Dear Heidi, I would love to see an explanation on how secretor status is inherited. Is it similar to the ABO system (but presumably simpler)? Thank you, Rose in Hungary

Exegesis:

Yes, secretor status is determined by two genes, just like ABO type. Each parent donates one of those genes to the child.

The two genes which determine secretor status are written "Se" (secretor) and "se" (nonsecretor). The important thing to remember is that "Se" is dominant to "se," just as type A and B are dominant to type O.

So, there are three possible combinations, or genotypes: SeSe, Sese, and sese. The first genotype is a secretor who has only secretor genes to pass on. The second is a secretor who carries a recessive nonsecretor gene. The third is a nonsecretor.

It's quite clear that a mating of SeSe + SeSe = child SeSe, a secretor. That's the only possible outcome, since each parent has only secretor genes to give.

By the same mechanism, sese + sese will always produce child sese, a nonsecretor.

The wild cards come into play with the combinations Sese + sese and Sese + Sese.

In the first, everything depends upon which gene the secretor parent donates to the child. If it is the Se gene, the child is a secretor... if it is the se gene, the child will be a nonsecretor. It's a toss-up.

In the second, Mom and Dad are both unsuspecting secretors with one recessive nonsecretor gene each. Just for fun, let's give them four kids. :-> Statistically, we expect their children to be: SeSe, Sese, Sese, and sese, not necessarily in that order. My man Bryan's parents happen to be in this situation. They are both secretors, as are their two younger children. Bryan, however, is a nonsecretor. Surprise! :-D

Rhesus type heredity follows the same pattern: two genes, with Rh+ dominant to Rh-. "Rh+" folks may be ++, or +- (one of each). "Rh-" always means --. As in the Nonsecretor Surprise, two "Rh+" parents may be scratching their heads upon the arrival of an "Rh-" little one... like a type A and B couple looking askance at each other on the birth of their type O baby. Knowing how blood type genetics works can circumvent loads of potential intrafamilial trouble! :-}

It's just another interesting thing about having kids: you learn things about yourself you might never have suspected -- biochemistry, too. :-D