This is a paper by Criss Waller.
I have yet to figure out how to post the tables and such on here so to see the full paper, please click on this link: http://members.cox.net/foxcroft/genetics.htm.
One of the things that makes Nigerians so fascinating is that they come in such a wide range of colors and patterns. As an amateur geneticist, a passage I read in a brochure on the Nigerian really got my attention-“ You never can be sure what color the babies will be until they are born.” “Why was that?”, I wondered. Wasn't much known about the color genetics of the goat?
As it turns out, the latter supposition was true. Little is known about goat color genetics, compared to other domestic animals. We know far more about color genetics in the mouse or the dog, or even the sheep, than we know about the goat. But some genetic principles are applicable to all mammals, and some specifics are known about caprine color. This knowledge can be useful to serious breeders, and is also interesting to know.
First, The Basics- Genetics 101
Yep, this is the stuff you probably learned in your high-school biology classes but may have forgotten!
Phenotype- what an animal looks like- is determined by genotype- what genes that animal has. Genes are the basic units of heredity. Each gene controls one characteristic of an organism, often by dictating how that organism assembles a certain protein. Genes are found in specific locations on chromosomes, which are sets of thousands of genes strung together in a specific order. These gene locations are known as loci (or locus, in the singular). Each gene has its own specific locus. Humans have 46 chromosomes, goats have 60. Each chromosome has thousands of gene loci. Each organism inherits one copy of each gene from each parent.
Genes often occur in several different types. These different types are called alleles. Each type will usually code for a different form of the same protein. Genes are usually known as recessive or dominant. A dominant gene will be expressed in the phenotype- or recognizable to an observer- even if only one copy, inherited from just one parent, is present. A recessive gene is noticeable only if two copies of it are present. In other words, both parents must contribute a copy of a recessive gene in order for it to be expressed in the phenotype.
Since recessive genes are expressed only if two copies are present, an organism can “carry” a copy of a recessive gene without it being shown in the phenotype. A simplified example is hair color in humans. Brown is dominant to blond. Thus, a person with brown hair may have two alleles for brown hair, but also could have one for brown hair and one for blond. The brown allele is expressed, and the blond allele is masked or hidden. If both parents carry the gene for blond hair, then two brown-haired parents can have a blond child. Because blond is recessive, it cannot “mask” brown. Barring random genetic mutations, two blond parents cannot have a brown-hared child.
The color phenotype of an animal is determined by the genotype of that animal. Colors are produced by the actions of specific proteins, including enzymes. Let's briefly discuss how such coat colors come about.
In all mammals, the pigments in hair and skin are made of the protein melanin. This protein is produced from tyrosine, an amino acid present in the food the animal eats. In the skin, small “pigment factories” called melanocytes turn the tyrosine into melanin, using an enzyme called tyrosinase as part of the process.
Melanin comes in two types. Phaeomelanin (sometimes spelled pheomelanin) is responsible for tan, cream, yellow, red and reddish-brown colors. Eumelanin is responsible for black, blue-gray and chocolate brown colors. Various genes control how much of each type of pigment is made and where it is deposited. White areas on an animal are areas where no melanin is being produced. There aren't always just two possible alleles of a gene. Sometimes there are many. This is the case with the main color loci in goats.
The following table outlines the important color genes in mammals, and what is known about their effects in the goat.
Sources of Goat Color Patterns
Before continuing, it's important to note that there are actually two very important components of the overall color of a goat. The first is the color pattern produced by the Agouti locus, discussed below, possibly modified by B, E or other modifier genes as discussed below. These color patterns include solid red, black, chocolate and tan, plus the familiar patterns such as buckskin and chamoisee. The second factor is spotting and other color modifications such as moon spots and roaning that cover the original Agouti locus pattern. Genetically, an animal that is buckskin with so much white spotting that it's difficult to determine the animal's pattern is every bit as much a buckskin as the animal with no white spotting at all. Spotting can be thought of as "paint" that covers the Agouti locus patterns.
Because of this, it's important to keep in mind that there are actually two ways to produce a goat that appears to be white. The first is goats that are genetically white, as discussed under the Agouti locus. These goats are often, especially when their hair grows out, noted to be a very light cream. The second way of producing a white goat is an animal that is so extensively spotted as to appear entirely white. It must be remembered that if an animal that appears all-white has only a few spots of color, that animal is not genetically white, but instead has an Agouti locus pattern other than white, masked by all the spotting. Even genetically white goats can be spotted!
The Agouti locus
The most important gene in mammalian coat color genetics is A, the agouti gene (named after the agouti color pattern in the mouse.) There are at least 14 different alleles of the A gene in the goat, and probably more. This gene controls the patterns of deposition of phaeomelanin and eumelanin in the coat. It produces all of the common color patterns that we recognize, such as chamoisee, buckskin, cou clair, solid white, solid black and more.
In the “Agouti series” in the goat, the most dominant allele produces a goat with only phaeomelanin pigment and no eumelanin. Such a goat, depending on the rest of its genetic makeup, is entirely white, red, yellow, tan or cream (with or without white spots- spotting is determined by different genes). The most recessive allele produces a goat with only eumelanin pigment and no phaeomelanin. Such a goat, depending on the rest of its genetic makeup, is entirely black or chocolate brown (with or without white spots). The alleles in between produce all of the other color patterns. In general, patterns with more tan are dominant over patterns with less tan.
Patterns at the Agouti locus, in general order of dominance (from Sponenberg, 1998, all pictures courtesy of Phil Sponenberg)
*I have not yet found examples of these color patterns in the Nigerian. If you have examples of goats displaying any of these patterns, please email me photos!
Interactions between Agouti locus patterns
A goat with genes for two patterns may display both of them, or the pattern with less tan may be masked by the pattern with more tan. Dr. Phil Sponenberg, an expert on mammalian color genetics, wrote, “The pattern of dominance at the Agouti locus is that all phaeomelanic (tan) areas are expressed. When a goat has alleles for two different patterns, each is demonstrated in the final color as the tan areas of both patterns. The patterns are superimposed, with all the tan areas being expressed.” It can be very difficult to determine the Agouti alleles present in such an animal.
The following is a table showing possible interactions between four of the known Agouti locus color patterns in Nigerians- buckskin, chamoisee, Swiss marked and peacock (cou clair.) In all cases, all of the tan areas of the patterns are expressed.
The Brown locus
The B, or brown gene affects whether eumelanic (dark-pigmented) areas of the animal are black or chocolate-brown. It does not affect phaeomelanic (tan or white) areas. There are four known alleles at the brown locus in goats. Browns produced by the B locus differ from the tans produced at the Agouti locus in that the brown coloration is found only in areas of the coat that would otherwise be black, and that the brown is chocolate-brown, not yellow or red-brown. Recessive red is an exception to this rule but is extremely rare. Almost all red goats are red due to alleles at the Agouti locus or to unknown modifier genes, not due to the Brown locus. Both chocolate shades are dominant over wild-type black. These colors are fairly rare but not unknown in Nigerians.
Patterns at the Brown locus, in general order of dominance
Mechanisms for Creating Solid-Colored Goats
Not all solid-colored goats receive their pattern from being genetically white/tan/red, or genetically brown/black. Other factors may come into play.
As one example, goats that appear solid brown or red in color may instead be Agouti patterned.
This goat is actually a chocolate buckskin. While she has no buckskin patterning visible in this picture, she was obviously buckskin-colored when young. The pattern became difficult to see as she aged, and is practically invisible when she is clipped. If a goat shows an Agouti locus pattern when young, it does carry that pattern, even if it is difficult to see as the goat gets older and invisible when the goat is clipped. Patterning is often much more obvious on an unclipped goat.
The mechanism for this solid coloring is intensification of the phaeomelanic (tan) areas to dark red along with brown dilution of the black areas. In these cases, the colored areas can be so similar as to obscure the patterns.
This type of patterning can be verified by breeding the animal to a black goat. If normally-patterned buckskins are produced, the animal must carry buckskin.
An additional solid pattern is seen where extensive white spotting covers any Agouti locus pattern. If a seemingly-white goat has even one tiny area of brown, red or black pigment, that goat is in reality not white at all. Extensive spotting is probably the most common mechanism of producing goats that look very white, as opposed to cream-tinged goats. Many cream goats may appear very white when clipped, however.
Examples of the Interaction between Agouti and Brown Patterns
The pictures below all exhibit a buckskin-patterned goat, with different intensities of base coloration.
All of these animals are buckskins with at least one copy of the A sc allele at the Agouti locus. The depth of color of the phaeomelanic tan areas is controlled by poorly-understood modifier genes separate from the color patterning produced by the Agouti locus. The Agouti locus produces the pattern of phaeomelanic (light) and eumelanic (dark) areas on the goat; the modifier genes determine how intensely pigmented those areas are. As we can see in these examples, the tan areas can range from nearly white to a rich red. The color of the tan areas does not affect the color of the eumelanic black areas, as the coloration of the black areas is controlled by the Brown gene discussed above.
This animal is a light tan with black markings.
This animal is also light tan, but the black markings are replaced by deep chocolate. This is due to the action of the Brown gene B d mentioned above, which turns all black areas of the coat to dark chocolate brown. Note that again the tan areas of the coat are unaffected.
This animal is a darker tan with black markings.
This animal is a darker tan with lighter chocolate brown markings produced by the B l gene. Again, only the black coat areas are affected by this gene.
This animal has phaeomelanic areas that are a rich red, with black markings.
In this animal, the eumelanic areas are chocolate brown.
This is an example of how extensive white spotting can mask Agouti locus patterns. This is the same animal as the first animal in this chart, with extensive white spotting. It is genetically just as much a buckskin as the first animal, as the white spotting is controlled by completely separate genes.
The Extension locus
The Extension locus, while very important in coat color in many other mammals, is of relatively little importance in the goat. Almost all goats are “wild-type” at the Extension locus. As mentioned in the chart of color genes in the goat, E (extension) can affects the expression of both types of pigment, phaeomelanin and eumelanin. . This gene is probably unimportant in the Nigerian.
Possible effects of the Extension locus in the goat, in order of dominance
Spotting and other markings
White spotting is produced by entirely different genetic mechanisms than color patterns. In addition, there are a large number of poorly-defined genes responsible for spotting, and little is known with certainty about their modes of inheritance. A goat can possess several different spotting patterns, and a goat of any color or pattern can display spotting (as mentioned above, even a white goat can be spotted!).
Most Nigerians show at least one spotting pattern. Even goats that appear solid-colored may still be spotted- if a goat has even a few white hairs somewhere in its coat, it's spotted. Solid-colored goats are relatively rare in the breed.
Spotting also acts to mask a goat's apparent color pattern. It can sometimes be difficult to discern what the goat's pattern would be without the spots!
Spotting genes postulated in the goat
*I have not yet found examples of these color patterns in the Nigerian. If you have examples of goats displaying any of these patterns, please email me photos!
Blue Eyes in the Nigerian Dwarf
Unlike the case in humans, blue eyes in the goat are dominant. Nigerians aren't the only blue-eyed breed; blue eyes are also reported in the Angora and fainting goat.
Because the blue eyes are dominant, a blue-eyed goat bred to a brown-eyed goat can produce blue-eyed kids. Two brown-eyed goats cannot produce any blue-eyed kids. Let's look more closely at the genetic possibilities for eye color inheritance in goats.
Let's call the gene for eye color Bl. Bl is dominant and codes for blue eye color. bl is recessive and codes for brown eye color. A goat could then have one of three possible genotypes:
Bl Bl- blue eyed
Bl bl- Blue-eyed and carrying the gene for brown eyes
bl bl– brown-eyed
Here are the possible matings that could take place, and the percentage of blue and brown-eyed kids expected from these matings:
One possibility that should be noted is that some goats have eyes in which only part of the iris is blue (this also occurs in dogs.) Such animals are genetically blue-eyed and can have offspring with completely blue eyes.
An Application Example
I bred a buck, Goodwood KW Frasier, to a doe, Covenant Kids Lady Patience. Frasier, like many Goodwood animals, is a black and tan buckskin with white belt and random white spotting. He also has frosting. Molly is black and white belted with random white spotting, ticking and frosting and some small spots of roaning.
Frasier's sire was Gay-Mor's RA Kingwood, a dark red buck with minimal white. His ancestors were buckskin and red and white. One, Goodwood Zippy Mariri, was mahogany in color and may have carried the A m allele. I have been unable to find a clear enough picture of her to confirm if she was a true mahogany (red and black hairs intermixed) or just a very dark red.
His dam was Goodwood Calliope, a buckskin with only buckskins and black animals close up in the pedigree.
Frasier thus has at least one allele at the Agouti locus for the buckskin pattern, A sc ., since he is a buckskin. This could have come from either parent. What is his other allele? It isn't for the dark red pattern of his father, as that dark red pattern is the dominant A wt . If Frasier had inherited it, he would be solid red (or tan) and the buckskin pattern wouldn't show. The other likely possibilities are that it is buckskin (from either parent), black (from the maternal side) and there is an off chance that it could be mahogany (from the maternal side). It is also likely that he carries some modifier genes to produce the dark red color. He has minimal tan areas, but those he does have are fairly bright in color.
Molly is much simpler- she's black, so both copies of her Agouti allele are A a.
Both animals are spotted with random white, thus both should carry two copies of the recessive white spotting gene. Both animals are also belted and frosted, so carry at least one copy each of these dominant genes.
So what do I expect in their offspring, and what will this tell me?
First of all, all of the kids should have some white spotting, and it's likely that they will be belted and frosted as well. This is because both parents carry two copies of the recessive spotting gene, so every kid will inherit two copies of that gene as well. Both parents carry at least one gene for frosting and for belting, and both genes are assumed to be dominant, so they will show up even if only one copy gets passed on. The kids may be ticked as well, but ticking often only shows with age, so we may not notice it in the kids.
If I get any black and white kids, then Frasier's second Agouti allele is for black. Black is all Molly carries, so it's all she can pass on. Since black is recessive, a black kid would need two copies of the allele to be black, and that second copy would have to come from Frasier.
I should get around 50% buckskin kids, since any kid that inherits Frasier's buckskin allele will be buckskin, as it's dominant over black. If I get any pattern other than buckskin, then that pattern is what's on Frasier's second Agouti allele.
So what were the results?
In two matings, we got three black and white kids and one buckskin. All kids had a white star. Two had white belting. All had some degree of frosting. One had roaning. So we now know that Frasier caries the A a Agouti locus allele for black.
The buckskin kid is interesting because of her dark red coloration, somewhat similar to her paternal grandfather. As mentioned earlier, it is possible that modifier genes are responsible for the intensity of the red coloration.
What are the practical implications of all of this?
• First of all, black animals bred to black animals will produce only black animals (with the extremely rare exception of .breeding two goats carrying recessive red on the B or E locus)
• Chocolate animals bred to chocolate animals will produce only chocolate and possibly black animals (with the extremely rare exception of .breeding two goats carrying recessive red on the B or E locus)
• Randomly spotted animals bred to randomly spotted animals will produce only spotted offspring.
• Animals that are solid white, tan or red can carry but not express other patterns on the Agouti locus. If bred to a black or chocolate animal, these patterns may be expressed in the offspring.
• Animals that are largely tan may be masking an Agouti locus pattern with less tan.
• Any non-black animal bred to a black animal who produces black offspring carries the A a (black) Agouti allele.
• The best way to determine if an animal carries but does not express a particular pattern is to breed that animal to a black (preferably solid black) animal.