Sunday, May 31, 2009

Identical twins and heredity

The question came up again on an online forum. And, once again, folks hopped into the discussion with both feet, readily giving incomplete and misleading answers with confidence.
The question?
Can the tendency to have identical twins be inherited?
The most common answer and the prevailing theory is "no."
Most OBs will tell their patients that identical twins are fluke, an accident of nature, and that their chances of having another set are no greater than any other woman's chance of having identicals in the first place.
But how does that explain my neighbor's daughter and her family? She has three sets of identical twin boys who come trick-or-treating to our house every year.
And what about the woman whose daughter took dance lessons with my daughter? She is an identical twin and she has identical twin boys of her own.
Then there is the woman I met at a local bakery. She looked longingly at my boys (who were screaming at my attempt to get some coffee, unwilling to be pacified by cookies) and told me that her two daughters each have a set of identical twin girls.
It just doesn't make sense.
The reality is that scientists have no idea why some women have identical twins and some don't. Evidence does exist that many sets are flukes. For instance, there is no history in my family or in my husband's of identical twins as far as we know.
My side boasts a set of twins and a set of triplets way back in the olden days, but they were fraternal. My mother-in-law remembers a set of triplets birthed by a distant relative, but they also were fraternal.
So our boys probably were an accident of nature, an awesome accident.
But in other families, the frequency is too great for simple coincidence.
The journalist in me demanded that I do some research.
This is what I found:
A 2007 study, led by Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, shows that identical twins form just after conception when an embryo collapses and splits in two. She discovered this by photographing growing embryos every two minutes in a lab using special computer software.
Her evidence negates previously held theories that the egg splits after it leaves its shell immediately before implantation and that, therefore, identicals either shared a placenta or had individual placentas that grew close together. The predominance of the previous theory explains why my OB insisted that our boys were fraternal until they were DNA tested.
Our boys each had their own placentas, which grew on polar opposite side of my uterus.
They were born in January of 2007, just before Dr. Payne and her colleagues went public with their research.
Dr, Payne's discovery has opened new paths for research into the potential genetic impact on identical twins. A study is currently underway that proposes that a male enzyme is involved. Scientist already know that the enzyme causes the embryo to collapse, but they are unsure who secretes it or why.
The study, as far as I know, is not yet public.
But I found this post-- written by a graduate bioengineering student who is helping to conduct the study at an unnamed university--on a Yahoo forum ( She says that the study also suggests some women carry a gene that prevents the enzyme from splitting the egg, and that men who produce the enzyme do not produce it every time:

"Thus far, the research shows that the enzyme is directly responsible for causing the splitting of the chromosomes, which results in the division of the cytoplasm which results in two eggs! There are a few (about 1%) that have alluded us so far and have shows no sign of the enzyme despite the fact that twins resulted. Thus, we have concluded that identical twinning can also be a random event. But in about 99% of the people tested, the enzyme is apparently the culprit. So, 99% is a darn good yield! So, according to our research, it is not a small percentage but almost the entire percentage."

This part of the post explains why we know so little:

"And yes, this study has been repeated already, but very, very little money is put into this research, so most of it is done on our own time. The reason for this is there is little to no medical advances that can come out of the research, just info and most of the money on medical and genetic research is for improving the outcome of diseases, etc. So, that is why it may be a while before this research is completed. We have to get about 5% of the public who have twins to complete our research (not all will go through tests, some will just answer questionnaires) before we can say that this represents the entire population, so you can see that this will take a while!"

So that's it.
The answer is that there is no definitive answer because the people in medical community, or rather the folks who fund their research, are just not not all that interested.
For now, identical twins remain a mystery.


Lexi said...

this is interesting. But here is what I thought... couldn't they limit the result of higher multiples by learning about this enzym. Maybe I am just looking for a reason lol becuase I am interested. But, if an enzym is involved. Couldnt they in turn test for that on patients using fertility treatments that sometimes result in higher multiples because of identical splitting? I don't even know how high that possibility is, but just a thought lol.

Anonymous said...

O.K. the study you quote only used 33 embryos in an artificial environment.

Also, anecdotal reports of people who have multiple sets of multiples are really irrelevant unless those sets of multiples are confirmed to be identical. Because your neighbor's three sets of boys look alike does not mean they are. Fraternals can look very idintical.

Twinsmom said...

Lexi, I'm guessing that there is an awful lot they could learn from that kind of research, but the incidence of identical twins is still so low, that my guess is it will be a long time before we see any ground-breaking research.

Thanks for your input.
From what I understand, previous theories of identical twinning were based on guesswork. This would be the first time anyone actually saw it occurred. I'm not sure how many of those embryos split, but scientists would only need to see one to determine what happened. I suppose it's possible that identical twins might form in other way as well. But right now, this is the best explanation they've got.
As for the anecdotal reports, I supposed it's possible that the mom of three sets of identicals might just be assuming that they are identical, but I've had several conversations with her and she seems like a bright woman. Bright enough to know whether her twins are identical. And, I must say, they sure do look it. I suppose the other two women could have been wrong as well, but in both cases, they asked me whether my twins were identical before they mentioned their situations. So it seems that they know the difference. Again, they could all be wrong, but I think my point was that we still don't really know whether there is a hereditary factor anyway.
Is there some reason you believe that there is no hereditary factor involved? If you know of other studies, I would be grateful if you could direct me to them.

Java with the Johnsons said...

hey...that is very interesting...i also have identical twin boys...and always get asked if twins run in the family. maybe some day we'll know for sure...cuz someone will care enough to find out.

Unknown said...

I'm very inclined to believe the study. I can't believe identical twinning is just a fluke. I think there isn't enough interest in researching this subject. I don't believe there isn't a scientific cause for identical twinning. I have identical twin girls. There are identical twins on my side and their father's side. At this point I believe in the enzyme theory. Thanks for the information. I have read it somewhere before, but I'm unable to find the article.

GLH said...

One thing that needs to be considered is the occurance of twins from conception. There are plenty of people out there who conceive twins but don't actually end up birthing twins. I have one friend who has three children, all were confirmed twins at the onset yet each time only one survived to birthing. If they succomb early enough the remaining tissue will absorb into the surrounding. So truly every man could produce this enzyme and yet every women produce the antienzyme. And still it would just be a random fluke that we have the occurance of twins at all!

sapna said...

can the enzyme produced in the male be the cause of heredity where in even the next generation land up havin twins?

Anonymous said...

Dear Author !

imkrystal said...

Thanks for the interesting article. I have 13month old identical twin boys and my half sister (we share the same dad) has 14month old identical twin boys. I was never sure if this was coincidence or not until I read about the families you know of with similar 'coincidences'. I would love a set of identical girls next (crazy?), so hopefully I have a slightly higher chance of getting them :-)

Dianne said...

I have a set of identical twin boys, and my sister also has a set of identical twin boys. Perhaps this enzyme passes from a father to a daughter as well? The male enzyme theory is okay if brothers have sets if ID twins, but in our case, not sisters? Hmmm. Or is it possible the lack of the ID twinning enzyme "blocker" is the hereditary trait? Thank you for your knowledge and forum to bring up these questions!

imkrystal said...

If you're right Dianne, if the lack of the twinning blocker enzyme was passed from father to daughter, it may explain why some families claim that twins "skip a generation". Which means our twin boys, will pass the enzyme to our granddaughters who will have the increased chance of twinning!