The "mailman" joke is aging badly

I have brown hair. Dina has brown hair. Our younger son, Zack, has brown hair. Our older son, Alex, has blond hair. Ever since Alex was born, we have occasionally had people make comments along the lines of "where did he get his blond hair?"

Some of these are perfectly innocent--they generally segue immediately into questions about our parents' hair, etc. They're genuinely curious.

But others, most often complete strangers, seem to find this an opportunity for comedy. "Where did he get his blond hair?" they ask in teasing tones. Wink wink. Nudge nudge. Often, they suggest that the mailman is responsible. This evening, we attended an orientation function for kids signing up for kindergarten in the fall at our local elementary school. A lady (who had seemed perfectly nice earlier) asked us, rather loudly, about Alex's hair, then followed it up with an amused glance at me. "I bet you'd like to know," she said. Ha ha. Mind you, this was in front of Alex.

How many possible ways could this joke go wrong for someone? I mean, in our case, there's no question that Alex is my child, and Dina's. But what if that weren't the case?

Let's take fictional family A. They've adopted a child with blond hair. Or fictional family B. The dad has died and the mother has remarried. Or fictional family C. The mom had a wild fling at one point and the child isn't her husband's, but they've reconciled and he treats the child as his own. I'm sure you could come up with more possibilities on your own. Is it a good idea, in any of these situations, to make a potentially awkward situation (for the parents) even worse? Is it ever a good idea to suggest to a child that his parents might not really be his parents?

For the record, our parents also have dark hair, but Dina's brother's hair was blond when he was young. My grandfather had blond hair, and my aunt is blonde. So Alex's hair is a little unusual, maybe, but nothing far from the family tree. I have absolutely no doubt that Alex is mine, nor should anyone who really pays attention for more than a few seconds--we've got too many traits in common (including, of course, dashing good looks, keen intellect, and charming personalities). But you know, if he hears a half dozen strangers asking how he can have blond hair when his parents don't, he might eventually start to wonder, every once in a while. He might start to doubt us. And making a child doubt his parents isn't worth the price of a joke. Particularly a stupid one.