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As if penetrance and expressivity didn't add enough variability to the equation, it's important to realize that there may be diseases, phenes, and traits caused by the effects of more than one gene. Some loci have a variety of different alleles that could participate in a gene pair. Nowhere is this more apparent than in coat coloration (Table 3.3.1). Epistasis is when the action of one gene depends on the action of another gene. Epistasis is wonderfully illustrated by the coat colors possible with different combinations of alleles from different loci. All breeds have all the loci mentioned, but they do not necessarily have all the possible alleles mentioned. Even within a given locus, some alleles are dominant over others.

Table 3.3.1 The genetics of coat color

Locus and allele	Effect
Agouti
A	Solid color
Ay	Fawn/sable
Aw	Gray/wolf
as	Saddle
at	Bicolor (tan points)
a	Recessive black
Black
B	Black
b	Liver
Color
C	Color factor
CC	Full color
cch	Chinchilla
ce	Extreme dilution
cd	White with dark eyes
cb	Blue eyes
c	Albinism
Dilution
D	No dilution
d	Dilution (e.g., blue Doberman)
Extension
Em	Black mask
E	Normal extension
e	Nonextension (yellow)
Graying
G	Born black, turns blue
g	Born black, stays black
Intensity
INT	Lightest tan
intm	Intermediate tan
int	Darkest tan
Solid
K	Solid color in pigmented areas
Br	Brindle
Y	Allows yellow pigment to show
Merle
M	Merle
m	Nonmerle
Spotting
S	Solid color
si	Irish spotting
sp	Piebald
sw	Extreme white piebald
S‐extension
Se	Black mask
sese	No black mask
Ticking
T	Ticking
t	Nonticking