There are a number of gray areas when it comes to twin pregnancy. Science doesn’t know all the answers yet but the theories are always intriguing. It is commonly accepted that fraternal twins run in the family, but this doesn’t mean we know exactly when someone will become pregnant with twins? Do twins skip a generation?
One of the most common questions I am asked is: “Do twins run in your family?”. I believe my Grandmother’s sister was a twin but I’m not sure we can claim a generational tendency. My boys are also identical and it is widely accepted (although many argue otherwise) that identical twins are not hereditary. Still, my older sister told me years before they were born that I was destined to have twins. She said: “Twins skip a generation and no-one in our generation has had them yet!”.
I thought it was time I delved into this one… If you’re a twin yourself, is it twin birth or twin grandchildren you should be preparing for?
How Expressed Genes Skip a Generation
Before tackling the chances of twins, it’s worth understanding why any trait may skip a generation. Genes are a section of DNA assigned to a particular trait. Variations in this piece of DNA are called alleles… eg A blue-eyes allele or a brown-eyed allele for the eye color gene. Some of these alleles are dominant and some are recessive, meaning you can silently carry a trait without physically being affected by it.
Red hair, for example, is a recessive allele and therefore requires both parents to pass it on to their children for it to show. This means two parents without red hair can create red-headed offspring, because they could both be silently carrying one recessive red hair allele. Hence, red hair is often skipped one or more generations before it shows again.
For more geeky info on DNA, check out my post: ‘Do Identical Twins have the same DNA?‘, fascinating stuff!
So the Twin Gene IS Recessive?
No. There is no actual twin gene. Fraternal twins are the product of hyper-ovulation; releasing more than one egg at once. The tendency for hyper-ovulation is down to variations in genes which have been passed on from parents, but it is not dependent on one single gene and there is no reason to believe it relies on a recessive allele.
Twinning is more complicated than the red hair example above. There are multiple reasons why a woman may hyper-ovulate, and age is also a factor. Don’t forget that not every egg released will become a baby, and therefore even with the allele for hyper-ovulation, a woman is never going to be sure she will have twins. The chances are increased but nothing is certain in conception.
Most importantly, we have to remember the boys. What happens when a man carries the hyper-ovulation allele?
Is a Male Twin More Likely to Have Twins?
Here’s where your head starts to spin. Not only have we got all the complications of conception, but we also have to consider those silent genes again. An allele being recessive is not the only reason it may not show up in a child. A man will not show traits he is genetically carrying which only apply to women, even if they are ‘dominant’. A man is not going to hyper-ovulate because he doesn’t have ovaries… even if he carries the hyper-ovulating allele.
This means a male twin (or singleton) carrying a hyper-ovulating allele is no more likely to have twins than a man with a regular ovulation allele. People often say twins run on the mother’s side of the family because only Mom can release two eggs. However, Dad can pass on his hyper-ovulating allele to his daughter, and she will be more likely to have twins. So in this case, twins may just skip a generation!
So… Do Twins Skip a Generation?
Sometimes, not always, and not in any identifiable pattern. The hard fast rule of “twins skip a generation.” is a myth. There is no reason a generation-skipping pattern would emerge in a family, but there are inevitably gaps in this complicated hereditary trait. Twins will continue to appear down the family line until the lottery of genetics chooses to stop passing on that hyper-ovulating allele.
Twins may skip one, two, three or more generations before they appear again because many different factors have to come together to create those two bundles of joy.