The coat color of an animal is determined by the pigments produced in the skin, hair, and fur. The production of pigments is controlled by genes that are inherited from the animal’s parents. Understanding how coat color is passed on from generation to generation has long fascinated biologists and breeders alike.
The Basics of Coat Color Genetics
There are two types of pigments that contribute to coat color in mammals:
- Eumelanin – Brown and black pigments
- Pheomelanin – Red and yellow pigments
The relative amounts of these two pigments produce the variety of natural coat colors we see. The genetics of coat color are complex, with multiple genes interacting to produce different pigmentation patterns. However, there are some basic principles that apply:
- Most coat color genes have a locus (position) on a chromosome.
- Alleles are variant forms of a gene that occupy the same locus.
- Some alleles are dominant and others are recessive.
- Offspring inherit one allele from each parent.
- The combination of alleles determines the coat color.
Let’s look at some examples of specific coat color genes to understand how they are inherited.
The Agouti Gene
One of the key genes influencing coat color is the Agouti gene, which controls the distribution of black pigment. The dominant allele (A) results in a variegated coat color pattern, while the recessive allele (a) produces a solid coat color.
There are three possible genotype combinations for the Agouti gene:
| Genotype | Phenotype |
|---|---|
| AA or Aa | Variegated coat |
| aa | Solid coat |
When a variegated coat color animal is bred with a solid coat color animal, the offspring will have variegated coats. This is because the dominant A allele from the variegated parent masks the expression of the recessive a allele from the solid parent. To produce solid colored offspring, both parents must have aa genotypes.
The Extension Gene
The Extension or E gene controls the production of black eumelanin pigment. The dominant E allele allows full expression of black pigment, while the recessive e allele limits black pigment production. TheExtension gene interacts with the Agouti gene to determine whether black pigment is distributed in a solid or variegated pattern.
There are two possible genotypes:
| Genotype | Phenotype |
|---|---|
| EE or Ee | Allows full black pigment production |
| ee | Limits black pigment production |
An animal with at least one E allele will be able to produce black hair, while those with two recessive e alleles will produce red/yellow coats. The ee genotype is necessary for true red coat colors.
The Agouti and Extension Genes in Dogs
Combinations of the Agouti and Extension genes produce four basic coat color patterns in dogs:
| Genotype | Coat Color |
|---|---|
| A_ E_ | Black/tan/saddle pattern |
| A_ ee | Clear red or yellow |
| aa E_ | Solid black |
| aa ee | Clear red/yellow solid |
Where the underscore represents any allele (A, a or E, e). Other genes further modify these basic patterns to produce more coat color variations like brindle, piebald, and spotted.
The Agouti Gene in Cats
In cats, the Agouti gene produces tabby coat patterns as well as red/yellow pigment. The dominant A allele produces variegated tabby patterns while the recessive a allele produces solid coat colors. Interaction with the orange gene determines whether the coat color is black/grey or red/yellow. The Extension gene is not operative in cats.
The possible Agouti genotypes and phenotypes in cats are:
| Genotype | Phenotype |
|---|---|
| AA or Aa | Tabby coat pattern |
| aa | Solid coat color |
A solid colored male cat bred with a tabby female will produce tabby kittens, since the dominant A allele from the mother controls the Agouti expression.
The Albino Series in Rabbits
In rabbits, a series of alleles at the C locus controls the production of pigment. The dominant C allele allows full color. The recessive cch allele produces the Himalayan pattern, while the recessive c allele produces albinism. There are four possible genotypes:
| Genotype | Appearance |
|---|---|
| CC or Ccch | Normal pigment |
| cchcch | Himalayan pattern |
| Cc | Normal pigment |
| cc | Albino |
The albino and Himalayan patterns require two recessive alleles to be expressed. Crosses between normally pigmented and albino or Himalayan rabbits will produce some normally pigmented offspring carrying the recessive allele.
The Spotting Gene in Cattle
Spotting patterns in cattle such as piebald and color-sided are controlled by the Spotting or S gene:
| Genotype | Appearance |
|---|---|
| SS or Ss | Solid colored |
| ss | Spotted |
The dominant S allele produces solid coat color while the recessive s allele causes spotting. Only homozygous recessive cattle (ss) will display spotting patterns. Crosses between spotted and solid cattle will produce some solid offspring carrying the s allele.
Interactions Between Genes
In addition to simple dominance/recessive inheritance, there are often complex interactions between multiple genes that control coat color. This includes:
- Epistasis – One gene masking the expression of another gene.
- Complementary genes – Two genes that work together to produce a phenotype.
- Polygenes – Multiple genes each contributing small effects to a phenotype.
For example, in dogs the K locus determines whether pigment can be expressed (domiant KB allele) or not at all (recessive ky allele). This epistatic interaction can prevent other coat color genes from being expressed.
In horses, the Agouti and Extension genes work together complementarily to produce bay, black, or chestnut coats depending on the combination of alleles.
Subtle variations in coat shades may be the result of modifier polygenes that enhance or suppress the effects of major genes.
Random Inheritance
Each parent passes on one of their two alleles randomly to their offspring. This results in diverse genetic combinations and a range of coat colors within a litter or generation. Some key principles of inheritance are:
- The offspring of parental genotypes AABB could be AABB, AABb, AaBB, or AaBb.
- Each allele has a 50% chance of being passed on.
- Recessive alleles (like b) may be hidden for generations unless paired with another recessive.
- The phenotype ratio often fits simple Mendelian ratios (e.g. 3 dominant : 1 recessive).
But chance variations can lead to coat colors popping up unexpectedly in litters. Breeders often analyze pedigrees to identify carriers of hidden recessive alleles that may produce surprises.
Breeding for Coat Color
Animal breeders select mating pairs carefully to produce desired coat colors and patterns in the offspring. To breed for recessive traits like solid coats or spotting, both parents need to carry the hidden allele either in homozygous (aa, ss) or heterozygous (Aa, Ss) form. Test breeding helps determine an animal’s genotype based on the coat colors produced.
Breeding colored variants can be controversial, however, if health or behavior issues are associated with certain genes. For example, the merle patterning in dogs is linked to deafness and vision problems. Responsible breeding balances coat color with health, function, and temperament.
The Future of Coat Color Genetics
Gene mapping and genomic studies continue to uncover new loci and alleles influencing coat color in different species. For example, the dun coat color in horses traces to the Dun gene, while tabby patterns in cats involve the Taqpep gene. A complex signaling network of genes affects pigment cell development and migration during embryogenesis.
Understanding the genetic basis of coat color variations has many applications beyond animal breeding. It provides insights into developmental biology, phenotypic diversity, domestication, and even human skin and hair pigmentation. From distinguishing species to producing designer pets, coat color genetics remains a fascinating and fast-moving field of study after centuries of work.
Conclusion
In summary, coat color is determined by the complex interactions between multiple genes encoding pigment production and patterning. While basic loci like Agouti, Extension, Spotting, and Albino series control the distribution of black, brown, red, and yellow pigments, many other genes modulate these effects. Inheritance follows Mendelian principles but the random assortment of alleles can produce unexpected phenotypes. Understanding the key coat color genes in different species allows animal breeders to select for desired aesthetic traits.
Genetic analysis continues to uncover new alleles and molecular pathways controlling mammalian pigmentation. The future promises even greater knowledge of how genetics determines the spectacular diversity of coats, manes, furs, and hairs that have evolved in the animal kingdom.
