Whether or not twins are genetic has long been disputed in science. If you’re a mother of twins, you’ve probably had many other women come up to you and ask whether or not there are other sets of twins in your family. Like blue eyes or a tall stature, most people assume that having twins is something that runs in your genes. It’s no wonder, considering many women that have twin babies do in fact have other twins in their families. However, this is not clear evidence, and you should first understand the nature of twins to learn why they are not necessarily genetic.

There are two different types of twins. Monozygotic twins—or identical twins—are born from the same egg when it is fertilized. These siblings are the same gender and they have nearly the same genetic makeup. These are the twins that you can’t tell apart right away, and this is in no way genetic. Scientists haven’t officially confirmed what causes the egg to split in two, but genetics are not to blame.

On the other hand, dizygotic twins are fraternal. They don’t look alike more than any other siblings born to the same parents might, and they can even be opposite genders. These twins are born from two separate eggs that happened to release at the same time. Most women release one egg to be fertilized, but women who give birth to dizygotic twins released two, and both were fertilized. This is where genetics might come into play.

Unless she is taking fertility treatments and is over the age of thirty, a woman probably produced more than a single egg because she has a hyperovulation gene. This gene is easily passed down to female offspring, which is why many people think twins “skip a generation.” If you have a hyperovulation gene, you won’t pass it to a son. However, if that son has a girl, you will have passed it on to her. Therefore, you and your granddaughter have a greater chance of having twins or multiples.

Genetics can be complicated, but the science of twins is actually quite simple. Women who have monozygotic twins had them by chance, but dizygotic twins could have been a result of hyperovulation. If your mother or grandmother had fraternal twins, you might have the gene—so don’t be surprised when you see two beating hearts on your first ultrasound.

Source: Fierro, Pamela. The Everything Twins, Triplets, and More Book. F+W Media Inc. eBook 2012

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