A common variation on the question, “Are they natural”, when people find out you’re having or have twins is, “Which side of the family do they run on?”
In some ways, it’s a more indirect way of asking the same question, because if the answer is, “Neither,” then it’s probably safe to assume fertility treatments were involved, right? Wrong.
If someone asks which side of the family twins run on, they’re revealing a common misunderstanding about how twins work, even the “natural” ones. Identicals don’t run in families; fraternal twins do, but only the mother’s side is relevant if you’re asking parents of twins.
The frequency of identical twins holds steady across many cultures and gene pools and does not appear linked to any inherited tendency. Thus, twins don’t run on either side (or it is irrelevant if they do) when twins are identical.
Fraternal twins are affected by a woman’s tendency to release more than one egg per cycle, which is an inherited trait to some extent. That doesn’t mean she releases multiple eggs every cycle, or that all daughters in a twin line will have that trait, but if the women in her family have a history of fraternal twins, her probability of twins is increased. That said, if someone asks a couple “Which side of the family do they run on?”, the only relevant answer is “on the mother’s side”, because if they run on the father’s side, that could be relevant to his daughters’ future chances of twins, but it’s only the person who releases eggs who can increase a couple’s chances of naturally conceiving fraternal twins, and that would be Mom every time.
Once fertility treatments are involved, it’s completely irrelevant if twins run on either side, but the person who asked the question knew that, right?
Continuing the theme of snarky answers I fantasize about giving but probably never will:
- We can’t tell because both of our families walk most of the time.
- Which side of of the family do your overly personal questions run on?
- The side with the lab coats.
- The totally awesome side.
- That’s an interesting genetic question. How familiar are you with the difference between monozygotic and dizygotic twins and what role inheritance plays or doesn’t play in the probability of either kind happening?
- [For grammar geeks:] The side that talks in long sentences without any commas or other punctuation.
- You know that’s a pretty interesting question and we’ve been getting it a lot ever since we found out we were expecting twins and lots of people are curious and I guess they think questions about genetics and infertility make for interesting chit-chat so they don’t think twice before asking…oh, dear me, please excuse me for running on like that. Hey…I’m running on. I guess that would make it my side.
- I wish I could answer, but my mom and all my aunts made us sign non-disclosure agreements before we were allowed to interact with all our twin cousins.
- On the outside.
- I could answer, but I’m afraid it would just make you feel like an ass for asking and I’d hate to embarrass you like that.
