-A curious adult June 25, 2014 That is a very interesting question! And one that many people wonder about. In fact, we answered a very similar question many years ago. Twin genetics depend on what kind of twins we are talking about. Having identical twins is not genetic. On the other hand, fraternal twins can run in families. Genetics can definitely play a role in having fraternal twins. For example, a woman that has a sibling that is a fraternal twin is 2.5 times more likely to have twins than average! However, for a given pregnancy, only the mother’s genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father’s genes can’t make a woman release two eggs. It sounds like fraternal twins do indeed run in your family! But, since your son is the father, his genes are on the wrong side of the family tree. So, your family history likely didn’t play a role in his wife’s twin pregnancy. The answer would be different if you were asking about a daughter. Also, although your son’s family history of twins can’t increase his wife’s chance of having twins, he can pass those genes down to your granddaughter. With your strong family history of fraternal twins, this just might increase the chances of your granddaughter having twins! But, your daughter-in-law is not necessarily having twins because of her genetics. Other things like environment, nutrition, age, and weight have also been linked to having twins as well. And there is always simple chance…every woman has a chance at having fraternal twins. It is just that some women have a higher or lower chance. Huh? Help Me Understand the Genetics! Wait a minute. One type of twins has a genetic basis and the other does not? And, only the mom’s genetics matter? How is that possible? Don’t worry. It makes a lot of sense once we break down the biology.

The important difference between identical and fraternal twins is the number of fertilized eggs involved. Identical twins come from a single fertilized egg. Fraternal twins come from two different ones. Identical twins happen when a single embryo splits in two soon after fertilization. This is why identical twins have identical DNA. They came from the same fertilized egg. Since embryo splitting is a random event that happens by chance, it doesn’t run in families. Genes are not involved. The same is not true for fraternal twins. Fraternal twins happen when two independent eggs are each fertilized by different sperm. This is why the DNA of fraternal twins is different. In fact, fhe DNA of fraternal twins is no more similar than the DNA any other sibling pair. Usually, a woman only releases a single egg at a time. Fraternal twins can only happen if a mother releases two eggs in one cycle. This is called hyperovulation. Unlike embryo splitting, ovulation is a normal biological process that is controlled by our genes. And, different women can have different versions of these ovulation genes. Some women have versions (called alleles) of these genes that make them more likely to hyperovulate. This means there is a higher chance that two eggs could get fertilized at once, leading to fraternal twins. The gene versions that increase the chance of hyperovulation can be passed down from parent to child. This is why fraternal twins run in families. However, only women ovulate. So, the mother’s genes control this and the fathers don’t. This is why having a background of twins in the family matters only if it is on the mother’s side. And why your son’s family genetics did not play a role in his twins. We went over a lot of this stuff in our previous answer, but your question got me thinking. Our last answer on twins was done so long ago. Has recent research discovered anything new on this fascinating topic? They have indeed… at least if you are a sheep!