MadSci Network: Genetics

Re: How did my nephew get blonde hair and blue eyes?

Date: Thu May 28 19:57:00 1998
Posted By: Christopher Carlson, Grad student Genetics
Area of science: Genetics
ID: 896235983.Ge

	Good question.  I actually have two answers for you, one based on the 
genetics everyone is taught in high school, the other a little more 
complicated and a little more accurate.

	First the oversimplified answer:   Both hair color and eye color are 
related to the production of melanin, the pigment which colors human hair, 
skin and irises.  Blue eyes and light hair are the result of low levels of 
melanin production, which is common in people of Northern European descent.  
In high school it is usually taught that dark hair and dark eyes are 
dominant to light hair and light eyes.  That is, if we assume a single gene 
governs pigmentation, there are two alleles D (dark) and d (light).  Since 
we each carry two alleles (one from Mom, one from Dad) there are three 
possible genotypes: D/D, D/d and d/d.  Because D is dominant, D/D and D/d 
both look dark, even though D/d carries one light allele.

	Thus, given that your sister has hazel eyes (and I assume dark hair), 
but her mother was blond/blue, your sister is probably D/d.  In other 
words, she carries the light hair/eyes allele even though she doesn't 
express it.  Her husband may not know of any blondes in his family tree, 
but apparently he carries the light allele as well, and when he and your 
sister had a kid they both passed on the d alleles they carry but do not 

	Now for the more complicated answer: what we learn in high school is 
pretty much wrong: eye and hair color are determined by a whole bunch of 
genes all of which are involved in the production of melanin, with the 
result that there are many gradations of color: in hair blonde, red, brown 
and black, in eyes blue, gray, green, hazel and brown.  Plus there are 
actually many gradations in between.  The pattern of inheritance for each 
color is actually rather complicated: red usually seems to be recessive, 
but families have been described in Ireland where it appears to be 
dominant.  So a blond, blue eyed kid isn't unusual for any pair of parents 
of European ethnicity; perhaps a little less likely for darker parents but 
not impossible.  

	To add to this mess, pigmentation tends to change over a persons life.  
For most Europeans this means getting darker with age (I fear that I'm 
ignorant of whether this holds for other races).  An interesting example is 
Central and Northern Italy, where many children start out blond, but most 
darken to brown with age.  I suspect that this partially explains your 
nephew: I'd bet that he'll go brown in his teens, although there's a 
significant possibility that I'd lose the bet.

	In conclusion, while it may be fun to tease your brother in law about 
the mailman, there's no reason for him to worry.

	Chris Carlson

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