Download 7. Hair color explanations - Ask a Geneticist

Ask a Geneticist
Name_________________________
Hair Color Inheritance
Date__________
by Dr. Barry Starr, Stanford University
1) I have a 4 1/2 month old that has bright red hair. My husband has a dark complexion
and black hair, but is freckled. I am a sandy blonde and a fair complexion and freckled. Our
first son is blonde. But, our second is as white as cotton and red headed. Is this common
when you mix a blonde and black haired? We have no red heads on either side of our
family. I'm tired of getting questioned!
-An elementary school teacher from Alabama
August 26, 2004
I have a kid with red hair too and everyone asks, "Does red
hair run in your family?" The question is made easier by the
fact that my wife and I do have relatives with red hair and you
can see red tints in both our hair. I can imagine how frustrating
the questions would be without having an easy answer...
The quick answer is that it is very possible (obviously!) to get a
redhead from blonde and black haired parents. I'll go into the
details below but next time you get a question about this,
maybe you could answer:
"This is actually pretty common. Because my husband and I are both freckled, we probably are
carriers for red hair. We had a 1 in 4 chance of having a redhead and we couldn't be happier to have
beaten the odds!"
Hair color in general is really complicated and poorly understood with the exception of red hair. Many
cases of red hair can happen by the simple dominant-recessive model I'll discuss below.
Before getting into the specific genetics involved, let's first talk about hair color in general. Hair color
is determined by the amount of two pigments called eumelanin and pheomelanin that are in your hair.
The amount of eumelanin in your hair gives you a range from blonde to black—a little eumelanin and
you are blonde, an intermediate amount, brown, and a lot, black.
Red comes into the equation with pheomelanin. The more pheomelanin in your hair, the redder it is.
OK, then, hair color is a mixture of how much total eumelanin and pheomelanin is in your hair. This is
sort of like mixing paints. For example, you start out with white, colorless hair. Strawberry blonde is
a little of each, auburn is medium amount of eumelanin and pheomelanin and a redhead is very little
eumelanin and lots of pheomelanin.
How does the body decide what type and how much of these melanins to put in your hair? Genes, of
course. We all have special skin cells called melanocytes. They ‘read’ the pigment recipes found in our
DNA and produce the different versions of melanins.
Humans usually end up with very little pheomelanin because of the product of a gene located on
chromosome 16 called MC1R. This MC1R gene codes for an enzyme (“melanocortin 1 receptor”) that
converts pheomelanin pignment into eumelanin which makes red hair pretty rare.
When someone has both of their MC1R
genes mutated, this conversion doesn't
happen anymore and you get a buildup
of pheomelanin, which results in red
hair (as well as fair, pinkish skin and
freckles).
A
n example of a nonworking MC1R gene.
So, your son may have two mutant MC1R genes, one from you and one from your husband. The fact
that you and your husband have freckles may strengthen this idea as I have read that people with a
single mutant MC1R gene don't necessarily show red hair but often are freckled.
So it may be a simple case of dominant and recessive traits. You and your husband each have one
working wildtype version but carry one mutant version of the MC1R genes making you freckled but not
giving you red hair. Your second son got two mutant MC1R genes while your first inherited either one
or no mutant copies. By this model, you had a 1 in 4 chance of having a redhead.
As to why you haven't seen it in your family before, it depends when it entered your family and simple
statistics. For example, let's say your family has a single copy of the mutant MC1R gene; no one has
two copies of the mutant gene. If everyone then marries people with two working copies of MC1R,
then any children produced will have either one or no copies of the mutant gene.
Even if they did marry someone with a single mutant copy, there is still only a 1 in 4 chance of
producing a redhead. It is important to realize that if you have four kids, this doesn't mean that one
will be a redhead for sure. Odds and probabilities do not work like that. It doesn't matter what came
before. Each new child has a 25% chance of being a red head.
You can see how even if there are no known redheads in your family, you can still be a "carrier" for
red hair. Hopefully this answer can help arm you against the inevitable questions.
2) My husband has bright orangey red hair with thick tight curls. I have stick straight
Auburn hair (although as a child it went from a light strawberry blonde to a flaming red
orange, slowly darkening to my auburn colour). I've heard that 2 redheads can only have
redheaded children. So far, seems true - although all are different. Our first has soft curls,
bright orange. Our second has straight, auburn. Our third really straight, light strawberry
blonde. If we have more will they be redheads too? Why the variations in shades? Is it true
2 redheads can only have redheads?
-A curious adult from Ireland
September 8, 2006
Great question. You are right in saying that almost always, two redheads will
have kids with red hair. But as your family shows, this doesn't mean that all
of the kids will have the same shade or darkness of red. They can have
strawberry blonde, almost orange, auburn and mixtures of all three.
The MC1R gene determines
whether hair will be red.
What do all these hair colors have in common? The pigment pheomelanin.
Other genes determine
Only European-Americans carry the gene for pheomelanin. In redheads, the the shade.
MC1R gene no longer works resulting in a build up of pheomelanin. So they end up with red hair and
freckles.
MC1R helps explain which pigment sticks around. Other genes decide how much gets made. Other
hair color genes are not as well understood as MC1R.
Here is one idea of how they might work…
Imagine that each of the eumelanin color genes comes in two forms: either on or off. When the genes
are on, they make some pigment and when they are off, they don't make anything.
So the more
genes that are turned on, the more pigment gets made and the darker the hair. In our analogy,
having more genes turned on is like adding more dye to our bucket of paint. Each “on” gene adds a bit
more dye. In your body, the melanocytes producing the pigmenst will contain lots of densely packed
eumelanin.
What about your case in particular? Let's say that there are four genes that determine how dark
someone's hair is. Remember you get one copy of each gene from your mother and one from your
father giving you a total of eight copies of these hair color genes. And which four you get from each
parent is random!
If one of these hair color genes is on, we'll represent it with H and if it is off, we'll represent it with h.
Using this system, someone with very dark hair would be HHHHHHHH and a very light haired person
would be hhhhhhhh.
You said you have auburn hair and you husband has red-orange hair. Let's say
you are HHHHHHhh and your husband is HHHHhhhh.
Someone with strawberry blonde hair inherits two broken MC1R genes and only a few of the on form
of the other hair color genes. Someone with darker red hair inherits the broken MC1R genes but more
of the on genes.
So in theory, a child could be anywhere between HHhhhhhh (strawberry blonde) to HHHHHHHH (very
dark red hair). The child with auburn hair inherited mostly H's. The strawberry blonde, mostly h's*.
And the oranger haired child inherited some of each.
And this is how it works for people who don't have red in their hair too. Only these genes are
determining how much eumelanin is in the hair, not pheomelanin.
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Medical Implications (from Paul Arnold, 9/4/2009, www.brighthub.com)
There is much more to this subject than its curiosity factor. There appear to be medical implications for
people with red hair. Although here, like many areas of science, the research is contradictory. In 2002
researchers from Louisville University showed that redheads are more sensitive to pain, and need more
anaesthetic during surgery than people with blonde or dark hair. However, in 2005 scientists from the
University of Edinburgh found that a MC1R mutation gives redheads a higher tolerance for pain.
Research into these aspects of red hair genetics continues.
Redheads should though be more careful about their exposure to sunlight as they are at an increased risk
of contracting skin cancer. One theory suggests that melanin, the pigment responsible for tanning (it
darkens as a protective mechanism) may be chemically different from the melanin of people with dark
hair. Researchers have found that UV rays interact with phaeomelanin, creating molecules that cause
damage to DNA, which can lead to cancer.
If you are a redhead, the advice is not to stay out of the sun, but to be careful about how much exposure
you get, and to cover yourself with a high factor sunscreen.
Post-reading Questions from Mr. Orend:
1) What are the two pigments that determine hair color?
2) What type of organic molecule are pigments?
3) What does it mean for someone to be a carrier for a genetic trait?
4) If a person has red hair as opposed to plain brown hair, what happened on a genetic
& molecular level?
5) Why is the hair color trait considered to be polygenic?
6) How are freckles related to hair color?
7) How is skin cancer risk linked to hair/skin color? What does that tell you about the
biological role of eumelanin?
Summarize how hair color is inherited using the following terms in a cohesive paragraph:
Phenotype
Genotype
Chromosomes
Dominant
Recessive
Polygenic
Gene
Allele
Mutations
Pigment
Eumelanin
Pheomelanin
MC1R
Enzyme
Melanocytes
Summarize how hair color is inherited using the following terms in a cohesive paragraph:
Phenotype
Genotype
Chromosomes
Dominant
Recessive
Polygenic
Gene
Allele
Mutations
Pigment
Eumelanin
Pheomelanin
MC1R
Enzyme
Melanocytes
Summarize how hair color is inherited using the following terms in a cohesive paragraph:
Phenotype
Genotype
Chromosomes
Dominant
Recessive
Polygenic
Gene
Allele
Mutations
Pigment
Eumelanin
Pheomelanin
MC1R
Enzyme
Melanocytes
Summarize how hair color is inherited using the following terms in a cohesive paragraph:
Phenotype
Genotype
Chromosomes
Dominant
Recessive
Polygenic
Gene
Allele
Mutations
Pigment
Eumelanin
Pheomelanin
MC1R
Enzyme
Melanocytes
Summarize how hair color is inherited using the following terms in a cohesive paragraph:
Phenotype
Genotype
Chromosomes
Dominant
Recessive
Polygenic
Gene
Allele
Mutations
Pigment
Eumelanin
Pheomelanin
MC1R
Enzyme
Melanocytes
Summarize how hair color is inherited using the following terms in a cohesive paragraph:
Phenotype
Genotype
Chromosomes
Dominant
Recessive
Polygenic
Gene
Allele
Mutations
Pigment
Eumelanin
Pheomelanin
MC1R
Enzyme
Melanocytes