One, two, skip a few, 99, 100!
I've heard people say that twins skip a generation.
I've also been asked by more than one person how this can be true since I'm a twin who is also a mother to a set of twins—and, therefore, no twins skipped any generation.
In my case, we're talking about fraternal twins in one generation followed by identical twins in the next, which will only confuse the matter, and we'll dig into that later.
...the truth is, the idea that twins skip a generation is a myth.
But even if we're talking about fraternal twins followed by more fraternal twins, the truth is, the idea that twins skip a generation is a myth.
So, why, if it's a myth, does this follicular fallacy seem to hold up in most twinning events?
Let's dig into the science behind this popular misconception.
The heredity of twins
In Canada, about 1 in 31 births results in twins or higher-order multiples.
This statistic is almost the same in the United States, with about 32.1 out of every 1,000 births resulting in twins.
Triplets or higher-order multiples occur in approximately 78.9 out of every 100,000 live births.
If you've conceived twins and shared the news of your double-bundle of joy, one of the first things people probably asked is if twins run in your family.
...one of the first things people probably asked is if twins run in your family.
Depending on who you ask, the idea of twins running in a family depends on which type of twins you're carrying: identical (monozygotic) or fraternal (dizygotic).
Identical twins are believed to occur by chance, when a single fertilized egg splits spontaneously and results in a monozygotic pregnancy.
Dizygotic twins, on the other hand, are more common, with about 70 percent of all twins being fraternal. They result when two separate eggs are fertilized by two different sperm.
Conceiving twins: A mother's role
On the mother's side of things, there are many components—both genetic and non-genetic—that can contribute to her having a higher chance of conceiving dizygotic (fraternal) twins.
If you're thinking about having a baby and are curious about your chances or odds of having twins, factors at play can include:
- A mother's genetics Having the hyperovulation gene (more on this below).
- Their age Being over age 35.
- Their BMI Having a pre-pregnancy BMI of 25 or more.
- Their race Having multiples is more common among Black and White women and less common among Asian and Hispanic women.
- How many pregnancies they've had The more pregnancies you've had, the higher your chance is of conceiving twins.
- If they've mothered multiples already If they've given birth to a set of twins before, they're more likely to have another set of twins again.
- Whether they've been undergoing fertility treatments Injectable hormones can increase the odds of multiples by up to 30 percent.
Homing in on the hyperovulation gene
If you're looking into your lineage, you may be wondering if there's actually a twin gene that exists, and if so, if you've got it.
While there isn't a single "twin gene" or even a single "hyperovulation gene," there are genetic factors that can make someone more likely to conceive twins, and you may have had some of those passed down to you by your parents.
Genetic variations in the genes that control follicle-stimulating hormone production, receptor sensitivity, reproductive signaling, and follicular development can all cause someone to hyperovulate.
When we talk about twins running in families, it's because of that little biological wonder called hyperovulation.
What is hyperovulation?
Hyperovulation happens when more than one egg matures and is released in a single menstrual cycle (note: hyperovulation doesn't necessarily happen during every cycle).
When those eggs both get fertilized by separate sperm, you become pregnant with fraternal twins.
Hyperovulation is believed to be the only hereditary factor that can genetically influence the conception of twins—though, some research suggests a potential link to embryo splitting in the case of monozygotic twins.
Twinning can be influenced by a father's level of insulin growth factor-2 (IGF-2), which is associated with cell growth and division.
The IGF-2 level is partly heritable, and when it's transmitted to children by their fathers, it's active (the same gene transmitted by mothers is inactive).
So, it's possible that identical twins could appear at a higher rate in some families if a higher level of the IGF-2 gene is being transmitted by the father. And with the number of sets of identical twins on my husband's side of the family, I'm inclined to believe it's due to more than just chance.
Getting back to hyperovulation and the role it plays in making fraternal twins possible, things can get even more interesting. If someone told you they were pregnant with a set of twins who had two different fathers, you'd probably think they're pulling your leg.
While it might sound like a made-up story, giving birth to fraternal twins who each have a different father is actually biologically possible.
It's quite rare, but it does happen. If you're wondering how, you can read more about hyperovulation and heteropaternal superfecundation here.
But we're getting away from the whole point of this article, which is whether or not twins skip a generation when they run in the family, so let's take a closer look at why people think this is the case.
Do twins skip a generation? A father's role
When it comes to twins, people have wondered if the dad's side "matters."
Of course it matters.
It takes two to tango, and it takes two to twin.
It takes two to tango, and it takes two to twin.
Genetically speaking, a father's role in a fraternal twinning event is what spawned the whole "skip a generation" myth to begin with.
Imagine this: Your grandma is a twin who has the genetic variants for hyperovulation. She gives birth to a singleton—your dad—and she passes on those genetic variants to your dad, so he carries "the gene" but because he doesn't have ovaries, he doesn't ovulate. So, the genetic factor for twins sits silent for his generation.
The father then has his own children and passes "the gene" on to his daughter. She, having inherited the genetic factors for hyperovulation from her dad, could then release multiple eggs in a single cycle and conceive twins.
With this example, the twins seem to have skipped from the grandmother's generation to the granddaughter's generation, with no twins showing up during the dad's generation.
That's where the concept (and the myth) that twins skip a generation came from.
That's where the concept (and the myth) that twins skip a generation came from.
If you really wanted to make this skip-a-generation scenario confusing, you could imagine that the father actually ends up with a partner who hyperovulates, and they could then conceive fraternal twins when she releases multiple eggs, but that set of fraternal twins wouldn't be "thanks to the father's genetics," since he isn't the one who ovulated.
To further confuse the matter, if the father and his partner DID have twins and they ended up being monozygotic, then that could, according to some research, be caused by the father's level of insulin growth factor-2, which is responsible for cell growth and division, if he passed along his higher level of that gene—so, then those twins would be thanks to his genetics (just not thanks to the "silent" hyperovulation variants he carries).
Twin myth debunked
While you may have heard or told others that twins tend to skip a generation, that concept is a myth.
At a quick glance of your family tree, it might look that way, but when you dig deeper into the genetics of it all, you can begin to understand how and why the genetic variants can be passed along but sit silent for a generation when they're passed down through someone's father.
This creates the illusion that twins can skip over a generation, when really, it's just the twinning traits are sitting silent, waiting to be inherited by the next branch of the family tree.
Do twins run in your family? Did they "skip'" a generation? Let us know in the Comments section below or connect with us on Instagram to share your thoughts!
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