Coat color in dogs is an excellent example of how complex interactions between genes may take part in the determination of a phenotype. Domestic dogs come in an amazing variety of shapes, sizes, and colors. For thousands of years, humans have been breeding dogs for particular traits, producing the large number of types that we see today. Each breed of dog carries a selection of genes from the ancestral dog gene pool; these genes define the features of a particular breed.
One of the most obvious differences between dogs is coat color. The genetics of coat color in dogs is quite complex; many genes participate, and there are numerous interactions between genes at different loci. We will consider seven loci (in the list that follows) that are important in producing many of the noticeable differences in color and pattern among breeds of dogs. In interpreting the genetic basis of differences in coat color of dogs, consider how the expression of a particular gene is modified by the effects of other genes. Keep in mind that additional loci not listed here can modify the colors produced by these seven loci and that not all geneticists agree on the genetics of color variation in some breeds.
1. Agouti (A) locus—This locus has five common alleles that determine the depth and distribution of color in a dog's coat:
aw Agouti, or wolflike gray. Hairs encoded by this allele have a "salt and pepper" appearance, produced by a band of yellow pigment on a black hair.
ay Yellow. The black pigment is markedly reduced;
so the entire hair is yellow. as Saddle markings (dark color on the back, with extensive tan markings on the head and legs). at Bicolor (dark color over most of the body, with tan markings on the feet and eyebrows).
As and ay are generally dominant over the other alleles, but the dominance relations are complex and not yet completely understood.
2. Black (B) locus — This locus determines whether black pigment can be formed. The actual color of a dog's fur depends on the effects of genes at other loci (such as the A, C, D, and E loci). Two alleles are common:
B Allows black pigment to be produced; the dog will be black if it also possesses certain alleles at the A, C, D, and E loci. b Black pigment cannot be produced; pigmented dogs can be chocolate, liver, tan, or red. B is dominant over b.
3. Albino (C) locus — This locus determines whether full color will be expressed. There are five alleles at this locus: C Color fully expressed.
cch Chinchilla. Less color is expressed, and pigment is completely absent from the base of the long hairs, producing a pale coat. cd All white coat with dark nose and dark eyes. cb All white coat with blue eyes. c Fully albino. The dogs have an all-white coat with pink eyes and nose.
5.10 Coat color in dogs is determined by interactions between genes at a number of loci. (a) Most Labrador retrievers are genotype AsAsCCDDSStt, varing only at the B and E loci. (b) Most beares are genotype asasBBCCDDspsptt. (c) Dalmations are genotype AsAsCCDDEEswswTT, varing at the B locus so that the dogs are black (B_) or brown (bb). (Part a, Robert Maier/Animals Animals; part b, Ralph Reinhold/Animals Animals; part c, Robert Percy/ Animals Animals.)
The dominance relations among these alleles is presumed to be C > cch > cd > cb > c, but the cch and c alleles are rare, and crosses including all possible genotypes have not been completed.
4. Dilution (D) locus—This locus, with two alleles, determines whether the color will be diluted. For example, diluted black pigment appears bluish, and diluted yellow appears cream. The diluted effect is produced by an uneven distribution of pigment in the hair shaft: D Intense pigmentation. d Dilution of pigment. D is dominant over d.
5. Extension (E) locus — Four alleles at this locus determine where the genotype at the A locus is expressed. For example, if a dog has the As allele (solid black) at the A locus, then black pigment will either be extended throughout the coat or be restricted to some areas, depending on the alleles present at the E locus. Areas where the A locus is not expressed may appear as yellow, red, or tan, depending on the presence of particular genes at other loci. When As is present at the A locus, the four alleles at the E locus have the following effects:
Em Black mask with a tan coat. E The A locus expressed throughout (solid black). ebr Brindle, in which black and yellow are in layers to give a tiger-striped appearance. e No black in the coat, but the nose and eyes may be black.
The dominance relations among these alleles are poorly known.
6. Spotting (S) locus — Alleles at this locus determine whether white spots will be present. There are four common alleles: S No spots.
s Irish spotting; numerous white spots. sp Piebald spotting; various amounts of white. sw Extreme white piebald; almost all white.
S is completely dominant over sL, sP, and sw; s and sp are dominant over sw (S > sL, sp > sw). The relation between of s and sp is poorly defined; indeed, they may not be separate alleles. Genes at other poorly known loci also modify spotting patterns.
7. Ticking (T) locus — This locus determines the presence of small colored spots on the white areas, which is called ticking:
T Ticking; small colored spots on the areas of white. t No ticking.
T is dominant over t. Ticking cannot be expressed if a dog has a solid coat (S_).
To illustrate how genes at these loci interact in determining a dog's coat color, let's consider a few examples:
Labrador retriever- Labrador retrievers ( FIGURE 5.10a) may be black, brown, or yellow. Most are homozygous AsAsCCDDSStt; thus, they vary only at the B and E loci. The As, C, and D alleles allow dark pigment to be expressed; whether a dog is black depends on which genes are present at the B and E loci. As discussed earlier in the chapter, all black Labradors must carry at least one B allele and one E allele (B_E_). Brown dogs are homozygous bb and have at least one E allele (bbE_). Yellow dogs are a result of the presence of ee (B_ee or bbee). Labrador retrievers are homozygous for the S allele, which produces a solid color; the few white spots that appear in some dogs of this breed are due to other modifying genes. The allele for ticking, T is presumed not to exist in Labradors; however, Labrador retrievers have solid coats and ticking is expressed only in spotted dogs; so its absence is uncertain. Beagle- Most beagles are homozygous atas BBCCDD&sPtt, although other alleles at these loci are occasionally present. The as allele produces the saddle markings — dark back and sides, with tan head and legs — that are characteristic of the breed ( FIGURE 5.10b). Alleles B, C, and D allow black to be produced, but its distribution is limited by the as allele. Genotype ee does occasionally arise, leading to a few all-tan beagles. White spotting in beagles is due to the sp allele. Ticking can appear, but most beagles are tt.
Dalmatian- Dalmatians ( FIGURE 5.10c) have an interesting genetic makeup. Most are homozygous ASAS CCDDEEsNswTT; so they vary only at the B locus. Notice that these dogs possess genotype ASASCCDDEE, which allows for a solid coat that would be black, if genotype B_ is present, or brown (called liver), if genotype bb is present. However, the presence of the sw allele produces a white coat, masking the expression of the solid color. The dog's color appears only in the pigmented spots, which are due to the presence of the ticking allele T. Table 5.3 gives the common genotypes of other breeds of dogs.
www.whfreeman.com/pierce Information on dog genetics, including the Dog Genome Project
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