Question of the Week

Why don't humans have tortoiseshell hair?

Thu, 31st Jan 2013

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Question

James Passas asked:

Hi Chris, †

 

Looking around at the neighbourhood cats, it struck me that most of them (and indeed most mammals) have patterned fur with several different colours.

 

This made me wonder why Humans tend to have uniform hair colour on their heads, faces †& bodies. Why is this?† Has there ever been anyone with multicoloured or patterned hair, or is it always a uniform colour? † †

 

Regards, † †

James in Cambridge.

Answer

Ian -   I'm Professor Ian Jackson at the MRC Human Genetics Unit in the University of Edinburgh.  We understand a bit about the genetics of human colour.  We know the gene that causes red hair for example and we know some of genes make long versus dark hair.  But in humans, we never see coloured patterns in their hair.  

Now in cats, and many other mammals, we see a whole range of patterns.  Cats have this stripy tabby pattern.  So the stripes are caused by a gene called amino peptidase q and when this gene is missing, the cats lose that stripy pattern and have a more blotched pattern where the stripes become irregular.  We humans have this amino peptidase q gene, it really isnít going to do the same thing in humans.  We donít actually know what itís doing.  

Cats also have that tortoiseshell or calico pattern.  That's caused by a gene on the X chromosome that makes orange pigment and females have 2 X chromosomes.  Females shut down one of the X chromosomes in random in different cells.  So, if she has 1 orange X chromosome and 1 black X chromosome, then you get a mixture of orange and black patches.  Itís the classic tortoiseshell pattern and tortoiseshell cats are always female.

Hannah -   Also, the pointed pattern in Siamese cats where the ears, nose and extremities are darker stems from a mutated colour gene that switches off at higher body temperatures.  

So, humans seem to have simple genetic rules for hair colour and they donít respond to the patterning genes in the same way as cats, which is why you donít generally get a tabby human.  

There are exceptions to this though.  People with piebaldism do show patches of white hair even at a young age.  Their melanocytes, so the cells that produce the pigment for hair colour have been mutated and switched off, and itís this that produces the colourless patches.

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Gorillas, Orangutans, and Chimpanzees all tend to have monochromatic hair, although other primates may have more colorful coats.  But, humans likely evolved from monochromatic primates.

Hair color patterns may reflect camouflage.  A small hunter such as a house cat may benefit from camouflage both for hunting, as well as preventing being prey themselves.  Of course, humans will often breed the animals for their coat, but something like the calico genes would have to exist in nature to be selected for.  Having a variety of possible colorations may help the cats to survive in a variety of different environments.

The large primates are herbivores, and thus need less prowess hunting, and also may not require camouflage for fending off attacks from predators.  Humans became an apex predator, but have found other ways to hide from their prey. 

Birds may use colors as part of their mating ritual.  It is a wonder that humans didn't evolve peacock feathers.  However, perhaps more than any other animal, humans have adorned our bodies with "art", not relying on nature alone. CliffordK, Fri, 25th Jan 2013

We also had this comment from Jenny Wing on Facebook:

Re: tabby-haired people question of the week from the last show.
A couple of years ago I worked on a paper evaluating the costs of targeted treatments for renal cell carcinoma. Sunitinib can cause changes in hair pigmentation and as it is dosed 3 weeks / 1 week off patients experiencing this side effect can end up with striped hair. There is a good photo illustrating this phenomenon in the Canadian Urological Association Journal, June 2007, Volume 1 Issue 2 Supplement in a review article "Sunitinib therapy for metastatic renal cell carcinoma: recommendations for management of side effects" by Kollmannsberger et al on page 10 of the article (page S50 of the supplement). Not exactly naturally produced, but a bit tabby nevertheless.
J. Wing (health economist) BRValsler, Thu, 31st Jan 2013

A few look like a tabby cat  ...


http://dermatlas.med.jhmi.edu/image/epidermal_nevus_syndrome_4_080216


https://en.wikipedia.org/wiki/Blaschko%27s_lines RD, Thu, 31st Jan 2013

Melding two zygotes can sometimes lead to chimerism - two sets of DNA in the same baby. If the different cells have different pigmentation genes, this pattern can be visible on the skin: http://www.ncbi.nlm.nih.gov/pubmed/18347288

There are also diseases which can affect skin colour in patches, eg photo at http://en.wikipedia.org/wiki/Vitiligo evan_au, Fri, 1st Feb 2013

I'm not convinced that the answer given here is the whole answer.
My cat's individual hairs are striped along the shaft.  This implies that the hair follicle is switching between producing pigmented and non-pigmented hair at different times.  That is giving her the overall tabby effect - not just certain areas being pigmented whilst others are not. TheAnalyst73, Wed, 6th Feb 2013

It is quite possible for a human to have multi-colored hair. I had a friend in school many years ago who was blond except for a spot of hair about 3 cm in diameter which was naturally black. He happened to have a birth mark on his head, and the pigmentation was incorporated into the hair. To me this suggests people with other conditions that cause large differences in skin coloration on the head might similarly have natural multi-colored head hair, but I myself only know of that one example. Of course, many people at some time in their lives naturally have multi-colored hair as their hair turns gray is certain regions before others. ps790, Thu, 7th Feb 2013

I have striped eyebrow hairs. As in, each individual hair alternates (every quarter- to half-inch) between pale blonde and med brown, just like the fur of a striped cat. The rest of my hair is all reasonably uniform. If there is really no gene for this in humans, I must have an extra or a mutation. Catwoman, apparently., Fri, 17th Jan 2014

Ok so im reading this articall and i call bull cause all my life ive been labeled as a calico because of my multi-colored patches of hair and growing up no one ever believed that my hair was natural and lead me to color my hair growing up because I always would be madefun of . So I would very much like to get in touch with someone that can explain this anomaly. Shanna Carrico, Thu, 8th Oct 2015

I have patternd hair blond(inner thigh) orange(back of the hands) and black (leg) hair a stripe in my beard and brown head hair the transitions are so subtle that no one has ever noticed except myself Chris, Sun, 15th Nov 2015

My daughter has natural tortoise shell hair. Several individual strands are multi-colored naturally. One strand has light blonde, red, orange, and dark blonde. She thinks she has tiger-striped hair and loves it. I can't find any instance of this inline anywhere... Casey , Tue, 5th Apr 2016

Myself and many of my family members have the same seemingly dominant trait of multicolored and multi textured hair. I have a mix, all over my head. Some hairs are black and thick, some are medium thick and brown, some are red, and some are very fine white blond. I have never know of anyone else besides my family having this trait. I constantly see researchers saying it is impossible. (Lol) Heidi C., Tue, 28th Jun 2016

Growong up we had a boy in the neighborhood who hadpatchy hair. It was brown, but he had also light patches and an almost white patch at the back of his head. Since he had short hair it was very visible. It wasn't dyed. Why would you, it wad too strange to be deliberate and it was always the same. But it looked good! I'd be interested to know how it occurred. Vanessa, Sun, 3rd Jul 2016

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