werner@aecom.YU.EDU (Craig Werner) (06/28/88)
Hair color is best understood in agouti animals. Mice are completely
agouti. The wild type mouse appears grey, but if one looks at it carefully,
one notices that each individual hair is striped. It is black at
the top, yellow (blond) for the mid-shaft, and black at the base. Cats
are a little bit conceptually easier. A striped wild-type cat, alternates
black (non-agouti) hairs with lighter agouti hairs, and one can see the
yellow bands on each individual hair if one examines it closely.
Alas, there are no agouti humans. While it is possible to observe
persons with striped hair, one can rest assured that the basis of this
phenotype is not genetic.
Occasionally, one will catch a mouse that is not grey, but rather
a dirty brown. In this case, the black pigment is replaced by brown.
Mice can be bred so they are nonagouti. In this case, they are either black
or brown. There are several strains of mice who are non-agouti Yellow.
The black pigment is called Eumelanin. Tyrosine is beta-hydroxylated
to dopaquinone, followed by complexation with proteins, and packaging into
granules within organelles called melanosomes. These are exported from
melanocytes in the keratinocytes that form the growing hair shaft.
The yellow pigment is called pheomelanin. It also originates from tyrosine,
but somehow during complexation and polymerization, ends up yellow instead
of black.
Brown hair is not simply less pigment. There may in fact be slightly
more. What differs between black and brown mice is the granularity of the
melanin packing within melanosomes. Brown granules are much finer, and the
net visual effect is that the melanin appears brown, not black.
It is rather a sore point, personally speaking, that there is no
such thing as a red-haired mouse. So called "Reds" of the mouse fancy
are really genetically brown Yellow mice, who posess some "umbrous" mutation
that allows some diffuse brown pigment to be expressed on the dorsum. THe
brown on yellow might look red, but I've never seen one. "Reds" may
in fact, be extinct.
However, red pigment, which begins as pheomelanin (which is why
red is recessive to brunette or black in most cases), undergoes complexation
with cysteine, whose sulphur groups are then available to bind iron. The
resulting bound iron is what gives red hair its distinctive "rusty" color.
--
Craig Werner (future MD/PhD, 4 years down, 3 to go)
werner@aecom.YU.EDU -- Albert Einstein College of Medicine
(1935-14E Eastchester Rd., Bronx NY 10461, 212-931-2517)
"That's not a philosophy, that's a bumper sticker."