|MadSci Network: Genetics|
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. Chris email@example.com
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