Genetics of comb type:

Comb type in chickens is due to two genes, the rose comb gene, R, and the pea comb gene, P. These two genes are on different chromosomes. The lack of these genes is represented with lower-case letters, r and p. More correctly stated, r and p (or r+ and p+ to indicate they are the wild-type genes) are the genes that replace R and P when they are not present. A chicken with a single comb is lacking both R and P genes and so could be represented as (r, r) for rose comb and (p, p) for pea comb. Some authors will combine this ‘notation’ and write (rrpp) to represent the genes for single comb. I prefer the first way of writing the genes for the purposes of this text.

A chicken with a rose comb will have one of the gene combinations: (R, R) with (p, p), or (R, r) with (p, p). A bird with a pea comb will have (r, r) with (P, P), or (r, r) with (P, p). Since one copy of the rose or pea gene is sufficient for that comb type, these genes can be thought of as dominant. However, they act together to create the walnut comb when both rose and pea comb genes are present.

Poultry with a walnut comb have at least one copy of both the rose comb gene and the pea comb gene. The gene combinations that give walnut comb are: (R, R) with (P, P), (R, r) with (P, P), (R, R) with (P, p) and (R, r) with (P, p).

To explore the genetics of comb type, let’s cross a pea comb chicken, (r, r) with (P, p), and a rose comb chicken, (R, r) with (p, p). Because two genes on different chromosomes are involved, there is more bookkeeping than if there were only one gene involved, but the principle is the same and no more difficult. We first have to consider the combinations of the rose comb genes of the two parents, then the combinations of the pea comb genes of the two parents. Then we realize that each of the rose comb combinations can occur with each of the pea comb combinations. In the end there are 16 combinations in all.

The four possible combinations for the rose comb genes from the two parents are: (R, r), (R, r), (r, r) and (r, r). The four combinations for the pea comb genes from the two parents are: (P, p), (P, p), (p, p) and (p, p). Since each of the four rose comb combinations can occur with any of the pea comb combinations, we now have to consider each of the rose comb combinations with each of the pea comb combinations (16).

The figure above shows how to make a helpful drawing. Make a list (column) of the four rose comb gene combinations on one side and the pea comb genes on the other side. The combinations of the first rose comb gene pair with all the pea comb gene pairs is shown in the figure by the connecting arrows. Considering the (A, a) of the drawing to be (R, r), the possible combinations of the first rose comb gene pair with the pea comb gene pairs are: (R, r) with (P, p) twice [we get this combination twice], and (R, r) with (p, p) twice. The second rose comb gene pair with the pea comb genes gives the same combinations: (R, r) with (P, p) twice and (R, r) with (p, p) twice. The third rose comb gene pair with the pea comb gene pairs gives: (r, r) with (P, p) twice and (r, r) with (p, p) twice. The last rose comb gene pair with the pea comb gene pairs gives the same: (r, r) with (P, p) twice and (r, r) with (p, p) twice.

So, of the 16 possibilities, four of them are (R, r) with (P, p) and is walnut comb, four are (R, r) with (p, p) and is rose comb, four are (r, r) with (P, p) and is pea comb, and four are (r, r) with (p, p) which is single comb. We have four out of 16 chances (25% chance) to get a walnut comb from this cross, four out of 16 chances to get rose comb, four out of 16 chances to get pea comb and four out of 16 chances to get single comb.

Genetics of shank/feet colour:

The shank/feet colour is controlled by genes that affect the skin at different depths. The visible colour is due to the combined effect of the different colours of the dermis and the epidermis. So, the shank/feet colours are a combination of upper skin and deeper skin pigmentations. The following table gives the shank/feet colours that result from the major gene combinations (the bird has two copies of each gene). It is important to remember that other genes can modify shank and foot colour. For example, the sex-linked barring gene, B, is a potent inhibitor of dermal melanin. The Barred Plymouth Rocks, for example, would not have light shanks and feet if it were not for the fact that they have sex-linked barring. The female Barred Rocks tend to have darker shanks due to the dose effect of the barring gene. The following table is intended as a guide but should not be considered to be absolute, since (as mentioned) other genes, such as sex-linked barring, can modify shank/foot colour.

Some Basic Shank/Feet colour Genetics.....

Genetics of dark skin colour:

The hypermelanic condition of some breeds, such as the Silkie breed, is due to a pigment cell activator, which was named by F. Hutt as "fibromelanosis" to emphasize the fact that the gene causes pigmentation of connective tissue. The inheritance of the dark skin phenotype involves the fibromelanosis gene, Fm, as well as dermal melanin inhibitors, such as the sex-linked Id dermal melanin inhibiting mutation. The fowl with Fm and wild-type dermal melanin, id+, will have darkly pigmented skin and connective tissue. The combination of Fm and Id gives a bird that has little or no observable skin pigmentation. There are other dermal melanin inhibitors that may have an influence on the degree of melanization due to Fm (or the degree of expression of Fm). Some genes influencing plumage colour have an effect on dermal melanin, such as the E-locus alleles, which may influence the expression of Fm. However, fibromelanotic Silkies exist with black, white, blue and partridge patterns.

Genetics of feather colour:

The genetics of feather colour and patterns is an active topic of poultry science research. Much of the work that was done prior to the late 1980s is now considered out of date. Because a number of genes interact to determine feather colours and patterns, it might seem to be too involved for the average enthusiast. I don’t believe that this is the case, however, the topic of feather colour and patterns may be beyond the interest and motivation of some enthusiasts.

White is actually all the colours combined and black is the lack of reflection of light in the visible range, so one might argue that black and white are not really ‘colours’ technically. However, if we count black and white as colours, chickens have only three basic colours: black, white and red (gold).

The colours of chickens are achieved by diluting and enhancing or masking black and red (gold). For example, Rhode Island Reds have the gold gene with the dominant mahogany (red enhancing) gene. A blue chicken is a black bird that has the blue gene which dilutes black. Two copies of the blue gene give a splash effect. A white chicken can be achieved in a number of ways by inhibiting black and red pigmentation with combinations of genes (dominant white, recessive white, silver, Columbian, Cuckoo barring).

Some perceived colours of feathers are due to the structure of the feather and not any pigmentation. The purple and the ‘beetle’ green sheen that can be seen in some poultry is due to the way the feather structure reflects light rather than the presence of a pigment.

First, we need to define a couple of terms. In poultry there are primary and secondary colour patterns. Perhaps it is better to define secondary patterns first. A secondary pattern is a pattern that appears on individual feathers. These are patterns like single and double lace, mottle, and so on. Primary patterns are colour patterns that involve the entire body of the bird. An example is the silver Columbian pattern. In the Columbian bird, black is restricted to the hackles, wing bow and tail. The silver Columbian is a white bird with some black in the neck, wing and tail areas. Because this pattern is not manifest on individual feathers, it is a primary pattern.

To ‘construct’ a chicken having a particular colour scheme, one begins with the ‘background’ or the E-locus gene(s). The other colour and (secondary) pattern genes essentially modify this ‘background’. Please refer to the table at the end and the pattern table below to see the choices and comments (other E-genes have been proposed but they are not yet well accepted). Some of these are: E, extended black or nigrum; ER, birchen; eWh, dominant wheaten; e+, wild type; brown, eb; speckled, es; buttercup, ebc; and ey, recessive wheaten. These genes cause recognizable chick down colour and influence the adult feather colour, sometimes male and female feather colours are influenced differently. For photographs of chicks with an assortment of E-genes the interested reader is directed to Poultry Breeding and Genetics, R.D. Crawford, ed., Elsevier, 1990 pages 115-117.

As an elementary exercise, let’s ‘build’ a white chicken. We can start with wild-type background, e+, and require our bird to have two copies of this gene. We can suppress the red in the chicken by adding the silver gene, S, which has the effect of changing red to white. Black is suppressed (changed to white) by the dominant white gene, I, however this gene is ‘leaky’ (see the table for comments) and allows black specks through. A good ‘helper’ gene in this situation is the Columbian gene, Co, since it is a restrictor of black. Although this set of genes is not the only set that will yield a white chicken, it is one of the ways a white chicken can be obtained.

The influence of one versus two genes for a colour trait:

Feather colour genes often display a ‘dosing’ effect: “Two genes are stronger than one.” For example, since the locus of the sex-linked barring gene is on the Z sex chromosome, females that have Cuckoo or sex-linked barring (the barring that Barred Rocks have) can have only one barring gene and have barring that is less well defined than the barring of males that have two barring genes. Also, Sil-Go-Link males that have only one silver gene (silver inhibits red) often have some red colour on their wings. So, in the Sil-Go-Link male, the one silver gene does not completely inhibit the red pigment. The silver gene is dominant but still some red is visible when only one silver gene is present. This 'dose effect' in which two genes for a trait reinforce or strengthen the expression of a trait is common in poultry.

Continue to the next page for more genetic patterns and diagrams

 

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