Allele - Two or more forms of the same gene (Example: Black or red)
Codominant - Neither allele dominates the other. If both are present, both are seen. The most common example is the calico cat. Black and orange are codominant in cats. If both alleles are present, the cat has both pigment colors.
Dominant - An allele is dominant if it produces its own effect and hides the presence of another allele. When a dog has a copy of the black gene the dog is black in color. Black is dominant to red, so you can't tell by looking if the dog has a copy of the red gene or not.
Gene - A unit of information on a chromosome
Heterozygous - "Hetero" means "different". A dog that is heterozygous for an allele has two different alleles for that trait. A black dog that is heterozygous for black has one black allele and one red allele (Bb).
Homozygous - "Homo" means "alike". A dog that is homozygous for an allele has two identical copies of it. A red dog is homozygous for red (bb).
Minus Modifier - A modifying gene or collection of modifying genes that acts to reduce pigment from whatever color or pattern the dog already has. A solid colored dog, like a Lab or Doberman, with white on a toe or a small collection of white hairs on the chest has minus modifiers pulling pigment from him even though he has no spotting pattern. Minus modifiers can further reduce pigment on a dog with a spotting pattern to make a collar extend beyond the withers.
Modifier - A gene that does not act on its own but exerts an effect on another gene. Modifying genes affect how much copper is present in a tan pointed dog. If the dog doesn't have tan points (black bi, for example), the effects of any modifiers of the tan point pattern will not be seen even though they're there.
Plus Modifier - A modifying gene or collection of modifying genes that adds pigment to whatever spotting pattern is present. A full Irish spotting pattern (like Lassie's) can be reduced to white feet, a partial collar, and little or no blaze, depending on their number and action.
Polygenic Inheritance - A trait is caused by two or more genes working together. In Aussies, the natural bobtail is polygenic. There are probably several muliple genes involved in determining exact iris color. The inheritance of a trait dependent on multiple genes is harder to predict than one based on a simple dominant/recessive relationship of alleles of the same gene.
Recessive - An allele's effect is not seen if another, more dominant allele is present. Red is recessie to black. For a dog to be red, he must have two copies of the red gene (bb). If he has a copy of red and one of black, the effect of red will not be visible and the dog will be black (Bb).
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