There’s both fact and fiction to the rumours that twins run in families and tend to skip a generation. Here's what you need to know about the science behind family heredity and twins.
Twin genetics depend on what kind of twins we are talking about. Identical twins vs fraternal twins. The important difference between identical and fraternal twins is the number of fertilised eggs involved. Identical twins come from a single fertilised egg. Fraternal twins come from two different ones.
Identical twinning and family heredity
Identical twins happen when a single embryo splits in two soon after fertilisation. This is why identical twins have identical DNA. They came from the same fertilised egg. Since embryo splitting is a random spontaneous event that happens by chance, it doesn’t run in families. Genes are not involved and there's no scientific evidence that being from a family with identical multiples has any impact on your odds of having twins. The same is not true for fraternal twins.
So, having identical twins is not due to genetics. On the other hand, fraternal twins can run in families. In fact, a woman that has a sibling that is a fraternal twin is 2.5 times more likely to have twins than average!
Fraternal twinning and family heredity
For a given pregnancy, the odds of conceiving fraternal twins are only determined by the mother’s genetics, not the father's. Fraternal twins happen when two eggs are simultaneously fertilised instead of just one. There appears to be a genetic link that predisposes some women to hyperovulation, which means releasing more than one egg during ovulation, or each menstrual cycle.
Heredity on the father's side does not increase a couple's odds of having multiples. This is true even if his family is full of multiples. Men, of course, don't ovulate, so even if they've inherited that hyperovulation gene from their parents, it doesn't increase their own odds of conceiving twins. It's all about the fertilisation of eggs and a father’s genes can’t make a woman release two eggs.
Do twins skip a generation?
You may have heard that twins 'skip a generation'. While it's not a blanket true statement, it is actually based on some truth.
Here's how: we know that twinning is due to factors on the woman's side, like hyperovulation. But it takes two to make a baby, or babies in our case! This is where the skipping is relevant.
A man's family history of twins can’t increase his partner’s chance of having twins. If a man has inherited the hyperovulation gene from his mother, he may pass this gene on to his daughter, but it won't impact on the chances of him conceiving multiples with his own partner. Men, of course, don't ovulate, so the chance of twins or more is up to the woman. The man's daughter, however, having inherited the hyperovulation gene through her father is then more likely to release more than one egg when she ovulates and therefore could conceive fraternal twins. In that scenario, it would seem as though the twins have, in essence, skipped a generation.