I have heard that there are some couples who cannot conceive a baby together but who could conceive a baby with other partners. Can a man and a woman be genetically incompatible? If so, is there any way to diagnose and treat this condition? --Kim

Knowledge of evolutionary theory would have us believe that evolution favors factors that increase genetic diversity. Mating with a genetically similar partner (such as a family member) does little to increase genetic diversity, and it also increases the risk that recessive genetic abnormalities will appear in any children. So one would hypothesize that being too similar genetically may lead to a couple's reproductive failure.

In fact, early studies do suggest that reproduction is hampered if partners are genetically compatible. That is, if you would be a good organ donor for your partner, the likelihood of successful reproduction drops.

One theory attributes this to HLA markers, the molecular "name badges" that all cells wear. HLA molecules on the surface of cells help the immune system identify cells as friend or foe. Some physicians believe that if partners share too many of these HLA markers, the woman's immune system might fail to make protective "blocking antibodies" necessary for a pregnancy to continue -- thus causing infertility. However, most reproductive specialists do not hold this belief, for several reasons. First, HLA markers do not appear on the surface of the placenta (the organ that links the fetus to the uterus) and studies repeatedly have failed to indicate that HLA sharing is a fertility factor.