Expert: John Locke - 1/2/2007

Question
I may be forgetful, but I seemed to remember that certain parental eye colors cannot produce children with certain eye color..?? is that correct?  If you have brown eyed parents, can you have a blue eyed child?  What if the mother has a form of hazel eyes?  Also, are there blood types that cannot produce certain blood types ?  Thanks for any imput.  Couldn't find anything on the net..  This is not for a paper, just curiosity.....

Answer
Thanks for using AllExperts, Scot. This are both questions we get fairly often, but they do have a well-defined answer.

Let me tackle blood types first, as they are easier than eye color. Blood type is determined by markers present on the surface of red blood cells; the white cells of the body recognize like blood cell markers and attack unfamiliar ones--hence the importance of matching the correct blood type during transfusions. These markers themselves are controlled by a single gene, with three possible alleles: call them Io, Ia, and Ib for short. Io is recessive, while both Ia and Ib are dominant.  The possible combinations are as follows (keep in mind that one allele will be inherited from each parents):

Io/Ia= Blood type A
Ia/Ia= Blood type A
Io/Ib= Blood type B
Ib/Ib= Blood type B
Io/Io= Blood type O
Ia/Ib= Blood type AB (this is an example of codominance)

I will leave it as an exercise to you to figure out which blood types are not possible from which parents. Here's a starting point: if either parent has Ia or Ib, the child cannot have blood type O. Beyond that, try making a table of possible combinations from the given starting blood types of the parents. I'm not doing that here for you simply because of space and formatting difficulties: it would be very difficult to read and would not illustrate the point very well.

As for eye color, the situation is more complex; indeed, human eye color is known to be a polygenic trait. Three genes control human eye color, and all three must be the same (that is, all six alleles must match) for the color to be pure: blue, brown, and green are pure eye colors. Any other combination results in a mixed eye color--hazel, amber, violet, etc., with the specific color depending on the proportion of blue vs. green vs. brown alleles. Again, recording all the combinations of three choices in six locations is quite daunting (there are 729 possible combinations, in fact); I will direct you to a site below that shows how these alleles interact to produce a given eye color:

http://museum.thetech.org/ugenetics/eyeCalc/eyecalculator.html