In the Jan/Feb 2002 issue, an article on color inheritance written by the late Carol Garmaker was republished, with the editorial comment that it "remains the simplest explanation we have seen to date".
Simple, yes. Correct, no.
Carol can be forgiven for getting it wrong. Anne Eskrigge had it wrong, and many of us - myself included - had it wrong for decades. The problem stems from the research published by Little in the 1930's on canine color inheritance, in which he placed dominant black on the agouti series (A locus) - along with the genes that create what we know as "salt & pepper" and "black & silver". In Carol's article she rightly wrote that a dog can carry only two alleles at any one locus. If all our colors were on the A locus, the only combination that could produce a three color litter would be that of a black, carrying b/s bred to a salt & pepper, carrying b/s. Carol illustrated this combination in her article.
The problem is that there are a number of blacks who produce 3 color litters when bred to other blacks and to black & silver mates. This should be impossible, and it has led to whispers about mismatings when it was reported. In the early years of the breed there were few incorporating all three colors. The gene for black & silver was rare. And there really wasn't a lot of evidence to be gleaned in other breeds... very few share the colors peculiar to Schnauzers. Today that situation has changed dramatically. I have seen the evidence for myself.
In the mid 1990's I imported the heterozygous black Can.Ch.Sandcastle Goodguyswearblack. "Clint" was sired by a homozygous black, out of a salt & pepper bitch, who carried b/s. At the time he lived here, the dogs at Minuteman were pure for salt & pepper. He sired only 3 litters and two were to homozygous s/p bitches. Then, Irma Wheeler sent a black & silver bitch to him to be bred. Irma has been breeding all three colors for nearly 30 years - she knows how to identify color in her puppies.
The resulting litter contained 2 blacks, 2 salt & pepper and a black & silver. Theoretically impossible, but - there it was. And there was no black and silver carrier here to "blame". So, how?
All canine color combinations are the result of the production of melanin. Mammals have two forms - emuelanin and phaeomelanin. Very simply, the first produces black pigment, and the second produces tan. The color rainbows we see among purebred dogs are the result of a host of genes that alter the shape and proteins of these base pigments, diluting them (black to brown or blue) or washing them out (turning tan markings to silver) or preventing their production entirely (white spotting or merled areas). Some genes alter both types of pigment, some alter only black or only tan. Some affect only coat - others change nose and skin color as well. So, while all genes are arranged in pairs - there are many pairs involved.
When genes are paired with others from the same series or "locus", the relationships are termed "dominant" and "recessive", depending upon which gene suppresses (in whole or part) the effects of the other.
But, when a gene or gene pair at one locus overpowers those at another, the relationship is termed "epistatic".
Little identifed several series in dogs: The A series, mentioned at the outset, the B (brown) series that converts black pigment to brown, the C (chinchilla) series, which removes tan pigment, but leaves black unaltered (MS are probably pure for chinchilla or we would see occassional true black and tans), D dilution (present at birth, as in blue dobes), E extension (where the recessive gene for the white schnauzer is probably located), G greying (probably responsible for greying of blacks), S spotting, which controls particolor patterns, M merling, and a few others.
The A series includes (in diminishing order of dominance) ay - sable, aw - agouti, at - bicolor. (As mentioned earlier, Little placed dominant black at the top of this series, which seems unlikely.) There is no evidence that sable exists in Mini Schnauzer show lines. Agouti produces banded hairs and may include our salt & pepper minis - but I'm not completely convinced. Salt & pepper is a bicolor pattern that is not present on classic agouti breeds. Black & silver is a bicolor with tan pigment washed out by the chinchilla gene.
There may be an intermediate agouti gene, or an independant, unidentified locus that produces banding or that removes tan points... whatever the case, it is a side issue - for the purpose of this discussion, I'll consider them to both be located on the A series. (A cross between a s/p Schnauzer and Keeshond would provide some answers, if anyone is interested in a little experimentation...)
But of importance to Mini Schnauzers - some modern researchers have suggested that there exists a separate locus for dominant black, and use the letter K to indicate it. (Dominant black is not on the A locus in other mammals studied, adding further weight to the argument.)
If true, a heterozygous black sire will be "Kk" at the K locus. If he carries both salt & pepper and black & silver he will also be "awat" at the A locus. To draw a punnit square to illustrate this, you must use the genes available at both loci in all their possible combinations. It is not as confusing as it looks at first glance!
(To simplify the charts, I'll use "s" to indicate the s/p gene, and "b" for b/s.)
K - dominant black
k - non black
s - salt & pepper
b - black & silver
The process of cell division that creates sperm or egg contributes one gene from every locus. My heterozygous black, "Clint" would have been Kksb - the combinations possible for his sperm cells were Ks, Kb, ks, kb
Bred to a black & silver kbkb, her eggs can be only kb:
Ks Kb ks kb
_________________________
kb Kksb Kkbb kksb kkbb
25% heterozygous black, carrying both Bred to a salt & pepper carrier - kksb, her eggs can carry either ks or kb:
Ks Kb ks kb
___________________________
ks Kkss Kksb kkss kksb
kb Kksb Kkbb kksb kkbb
25% heterozygous black, carrying both To another black of identical makeup:
Ks Kb ks kb
___________________________
Ks KKss KKsb Kkss Kksb
Kb KKsb KKbb Kksb Kkbb
ks Kkss Kksb kkss kksb
kb Kksb Kkbb kksb kkbb
25% heterozygous black, carrying both So, in order to work out what color combinations are possible, it is important to know what the makeup of your black dog or bitch truly is - sometimes that will be clear by pedigree analysis, sometimes one will have to wait until you see what you get!
But clearly, it is not as simple as previously thought.
Catherine McMillan
For an excellent reference on the internet about the latest thought in canine color genetics visit: http://bowlingsite.mcf.com/Genetics/ColorGen.html