Have you ever heard it said that twins skip a generation? This is a commonly held myth about twins and multiples. Many times, this claim is stated with little to no understanding about what it really means. Perhaps people think that it means twins don’t have twins, yet the children of twins do. But more likely, they really haven’t thought it through.
The statement is based on the assumption that twinning is genetic, that twins run in families. However, if that was truly the case -- if there was a twin gene, so to speak -- then twins would occur consistently in those families that carry the gene. Everyone who is a twin would also produce twins. Yet, there are very few incidences of twins across every generation of a family’s lineage. But for families who do exhibit an abundance of multiples, the “skip a generation” theory may have been proposed to explain the gaps.
Like many myths about multiples, there’s a kernel of truth in the misconception about twins skipping a generation. Indeed, some twinning can be attributed to genetics. Sometimes twins do run in families. While there is not technically a “twin gene,” there is a genetic component that makes some women predisposed to conceiving twins. It’s suggested that there is a gene for hyperovulation, the tendency for a woman to release more than one egg in a menstrual cycle. If two - or more - eggs are fertilized, dizygotic (fraternal) twinning can occur. However, monozygotic (identical) twins are thought to be random; no genetic component has been identified that increases identical twinning. (Click here to learn more about the difference between identical and fraternal twins. )
So, for families with a preponderance of fraternal twins, the hyperovulation gene may be the cause. However, without confirmation via genetic testing, this is only an assumption. But, if the cause of twinning is related to hyperovulation, only the mother impacts the chances for twins. The father’s role is irrelevant - the twinning results due to the mother’s ovulation. Thus, it’s believed that twins run in families “on the mother’s side.” A mother who hyperovulates might produce twins. But would her twins also have twins? Or would it “skip a generation,” so that her grandchildren produce twins?
A basic review of genetics might explain why this misconception about twins skipping a generation has persisted. We’ve established that hyperovulation can produce twins. But only women ovulate. However, males carry both X and Y chromosomes, and can pass either to their offspring. So a man might be a carrier of the hyperovulation gene, and pass it on to a female offspring, who does hyperovulate, and might have twins.
Let’s look at a family example to understand this further.
First Generation: Adam and Eve. Let’s assume that Eve has the gene for hyperovulation. They have six children. Two of them are fraternal boy twins, Jesse and James.
Second Generation: Jesse and James grow up. They each produce three children, but no twins. Although they carry the gene for hyperovulation, they don’t ovulate. Therefore they don’t influence the possibility that their partners will conceive twins. <---- Twins “skip” this generation.
Third Generation: Jesse’s children; James’ children. These are the grandchildren of Adam and Eve. Jesse’s oldest daughter -- gives birth to fraternal girl twins, Thelma and Louise. James’ youngest daughter gives birth to fraternal boy/girl twins, Luke and Leia. Jesse and James may have passed the hyperovulation gene to their daughters.
Fourth Generation: What do you think will happen? The female offspring may have twins. Or they may not. The male offspring don’t contribute to the chances of their partner’s producing twins. But they may have twins or multiples for a variety of other reasons.
From this example, you can see why it sometimes appears that twins skip a generation. But certainly it’s not a hard and fast rule. With many other factors contributing to twinning, the hyperovulation gene is simply one factor that influences the process.http://www.nytimes.com/2007/10/02/health/02real.html