could it not be possible for an egg to split into two (like chicken eggs sometimes do) and each part be fertilised by one sperm?
some twins are very similar but not identical. could this be the explaniation?

This chimerism phenomenon has been related to "vanishing twin" phenomenon, where an embryo or fetus dies in the womb and disintregrates, often leaving no trace, but there can be signs on the placenta. In this case there may have been a vanishing triplet, a female, whose blood and stem cells carried in the blood, mixed with her identical twin brothers' blood before she died and faded away. The sole survivor of a twin pregnancy may carry the genetic code of his or her twin in blood or other tissues. I am surprised that this eventuality was not mentioned in the report because it is a well-known cause of chimerism.

Althea Hayton, Project Director, wombtwin.com.

I am a fraternal twin butI look so like my sister that even close granchildren do double takes. We played the twin pranks as kids & never got caught. I wonder in like of this new discovery if we are in someway a similar circumstance to the semi-identical twins. We are 63 now but still look very much alike.
Want more information on who to talk to about this occurrence. Or maybe a e-mail address of someone in in authority.

I found this article extremely interesting as I have always believed my twin daughters were not identical or fraternal but somewhere inbetween. They were declared fraternal after examination & testing of the placenta (there was only 1 placenta at birth). My daughters who are now teenagers look
very similar - same height, same hair colour, same teeth, similar interests & talents. However there are slight differences in their looks
that their close friends and family can see. I wonder how common this new discovery of an inbetween fraternal/identical twin is and if it is possible to find out for sure.

I'm wondering if this is really as rare as they seem to think. I also believe my twin and I are in this category. We are "fraternal" but look very much alike, much moreso than we look like any of our other 11 siblings. We did a twin study with NIH many years back and scored almost identically on their barrage of tests. Unfortunately the study did not include DNA testing. If someone wishes to contact me about this, you are welcome to.

My twin boys were a surprise to me and my husband. I didn't know I was having twins untill the other one was born.But when they were born there pediatrician insisted they were identical, and I thought they had some differences between them. One twin is a little heavier than the other and different head shapes,but they still insist. We had no testing done because we really didn't care as long as they were healthy.The twins were also born in one placentea,so I wonder now if they are semi.

Excuse me if I didn't understand the article well, but it's not clear to me the uniqueness of these chimaeric twins. In fact, the article doesn't even mention if they come from one or two placentas. As I see it, they're two chimaeras (rare case, granted, but unique?)

I am wondering about my girls to... they look just alike, I was told they had to be frat... but they are the same high and same weight... but one of them has a hemangioma and the other does not...

also Althea Hayton, Project Director, wombtwin.com there is no way for a female to have a male idenical twin... they would not be idenical then... it would have to be frat :-)
Michelle

Michelle,
I think Althea Hayton was saying a triplet that has one fraternal female baby from one egg, plus two identical male twins from the other.

I believe that me and my twin sister also fall into the "semi-identical" twin category. My mother was told that we were identical because there was only 1 placenta. My twin sister has dark hair, brown eyes, dark complexion and a petite build. I have fair complexion, green eyes, light-colored hair and a larger build. I look just like my Father and my twin sister looks just like my Mother. I am 5" taller and 50 lbs heavier than my twin. We always tell people that we are fraternal twins because we look so different and no one would believe us when we said that we were identical twins. We are also COMPLETELY different in our careers and our hobbies.

I have fraternal boy/girl twins that are 6 years old. I also think there is another "catagory" of twins. My boy twin has a missing bottom lateral incisor tooth and my girl twin has a double bottom lateral incisor. I have tried to research this several time since they were born with no luck. I was thrilled to see this article because it shows they are finally admitting that it isn't as simple as either fraternal or identical. I have always thought that it probably occured when an egg split and then was fertilized by 2 sperm.
My twins look like siblings but do not look very similar. Both are blonde but one is platinum and one is sandy blonde. One has very light blue eyes and one has blueish-hazel eyes. They are the same height and weight. One twin is right handed, the other left handed.
Any thoughts?

Two things... 1) there are any number of twin pairs out there who are certain they are "fraternal" because the doctor said so because they were dichorionic, but who think they look too much alike to be dizygotic because they are, because they are actually monozygotic and their doctor just doesn't know that monozygotic twins are often dichorionic [twins of African ancestry, ~50%; European ancestry, ~33%; Asian ancestry (including 'Hispanics')~18%]. Plus, how many times have you heard things like "She looks just like her sister (or even her mother) did at the same age!"? Well, most cotwins are actually the same age... We also now know that monochorionic twins are sometimes dizygotic, and most of us have long known that “a single placenta” means nothing –roughly half of dichorionic placentas arrive fused.
2) on second thought about my comment that Mr Whitfield included in this note: these kids are chimeric, which was discovered only because one of them has unusual sexual development. As I have argued, also in Human Reproduction [21(3):579-591, 2006] there are a great many more such people, at least ten times as many sole survivors as twins born together, who will never be found because they're perfectly normal and will never be investigated the way these children were. And that crowd might actually include more of this maternal pronuclear endoreduplication type of twin pair... so it might well not be as rare as we believe it is. I certainly should know better.

Does this mean that to male doners were involved or that 2 sperm form the same male cause this?

I have twin daughters and found this very interesting becuase we are not sure if they are identical or fraternal. We have not done DNA testing because of the expense. Our girls look a lot alike but different too. The weird thing is my husband is a twin, his father, his grandfather but supposedly twins doens't run on the father's side. Explain this VERY coinsidental case! We believe the father's side produces a "supersperm" which tells the egg to split. What do you think? Very intersting stuff, I believe there is a lot to learn about twins!

One thing to remember, if you have identical twins, they have identical chromosomes to start with in utero, but a lot happens in the womb with horomones and so forth that they dont' turn out exactly the same all the time. YOu can have identical twins and they look a lot different becuase of what takes place AFTER conception. Hope this helps!

Is there a web site or group out there about this subject? I am finally close to getting some answers to my questions, I don't want to start over again! Thanks.

This is certainly not a new discovery - while it may be rare. My pediatrician told me that my female twins were of this "third" type 21 years ago!.

My twin brother and I were told that we could be semi identical back when we were teenagers. We're diferent in height but very similar in looks. We drove our 2nd grade teacher crazy. Our dentist found a cavity on the same tooth.

My twin daughters are 24 yrs. They’ve had no DNA testing. It wasn't clear, at birth, whether they were identical or fraternal. They also have a strong attachment to each other and have felt each other's physical pain with injuries. It has been suggested, several times, that my twins fall into some 'other' category. Physically, they're remarkably the same and yet there are subtle differences. In addition, they bear scarce resemblance to their dad.

My mother said that there was only one placenta when she gave birth to my twin sister and me. She attributed it to a two-headed sperm fertilizing one egg. That was 54 years ago. My sister and I definitely do not look alike, and I have always assumed that we were fraternal and that my mom was wrong about the placenta. After reading this article, maybe she wasn't wrong.

Excuse me for being thick but does this suggest that the zygote contains 76 chromosomes at some point? If this is so how is it postulated that two functional diploid cells result?

I was wondering about those twins immunity... Do they have one side reaction agains the other, like a weak autoimmune?

My twin and I are 44 years old,Fraternal twins as there were two placenta. However, we are identical in looks, likes, behavior, and personality. Our children would get confused as little ones when they thought they were talking to their mom and it was their Aunt. We also each have a son who were often mistaken as twins.

I have always questioned whether or not we came from one egg, fertilized by two sperm. It is the only way I could figure out how to explain the two placentas.

WE have always wanted to have our DNA tested but don't know how to go about it or if it is even worth it.

Slightly related to an egg that divides, isn't it interesting with all that's now know about fertilization and development that if it were possible that an unfertilized egg were to develop into a healthy baby that it could never be male. Of course what I was getting at is the so called "virgin birth" of christian doctrine held by many churches. No combination could produce and XY male from an unfertilized egg that spontaneously develops into a baby (not that it has ever been documented that spontaneous development in humans is even possible). Remarkable since none of this knowledge about genetics could have been known 2000 years ago aand only became started relevant for out times.

I think my sister and I may be this!!!

I have been searching the internet for studies regarding semi-identical twins, polar twins, or half twins. My sister and I were told we were identical when we were born, but looked nothing alike. As we age (almost 23) we begin to look more alike. We would both like to be tested to find out for sure what category we fall into. Has anyone found a study that we could participate in?

Hi, I am an "identical twin" but have always believed that my sister and I are NOT identical. My mom gets so upset when we say this because she says she had us "tested" and there was 1 placenta. We have NEVER been able to fool ANYONE at all! Of course if you didn't know there were 2 of us, you would most likely think we were the same person if you saw us seperatly..but only because of "similar" characteristics..Blonde hair, roughly same height...roughly same weight...same eye color. How IDENTICAL are twins SUPPOSED to be if they are identical? Will they share the same allergies, same medical conditions? My twin has always been bigger than me. Her body shape is completly differnet with no waist while I have a very hourglass figure. I am 1 inch or more taller, and she wears 1 1/2 size bigger in shoes..The oddest thing..parts of the body you would think would be EXACT are not..hopefully this isn't cut out, but our chests are completly differnt, and this always weirded me out. She got her period almost a year before me..and we are really NOTHING alike. I have a lot of medical conditions that she does not have..and she is extremely talanted with art and drawing and painting..I have no such talents. ALSO, everyone alwasy has told us that I look JUST like my dad, and My sister looks like my mom. My twin also resembles my mom when she was younger..but I don;t look anything like my mom's childhoos pictures/ If anyone has ANY info on this, or would like to contact me..please feel free.

Thanks.

It is my sincere hope the hermaphroditic twin is not surgically assigned either sex before s/he reaches an age of informed consent. Too many intersexed and hermaphroditic individuals have been misjudged and mistreated for too many years. It is the individual's own choice, not the choice of poorly informed or uninformed parents, and certainly not any doctors' decision.

As a parent of twin girls, I've always been interested in the subject of twins and have kept information I have uncovered about the subject. I have before me a 20 year old edition of Newsweek magazine where they featured stories about twins including what they termed as half-identical twins. They stated it was a rare type, occuring when a precursor to an egg splits evenly and is fertilized by two sperm. They went on to state that the fetuses have about half of their genes in common - those from the mother. So, I'm a lititle surprised that this in-between set is being touted as a NEW discovery when it sounds like the same topic discussed 20 years ago.

In any case, the subject will always be fascinating to me and I'll keep this latest article tucked within the yellowing pages of my Newsweek "All About Twins" article.

Janice Williams

Many of the comments here from people who are confused about twin zygosity talk about 'one placenta' or 'two placentas' ... Separate (dichorionic) placentas fuse into a single mass about half the time. The placental mass, whether one or two, carries no information at all.
What matters is the state of the membranes - the amnion (thin, inner membrane) and the chorion (thicker, outer layer of the membrane).
A single chorion has long been considered proof of monozygotic twins - we now know that is not absolute, but it is still strong evidence.
On the other hand, two chorions, whether they're on a single placental mass or separate, is true of almost all dizygotic twins, but it is also common in monozygotics.

So... Much of the confusion comes from doctors saying "these twins are 'identical' because there is a single placental mass, or "they're 'fraternal' because there are two placentas.
Both of those statements might be right more often than not, but both of them are also often wrong.
If twins come in a single chorion, they are probably monozygotic. If twins are of opposite sex, they are almost certainly dizygotic. If twins are same sex and dichorionic, testing will be required to decide.
And the testing can sometimes also be wrong, because mixed cell types don't have to show up in blood or skin or any given tissue.

Like so many others who have commented, I have fraternal twin girls who are very much alike. Strangely enough, my sister also gave birth to fraternal twin boys who are very much alike. Both sets look very much alike; same hair, height, weight, body build, same interests and talents. I found this article very interesting because I read an article 20 years ago that stated scientists have suspected there might be such a thing called semi-identical twins. My sister and I have joked over the years that our twins must be just that, semi-identical. It just seems strange that so many "fraternal" twins can be so much alike but with enough differences that they are not exactly identical. I wonder just how rare semi-indentical twins really are.

The connection to chimerism is fascinating. Some of us believe that the documentary, "I am my own twin" which describes two women with 2 sets of DNA...could explain gender identity confusion, and homosexuality if one of the twins is male and the other female growing up as one individual. Do other agree that this is so? Anita

I also have twin sons, supposedly fraternal, who could pass for identical twins. I'm also pretty sure that I read the same article in Newsweek 20 some years ago. My grandmother on my dad's side gave birth to twins, which, as I understand it, gave me a better chance of having twins. This made me think that the boys are most likely fraternal, but then again they have always been so similar in looks (people have always gotten them mixed up), body type, and yes, even their teeth were pretty much identical, that I couldn't help but wonder. The boys also have a younger sister who really doesn't look that much like either of them. When I read the original article in Newsweek, way back when, it seemed like a good explanation, but then we never heard another word about it until now. I will definitely be interested in seeing where this goes.

Like many others in the blog, I have twins very similar, now 18 years-old, that could participate in some sort of study about fraternal twins.
Aldo Calzolari
acalzolari30@yahoo.com.ar
Argentina

I cant understand the cytoplasmic signalling that trigger the entry of only one sperm nucleus to the ovum. In case of semi identical twins this signalling pathway may
altered in some way because of its own unpredictable cellular genetic environment.

I strongly believe the semi-identical twin scenario as I have a set of identical twins who look nothing alike. I have been informed and reassured by the hospital that they are identical and yet I have always suspected that they have got it wrong. I would be very willing to participate in any further study related to this in order to find out more.

I am also a semi-identical twin. My sister and i started to suspect we were not identical in our thirties. So we took our own finger prints. 6 fingerprints are exactly alike while 4 prints are different. So some parts of us have the same DNA but some do not. Wierd huh? If you are a twin try taking your fingerprints to see what happens.
Sheryl

I have twin girls who are 18months old. We had genetic testing done. I was told 6 of the 7 markers were found in-common and the lab suggested that they may be half identical although they are labeled fraternal. I would love to know the real answer before my days on this Earth are done. Is there anyone out there doing research to find confirmation?