The study of the genetics of canine coat color predates molecular genetics (using DNA to understand genetics). Not everything is known about coat color, so this is still an active area of scientific research. Here I present my understanding of the molecular genetics of Norwich terrier coat color. My understanding is based upon my reading of the literature on the subject, my own observations, and DNA testing of 117 Norwich terriers. This is still a topic about which I am learning; if anyone has more information on the subject I would be happy to hear about it.
The genes that control canine coat color were historically called the A-Locus, B-Locus, D-Locus, E-Locus, K-Locus, etc. If you do a DNA coat color panel test on your dog, the results will be reported using these historical names.
For Norwich terriers the only ones of interest are the A-Locus, the E-Locus, and the D-Locus. For all the others, molecular genetics has shown that they are "fixed" in Norwich ... meaning that their values are all uniformly the same for all Norwich; thus they do not affect the different coat colors we see in Norwich terriers.
On the Norwich that I have DNA tested, the distribution is
ay/ay = 19 percent (red)
ay/at = 47 percent (red or grizzle)
at/at = 34 percent (black-and-tan)
The individual allele distribution is
ay = 42 percent
at = 58 percent
On the Norwich that I have DNA tested, the distribution of the E-Locus is
Em = 39 percent
E = 57 percent
e = 4 percent
Of the Norwich that I have DNA tested, the distribution of the D-Locus is
D = 95 percent
d = 5 percent