She gets SO much mail, she now warns people she'll respond to very few so I feel fortunate to get her reply. And I wanted to share it (although you may find out more than you wanted to know; it'll take me several readings of her response plus putting it together with her longer article on Lab genetics on her website).
Thanks for writing. With regard to each of your questions:
>Does the Dudley/pink nose only occur when the
>chocolate gene (b) occurs in a double dose?
No. Dudley/pink is a condition in which there is complete absence of
pigment in the skin and should not be confused with fading of pigment
(which I describe below). Dudley/pink is an extremely rare condition
in Labs and would occur due to presence of a homozygous (i.e.
"double") recessive allele of the "C" locus [most likely c(d)]. You
would see this condition occurring in a Lab whose coat color is
yellow, but the presence of a homozygous c(d) would cause the yellow
coat to appear as a "true" white. This allele would have no impact
on black or chocolate/liver expression because alleles of the C locus
only affect phaeomelanin (red/yellow pigment) which is produced by
the recessive "e" of the E locus (yellows are homozygous "e" [i.e.
"ee"] at the E locus.
>And the liver/brown colored nose "snow nose"
>only when the chocolate gene (b) occurs in
>a single dose along with a gene for Black?
>And the black nose, black "eyeliner" occurs
>only when the Black gene occurs in a double
It's a bit more complex than this, and it is important to understand
that "snow nose" and the liver/brown colored nose are not the same.
First, the B locus alleles will determine whether a Lab has black or
liver (aka chocolate) pigment. To have chocolate pigment, a Lab must
be homozygous recessive "b" at the B locus (i.e. "bb"). The Lab will
have black pigment if it is homozygous or heterozygous for "B" (i.e.
"BB" or "Bb"). The liver/brown nose, however, is different than what
is referred to as "snow-nose". Liver/brown (aka chocolate) is
dilution of black at the melanosome level and occurs when the dog is
homozygous (double) recessive "b" at the black locus. A yellow lab
can have a liver/brown nose (as well as eye rims) if it is homozygous
"b" at the B locus...but this is not "snow nose" because it is not
due to fading and such a yellow Lab will always be liver/brown
pigmented and will never show black pigmentation. "Snow-nose" is a
different condition entirely in which a yellow dog can produce black
pigment at one time or another, but this pigment is susceptible to
fading based on environmental conditions. Fading occurs only in
yellow Labs (and just to make matters more complicated, fading of the
liver/brown pigment in a "bb" yellow Lab can also occur) . Coat
color and pigment is first and foremost determined by alleles at the
E Locus. Blacks and chocolates both have the dominant "E"
allele. The receptor of the E locus that produces yellow in the fur
of a yellow lab (homozygous "e") is a recessive mutant that can only
produce red pigment (phaeomelanin, which is diluted to yellow by the
alleles of the C locus) in the fur. The same receptor in the skin of
that same dog can produce black (phaeomelanin is not produced in
skin) but is dependent upon the alleles of the C Locus, which produce
tyrosinase, an enzyme needed by the mutant receptor to produce black
color. Fading does not occur in black Labs or chocolate (i.e.
brown/liver) Labs because the eumelanin (black) pigment produced by
the E locus receptor in black or chocolates (encoded by the dominant
"E" allele) is not dependent on tyrosinase encoded by the C
locus. Only the pigment in yellows fades because production of
tyrosinase is influenced by environmental and some physiological
variables. For instance, in cold temperatures tyrosinase is less
stable and without tyrosinase, the recessive "e" receptors stop
producing the black pigment. In the case of the Dudley/pink nose
referred to in the first question, the c(d) allele does not produce
tyrosinase at all, so the mutant receptor encoded by the "ee" alleles
of the yellow dog cannot produce any pigment.
I hope this answers some of your questions. Please feel free to
contact me if you have additional questions.
Puff [YF, AKC field line (from competing HT/FT breeder) 62 lbs, dob: 8-'01]