MadSci Network: Genetics

Re: How do two blue-eyed parents produce a brown-eyed child?

Date: Thu Jun 22 01:03:48 2000
Posted By: Christopher Carlson, Grad student Genetics
Area of science: Genetics
ID: 961023727.Ge

Hi Ashley,

	I'm glad you understand the basics of Mendelian inheritance.  Your 
question is perfectly reasonable in light of how genetics is taught in most 
bio courses: first you study green/yellow smooth/wrinkly peas, and then a 
remarkably naive attempt is made to extrapolate to humans.  The usual traits 
used to illustrate mendelian inheritance in humans include hair and eye 
color, PTC tasting, tongue curling, hitchhiker's thumb, etc.  Frankly, none 
of these traits are clearly mendelian; they are simply traits for which two 
or more common variants exist in the human population.

	The basic premise of Mendelian genetics is that you are dealing with a 
trait which has either two or three phenotypes, with a single underlying 
genetic determinant which has two alleles.  This works in the case of the 
peas: the peas are either yellow or green.  There are no chartreuse peas, 
intermediate in color between yellow and green.  Likewise, there are no 
lightly wrinkled peas: the pea is either wrinkled or smooth without 
intermediates.  Both systems are determined by a single locus with two 
alleles, one of which is dominant to the other (I can't remember which, so 
I'm arbitrarily assigning green and wrinkly as dominant).  If you cross a 
purebred smooth yellow strain with a purebred wrinkled green strain, and 
then intercross the wrinkly green F1 progeny, the F2 progeny will have a 3:1 
ratio of dominant phenotype (green or wrinkled) to recessive phenotype 
(yellow or smooth) for each trait.

	The key here is the notion of dominance.  Green peas can have yellow 
offspring because green peas can carry a recessive yellow allele.  Yellow 
peas can't have gree n offspring.  Now let's get back to your brother in 
law's problem.  In school we all learn that brown is dominant over blue, and 
dark skin is dominant over light.  Therefore blue eyed parents (recessive 
phenotype) can't have brown eyed kids (dominant phenotype).  This is hooey.  

	Are there really only two eye colors?  Far from it.  There is a 
virtually infinite spectrum of eye color, stretching from the deepest shades 
of brown through amber and green to the lightest blue.  All of these eye 
colors are the result of pigment production in the iris: there are two 
pigments (one brown and one yellow) produced in the iris.  The amount and 
the ratio of each pigment produced determines the eye color.  There are a 
number of enzymes involved in the production of each pigment.  If any one of 
these enzymes is defective, the end result is lack of pigment production and 
light (blue) eyes.  However, not all people have blue eyes for the same 
reason: some have defective enzymes (albinos) while others simply have low 
levels of enzyme (Scandinavians).  Furthermore, not every albino is 
defective in the same gene, so it is possible for two albinos to have 
pigmented children (trust me, I didn't believe it either until my clinical 
genetics prof showed me a picture of two African albinos with their very 
dark child).  This is because the kid only inherits one functional copy of 
each enzyme in the pathway from each parent.  By the same token, neither 
does every fair skinned person have low levels of pigment production for the 
same reason, so it is definitely possible for two blue eyed parents to have 
a dark eyed kid.

	That said, in the genetics community the accepted estimates for the 
rate of non-paternity in most cultures (including the US) are between 5 and 
10%, so it's not necessarily impossible that your brother-in-law has been 
cuckolded.  It simply isn't proven by eye color, which is why paternity 
testing is based on blood tests for truly mendelian traits like the ABO 
blood type.  I'm not recommending that your sister and brother-in-law have 
the kid tested: if he loves and respects your sister, the mere fact that the 
kid is probably and possibly his should be enough.


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