MadSci Network: Genetics |
Hi Abby,
It is "common knowledge" that redheads have hot tempers,
but I
haven't ever seen a controlled study that tested the
correlation.
So while I personally like to use that excuse, it probably
isn't a
valid one. : )
However, there are a few things that are clearly associated
with
red hair. Redheads are generally pale, freckle easily, and
tan
poorly. Like anyone with fair skin, we are likely to
sunburn
without sunscreen, and we're at increased risk for the
eventual
development of skin cancer.
There are a few strange syndromes associated with red hair,
but
they are very rare. One of them is the
Brittle Cornea Syndrome, which is seen very
infrequently in
Tunisian Jews. It is inherited in an autosomal recessive pattern
(you need two mutant copies of the gene to be affected) and
includes several apparently unrelated traits such as
brittle
corneas, blue sclera (the whites of the eyes are bluish), and
hyperextensible joints. In some families the affected
individuals
always have red hair!
All of this should make you think about the genetics of
hair color,
which is a complex and not well understood subject.
However, a few
things are known about the
genetics of red hair. One genetics textbook I read
explained
it this way: a person with red hair has genes for light
colored
hair (blond), and also possesses separate genes specifying
red
hair. This combination makes red hair -- redheads are
blonds in
disguise! Someone with dark hair might also have the red
hair
genes, but their dark hair genes overrule the red genes and
so they
don't have red hair. This would also explain why
dark-haired
parents can sometimes have redhead children.
Nothing else I have read directly contradicts this model,
but it
looks like it is more complex than that. In 1987, a Danish
group's
research suggested that there is a gene on Chromosome 4
that is
involved in red hair color. More recently, a group at the
University of Newcastle upon Tyne, UK, found that
variations in a
specific gene that encodes a hormone receptor are often
associated
with red hair.
The gene that they are looking at is the MC1R gene located
on
chromosome 8. They found that redheads often have
mutations in
this gene. The MC1R gene is involved in telling certain
cells to
make a brown-black type of melanin called eumelanin. The
lack of
this gene would result in decreased production of
eumelanin.
People with a mutation in the MC1R gene would instead make
another
type of melanin called pheomelanin. Pheomelanin is a
red-brown
pigment and provides less UV protection than eumelanin
does.
This all starts to make sense if you examine the hair of
redheads.
Unlike brown or black hair, the pigment of red hair is
almost all
pheomelanin, with very little eumelanin! It can't possibly
be that
simple though, because there are plenty of redheads who
don't have
mutations in the MC1R gene. Other genes, like the unknown
gene on
chromosome 4, must also play roles in determining red hair.
Hope this information was helpful!
:)Carolyn
Selected references:
Barsh, GS. The genetics of pigmentation: from fancy genes
to
complex traits. Trends in Genetics. 8, 299-305 (1996).
Eilberg, H and Mohr, J. Major locus for red hair color
linked to
MNS blood groups on chromosome 4. Clinical Genetics. 32,
125-128
(1987).
Valverde, P et al. Variants of the melanocyte-stimulating
hormone
receptor gene are associated with red hair and fair skin in
humans.
Nature Genetics. 11, 328-330 (1995).
Try the links in the MadSci Library for more information on Genetics.