks because of the dermal melanin inhibiting property of Cuckoo barring. Without this, the breed would have near black shanks with yellow soles.
White Leghorn
Homozygous for either E, ER or e+, I/I, w/w, o/o
B, S, Id
 Lines of Leghorns have been found with different alleles at the E locus.
Delaware
eb/eb, Co/Co, w/w
B, S, Id
 With polygenes for red ear lobe and brown eggshell colour. Delawares have Barred Rock and New Hampshire genetics.

 

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