Causes of Identical Twinning

Causes of Identical Twinning FAQ

Is there a genetic component to identical twinning or is it a random act of nature? Well, it looks like the jury is out on this one but there appears to be some new research in this area. Here is a compilation of what was discussed on the twins list. -- twinfaqs@yahoo.com

From: MFoley@lnd.com
Everything I have ever read has indicated identical twinning is a random event, although I seem to recall some discussion awhile ago regarding anoxia (loss of oxygen ?) and identical twinning in lower animals.
That being said, on my maternal side we have had 3 sets of identical twins (each skipping a generation). LOL, that is as far as we can trace back Probably the most interesting aspect of this is that each set has been male, and they look remarkably similar. My husband and I both have red hair, but our ID twins have very dark, almost black hair. I saw a picture of the next oldest set of ID twins in our family and couldn't believe it. They were the spitting image of my guys. Maybe this is a recessive gene thing :-)
Mary

From: mindy@vegas.es.hac.com
Here is one story from my husband's workplace. A woman that he works with claims to be one of ID twins and her father is one of ID triplets, in fact the first ever ID triplets in NJ! The woman also has two sister that are pregnant with twins (frat or ID unknown at this time) right now! She is does have an internet email address. I could ask her if she would answer any questions you might have if your interested??!
Mindy

From: <cembar@infobahn.icubed.com>
Identical twins in almost all cases do not run in families. Of all identical twins in the world, there are probably only a handful of families in which it can be said to "run". And geneticists do not know why.
The reason identical twinning does not run in families is because, in point of fact, identical twinning is a case of "mis-splitting", it is an accident. In the early stages post-conception, the cells divide and divide. In the case of ID twins, during one of these divisions, the ball of cells breaks completely apart and forms two balls of cells and, in turn, two individuals.
If there are 2 sets of identical twins in your family, that in no way means that it runs in the family, it's just chance. In order for it to run in a family, there would need to be many cases of identical twinning among first degree relatives (e.g.: you, your mom, your sister-not, however, your brother's wife). Even if a woman has 2 sets of identicals herself itis most likely chance. Also id twins have no greater chance of having identicals than anyone else, although once a woman has carried twins, even id twins, her chances do increase slightly of having another twin pregnancy because her body is "used" to carrying twins. Of course some women who ovulate more than once a month are far more likely to have more than one set of fraternals.
And that every other generation thing is just myth.
Ayn

From: jacobsk1@westatpo.westat.com
My paternal grandmother was a fraternal twin. My father is an identical twin. It looks like mine will be identical (based on sonograms) And, I have a set of cousins (mother's side) that are identical
It isn't enough to be statistically "interesting", but the odds must sure be pretty low! Have any RECENT studies been done in this area to confirm the "fact" that heredity doesn't play a part with identical twins? It just seems odd to me that there would be a predisposition to release multiple eggs for some families, but not a predisposition for a single egg to "mis-split".
Kathy

From: <kmoutsos@bih.harvard.edu>
Wow that's really interesting.
My husband has an identical twin brother and our boys are identical. We're always hesitant to mention this because people assume it's hereditary and then they mention that they thought it skipped a generation.
A paternal first cousin of mine and his wife have fraternal girls. My mother has mentioned that her father,who had twin sisters who died in infancy, told her that if she ever had a daughter, she would have twins. His prophecy came true but if there was a hereditary link my mother should have been the one with twins.
Kelly

From: <handerso@suffolk.lib.ny.us>
A co-worker of mine just told me the other day that her niece is pregnant with twins. Her niece's father is an identical twin, the niece's aunt (on her mother's side has identical twins, and the niece's husband is a fraternal twin. Well, according to the theories, none of these twins in her family should have influenced her chances of having twins (identicals aren't supposed to be hereditary, and the father being a fraternal twin should have no influence on the number of eggs his wife ovulates).
It will be interesting to see if she has fraternals or identicals--judging from what I have heard from others, I would be surprised if identicals are as random as 'they' claim.
Heather

From: <cembar@infobahn.icubed.com>
Sorry, but twinning (of the fraternal type) is only passed through the mother's line. So your grandfather and father being twins (not to each other, but you know what I mean) is in no way related, nor is your own instance of twinning.
If identical twinning did run in families, since there are more people today than say 50 years ago, we would also see an increase in identical twinning, yet the rate for ID twins has remained fairly constant (about 1/0 births) over the last 100 years. During the same 100 years the rate of fraternal twinning has doubled (thanks mostly to IVF and fertility medication).
If there were some sort of strong genetic component to identical twinning, women with ID twins would be more likely to have a second set of id twins-this rarely happens.
I don't know of any current studies, off the top of my head, but if you have a medical school near you, you could go to their medical library and glance through some books on multiple pregnancy. The whole issue of the genetics of it is usually covered in one of the first few chapters. If you have Web access, MedLine is also available and you may be able to do a search.
If there is a genetic component to ID twinning, it is very likely to be a recessive gene, or, like I said, the ID twinning rate would go up.
Ayn

From: <alsup@gate.net>
I was always positive that id twinning was genetic. Is there anyone on this list that has id twins randomly while taking fertility medicine?
Diane

From: LangdonGV@aol.com
"Has there been any study to suggest that maybe it's the sperm's doing in the case of ID twinning? :-)"
To the best of my knowledge there has not been any proof of such but my theory of id twinning from the paternal side it MEGA, SUPER sperm.
So....can you see/hear the talk.... male ego super boost when that report comes out.
Nancy

From: JGorger@aol.com
I am not contradicting you at all, but just to let you know, there has been a ton of research done on this subject at the universtity where I am getting my masters in Bioengineering. The theory we are all sitting on now is that there is an enzyme in the male sperm that causes the embryo to split into two separate embryos, resulting in identical twins. I was so fascinated by this and it makes a lot of sense. This does not exclude the theory of random splitting at all (as you alluded to in your post) b/c there is evidence to that also. But, for the families that feel that I/twins run the in the family, there is something disctinct about the sperm. It is so fascinating.
This proves, if the enzyme is the culprit, that there is a genetic or familial tendancy toward identical twinning. My husband has identical twins on his side and my grandmother is an identical twin. This may also be the reason that some identical twins seem to skip a generation b/c the man has to pass the enzyme gene onto his son, he would never pass it onto his daughter,since it is in the sperm and therefore the "Y" chromosome. I subscribe to this theory when it appears that twins/multiples are running in families.
Also, a man could have this gene but never give rise to twins (they have seen this already) b/c there is something in the woman's egg that basically stops the enzyme from splitting the egg. Interesting huh! This strange stopping gene in the female may also be responsible for I/twins skipping a generation b/c one female may have the gene but the next generation female may not and thus the twins come out. I love this stuff. It gets really interesting from a iotechnical point of view. Sorry I ranted on about it.
I thought that since you were so knowledgable on the subject you might be interested in this info. Again, it is just a theory (not like fraternal twinning which is a biological fact of more than one egg) but a fun one and we have had a great deal of success in our studies.
BTW, this is not even what I am doing my thesis on, but I am so interested in it (for obvious reasons!) that I am always going into the lab and in on the studies. Cool huh!
Take care and thanks for helping educate people. The more they know, the better chance we have of less people running around saying, "Are your B/G twins identical?" LOL!
Jamie

From: JGorger@aol.com
If there is indeed a genetic component to identical twinning, it would NOT make the rate go up, as there is no way that we as genetisists and biologists can influence the way that genes work or increase or decrease their frequency! Identical twinning could be remaining constant b/c there is a genetic component, not just because of chance. Also, if this is so, women would no more have a better chance of having another set of identicals any more than they do of fraternals. MOST of us do NOT have two sets of fraternals any more than we have two sets of identicals. It is vastly b/c of the genetic component (according to research done by us and others) that makes it a constant thread in history, as would fraternals had infertility treatments not become commonplace! Just for your info.
Jamie

From: MFoley@lnd.com
Are you saying the "1-in-80 births and 1/3 of that" numbers we always see for identicals can't be random because of its consistency? I admit to wondering about that. It would seem to be if it were truly a random event there would be "bumper crops" of ID twins some years and none or nearly none others.

From: <motfot@tdl.com>
>> Sorry, but twinning (of the fraternal type) is only passed through the mother's line. <<
This has not been proven. It is possible that a male can be a carrier of the genetic basis of multiple ovulation in a woman and may be able to pass it on to his daughter. I don't think that it is known how or why some women tend towards multiple ovulations (the specific gene has not been determined) but it *seems* to be genetic for fraternal twinning. Consider that females carry the male baldness trait and the hemophilia gene that get passed on to their sons.
>> The theory we are all sitting on now is that there is an enzyme in the male sperm that causes the embryo to split into two separate embryos, resulting in identical twins..........Also, a man could have this gene but never give rise to twins (they have seen this already) b/c there is something in the woman's egg that basically stops the enzyme from splitting the egg. <<
I have heard about this and it's interesting to know that there is more research being done on this theory! It really is a fascinating topic! But the fact that there has to be just the right combination of male/female complimentary enzymes for identical twins (or higher) to result seems to contradict the 'running in families' idea, doesn't it? Each male would have to mate with the 'right' female to get a series of monozygotic children in a family.
Hmmmmm...
And one thing I've wondered about is why does this splitting typically only happen once? I know there have been identical quadruplets (saw them on Oprah :) so it has happened that the first fertilized egg keeps splitting and splitting and .... but there are many more twins than identical higher multiples.
Wonder what *stops* the splitting from occurring more than once? Especially if the enzyme is still present?
Terri

From: mporter@world.std.com
I know of 2 couples who had identical twins via IVF.
Aliza

From: PPFQP@aol.com
Jamie: thanks for the info on the sperm enzyme -- is there a test that theydo to see if a man's sperm contains that enzyme?
Here's our story. My grandmother is a fraternal twin (she has a twin brother). She has fraternal twin cousins (via her mother's sister). My mom had a twin pregnancy and we believe that I started out as half of a twin pregnancy. She had
some sort of testing done in the 70s where the doctors told her that she "hyperovulates" or something -- releases more than one egg a month. That is supposed to be a genetic tendency that can be passed from mother to daughter.
So here I am. If all the theories hold true, I'm right in line for fraternal twins: "running in the family," "skipping a generation," etc. When I got pregant there were also some other factors that are supposed to be conducive to fraternal twins: getting pregnant right after going off the pill and getting pregnant after a long period of abstinence (hey, my husband was out at sea!!!)
But my girls are identical (monochorionic, one placenta) -- how did that happen????
Okay, so what's the prediction for my girls? Assuming they've inherited the tendency to hyperovulate from the genetic connection through me, my mom, my grandmother, etc., then will their dad's "super sperm enzyme" carry into them in any way? That is, if they have sons (twins???), will they pass that sperm enzyme onto them, and then their sons could go onto have identical twins????
We think having identical twins is a beautiful, incredible blessing, and I'm not sure I ever want to truly know *why* identical twins happen, as it might diminish the mysteriousness of it. But it sure is fascinating!
Pam

From: <cembar@infobahn.icubed.com>
While it's true that one set of IDs don't increase your liklihood of another set of IDs. One set of fraternals does. There is even a woman in my club who had two sets-both conceived w/o IVF or drugs. The majority of women who spontaneously conceive fraternals ovulate more than once, at least during some months. This, in and of itself, increases their liklihood for fraternals. It is also true that even carrying IDs increases your chances for fraternals, but only about 3-4% over the general population, simply because your body is "used" to carrying twins.
Ayn

From: JGorger@aol.com
Mary,
No, what the genetic component means is that twinning is consistent throughout time/history. This really means that bumper crops will NOT happen. As long as there is no inbreeding rates will remain relatively constant. It is more likely that random events such as mutation which is INconsistent would allow these "bumper crops" to appear on occasion. This is basically genetic theory, not really our own. I am as fascinated as you!
Jamie

From: JGorger@aol.com
Hum, Terri, you got me! It seems that this is why identical twins may run in families, but not every time. This gene may actually be dominant, not recessive, but like you said, in order for I/twins to run in families, you May need the right combination. But, you may not. we have seen an absence of the female component and still seen identicals run in families. As for higher order multiples, the consensus around here is still basically one of enzyme-out-of-control! That is, it may be that the enzyme, in rare or few circumstances, does not know when to stop. To be slightly technical, genes are either turned off or on. Each gene has a specific purpose and in higher order multiples, the gene that turns on or off the "enzyme" gene (these genes are called regulatory genes b/c their entire job is to turn genes off and on) may be messed up and not know when to turn off and thus the egg continues to split, i.e. Dionne Quintuplets or those gorgeous -year-old identical quadruplets often seen on Oprah and Maury Povich!
We are still learning so much. It could take years or more until medical science actually accepts our research. There is not much energy put into it b/c no one really cares and unless there are medical advancements that can come from it, research is largely ignored. But thanks for the support!
I amso glad that SOMEONE is interested in this research! What better place to sound off our theories than this list!
Jamie

From: JGorger@aol.com
According to our research, having ID's would increase your likelihood of having ID's again as much as fraternals...but like I said, MOST people dont have more than one set of twins. Of course, some do. We have, out of severeal hundred in our club over time, only two women that have had two sets of non-IVF fraternals. You can see that the liklihood of having either a second set of Fraternals of Identicals is small, very, very small. This would only probably be true for Identicals if the woman had all her kids with the same husband though, unless the subsequent husband also carried this enzyme gene. Of course, this info is only in the research portion, not yet a fact, so please bear in mind that this is still hypothetical. Although, I firmly believe that it will eventually be proven beyond a doubt and that our research team will be famous for inciting a new theory regarding the inception of identical twins!
Jamie

From: JGorger@aol.com
Yes, we are in the process of publishing uur findings. I would say that probably within the next year or so you would be able to find it in a research library, such as at a University near you. Also, once it has been more accepted, you might even find it in Twins magazine! We will try to have the researh published in laymen's terms in Twins magazine in a year or two.
It is exciting. I will keep everyone updated as to our progress. I am sort of surprised that everone is so excited! I had no idea. I thought, for some stupid reason obviously, that people would find this boring! I misjudged everyone. Sorry!
Jamie

From: <cembar@infobahn.icubed.com>
I agree with you about the fact that id twinning having a genetic component may not increase its frequency, however, that would depend upon what mechanism id twinning occurs by. As you know, several diseases, which have a genetic component, when introduced into a population run rampant, others do not.
The work in the lab at your Univ. sounds interesting. As you wrote, they are trying to publish, let me know where, our medical library has just about everything. It will be very interesting to see what happens and if the study can be replicated. So many things in genetics are seldom what they seem. However, I would think that id twinning running in families is still uncommon. I think it would be misleading to tell people that 2 sets of ids in a family or the fact that your grandmother was an id twins means that it runs in your family.
Ayn

From: <cembar@infobahn.icubed.com>
Jamie: You seem to be far more on the cutting edge than I am about this ID twinning thing-that's what I get for relying on OB books (even the 96 ones) and high risk pregnancy books for my info. If you have access to even a small part of a biblio., could you e-mail it to me? If you have one on your computer, you can attach a file to e-mail that you send. I just can't be out-of-date!!!
Thanks, again, you're a wealth of information.
Ayn

From: <steltech@icon-stl.net>
Just food for thought......
One of our babysitters is an identical twin. Her parents divorced; her father remarried and had ANOTHER set of identical girls with his new wife! Honest this is true....I've seen all of these girls!
I know everything is circumstantial, but something tells me that little spermies can have *ALOT* to do with identical twinning!!
Kim

From: JGorger@aol.com
I am on the cutting edge only because it is still research. I cant wait until my name is associated with this research some day but there are still many scientists, embriologists and genetisists who do not believe in our theory, which, as you know, may ALWAYS be the case! Not everyone will agree with our research even when it is scientifically accepted. Also, this research, even though I am giving you some details, is mainly not public info yet. That is, no one except the university and we researchers have access to the details of the study. So, what you see here is all you can get right now, but never fear, our results will be public someday, whether or not they are finally accepted. Thanks for supporting me on this, it is excting for all of you to have taken such a keen interest and no, you dont have to believe what I am telling you! Not everyone will subscribe to this theory and that is fine, that is what research is all about! But, thanks for asking questions!
Jamie

From: JGorger@aol.com
I never told anyone that just because twins run in my family that it means that all twins run in the family!!! I was not being misleading at all, please go back and re-read my previous posts. All I said was that the studies we are conducting, which include hundreds, at this point, of families where there is more than one set of identical twins in the family (not necessarily the immediate, meaning same parents, but grandparents, cousins, brothers, sisters, etc.) conclude that twins do indeed run in the family. I suppose you just dont realize that studies are not done on hunches, like "hey, twins run in my family, so it must be the truth, let's try and get everyone to believe it!" They are performed via tons and tons of research and then once a thesis is established and accepted by the university, then we are funded to continue the research with human (if that be the case) subjects.
No one decided, certainly not me because this was not my research project and was not started by myself, that just because identical twins run in my family, that we should do a research project on it.
Jamie

From: JGorger@aol.com
<< Now, does the research seem to indicate that this enzyme plays a role frequently in id twinning, far more frequently than geneticists have believed up until now, or is this still for only a small precentage of id twins?
Also, you said the chance of repeat ids is as great as the chance of repeat fraternals, yet both you and I know women with 2 spontaneous sets of frats. (Of course, that's hardly a statistically sig sample) But,
I've never met anyone with 2 sets of ids, nor have I heard of it. >>
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 darm good yield! So, according to our research, it is not a small percentage but almost the entire percentage.
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 reasearch (not all will go through tests, some will just answer questionairres) before we can say that this epresents the entire population, so you can see that this will take a while!
I have been working on this theory for some time. Ask people on this post nd I guarentee that you will meet peolpe who at least know of someone who as two sets of identicals. I know 2 people out of hundreds that have more han one set of fraternals and about that many in our research that have two ets of indenticals. There is a woman in Australia that has three sets of identicals (yes, with the same hubby!)! I think that hyperovulation is more frequent than identical twinning because it depends entirely on the woman and not on the husband, but identical twinning can sometimes be related to both, about 90-10 to 80-. It may also depend on many different things. The divorce rate for parents of multiples is very high. Identical twins tend to have more birth complications that fraternal twins, so that also can incease the divorce rate, as parents of sick children/babies seem to divorce more readily. If that being the case, the husband that caused the identical twinning is no longer in the picture and twins thus may not recur.
Also, many, many parents who have twins do not have more children, for obvious reasons. This also, that this enzyme may or may not manifest itself at every conception. There could be numerous reasons that the gene that turns the function or production of this enzyme on or off, as in diet, drugs, health, etc. There are so many factors and we have only scratched the surface.
It will be years before we can tell you what drugs, etc. influence the expression of the gene or the action of the enzyme.
Also, we see this enzyme in different races more frequently than in others.
We very, very seldom see this in the black race, but see it almost entirely in the oriental race. We see it about 50-50 in the caucasion race. It is almost non-exsistent in the Jewish race. So you see, many factors can increase or decrease the frequency of twinning for identicals, just as it can in fraternals. There is so much more resaearch to be done, that we have only scratched the surface of this, as you can see. Our research is in the preliminary stages.
Hope this helps.
Jamie

From: <etosie@eto.ericsson.se>
I bought my second hand stroller from a girl with two sets of fraternal twins (ant thwn she tied her tubes!). Her sister had also two sets of fraternal twins. They ahd been tested, and it turned out that they both had ovulation on both sides at once, and only every second month! Interesting!
This means also that to have two sets of twins is more commen with fraternal that with identical. It probably is not so that if you are dispositioned to have identical twins, it is the ONLY thing you can get. But who knows? I agree - there is so little reseach being done on this!
Maybe one 'part' just dissolves after a day or two. I have heard some statistics of how many 'twins' that actullay are assumed to be there the first days/weeks...bur before anyone notice they 'disappear'....
siri

From: JGorger@aol.com
You are very accurate in your uderstanding on the subject of more twins are actually conceived than born due to the vanishing twin syndrome and other reasons. It is my own personal (not theoretic/research oriented me) opinion that we see more fraternal multiples run in families (same mom/dad or immediate family) for a few reasons.
One is that hyperovulation is more common in women than the enzyme is in men. Which would mean that more women hyperovulate than men who produce this enzyme. Again, other factors are involved in both like age, diet and drugs,etc., but more than 50% of ovulating women (women that ovulate normally for the most part) actually hyperovulate, but they may NOT do this every month, or every year or whenever, depending on age, diet, drugs, etc....you get it by now! It is difficult for me to generalize the subject and I am glad people are bringing up other influences which are greatly important to our research , Dont worry, we wont leave these important isssues out. They were well established at the beginning, but we only concentrate on one thing at a time, called a controlled experiment.
Hope this helps. Please feel free to respond in like form!
Jamie

From: <brei@edcc.ctc.edu>
Thanks to everyone who responded to my request for anecdotal information about identical twins in families. My students were very interested in the response. In my further investigations on the matter I've come across one source that admits there appears to be a *tendency* to have identical twins in some families. Not sure what that means. Also, I came across a third type of twin where it appears one egg splits then is fertilized by two sperm. Thus boy/girl twins could result who would share an unusual amount of genetic material. I know a set of boy/girl twins who are "look-alikes," but maybe they come from a family where all the kids look alike. In mine, we scarcely look like we're related. Hmmm.
Jamie, I'll look forward to seeing the article about identical twinning in TWINS.

From: <motfot@tdl.com>
FYI, I recently read an abstract about observations of spontaneous monozygotic twinning in a 'test tube' (preparing for an IVF procedure).
It was observed that the covering of the egg (the zona pellucida) had surface textures that could be divided into 2 different types; a smooth, firm, thicker surface layer and another that was 'spongy', relatively rough, and thinner. Those eggs with the thinner, spongy surfaces were found to be more easily fertilized (the sperm had no problems entering through the layer whereas the thicker layer was more resistant). They were invariably the ones that resulted in twinning--though not every time. Maybe this is the feature that, combined with the male enzyme, nearly guarantees monozygotic twins?
Mechanically, it would make sense that the thinner (weaker) surface might 'fail' which could cause splitting.
Terri

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