Well, John, I am a fraternal twin. Everyone thinks my brother and I look “exactly” alike, especially in pictures of when we were young. If you but the two of us side by side, however, you can see the subtle differences in our faces and general “head shape,” I guess. Up until my early teens, I always thought I was an identical twin because no one except close friends and parents could tell my brother from me. Then one day my mom said “No, you’re not identicle, you’re fraternal twins. At least that’s what the doctors told us.”
As to your girlfriend and her sister, I suspect that they are probably fraternal, but look so much alike (as in my case) that they have always been mistaken for identical twins. Identical twins MUST be identical in all ways physically (barring tragic trauma and wild hairdos) by definition.
Sachin K.
“The most merciful thing in the world, I think, is the inability of the human mind to correlate all its
contents.”
-H.P. Lovecraft, “The Call of Cthulhu”
There well may be more knowledge on the matter now than back when I learned about this, but I understood that a Barr body was intracellular, stained with DNA-reactive dyes, and was in fact an inactivated, condensed X chromosome.
Studies showed that XY human males didn’t exhibit a Barr body, XX human females had one Barr body, but phenotypic females who are genentically XXX had two Barr bodies, and so on. This led to the theory that each human body cell only has one X chromosome active at a time.
(And in Queensland XXXX is a beer, but that’s another matter - sort of like the old Ogden Nash “The one-l-lama he’s a priest” poem!).
I believe Sachin K. might have too hastily agreed with the doctor’s assessment of dizygosity. About 25% of monozygotic twins in the U.S. are dichorionic, however, many physicians are not aware of this and incorrectly assume that the presence of two placentas automatically means the twins are fraternal. Marked phenotypic variation is also quite common among monozygotic twins. In a small twin study we just completed, 47 twins and their mothers all agreed that the twins were dizygotic, however, DNA results found that 6 of these pairs were actually monozygotic despite obvious differences in the twins’ appearances. Finally, according to one researcher it is possible for monozygotic twins not to be genetically identical because of variations in how the zygote divides DNA prior to division.
The explanation I got for the identical twin brothers who looked different, at least by the age of forty: “We’ve been weatherbeaten in different directions.”
Just to confuse the issue a bit more: It’s possible to have identical twins that are outwardly of different genders. It’s been found that something like 1 in 1000 “women” are actually XY, but their maleness failed to develop, the remained female. (All fetuses start out female.) These individuals are female to any physiological examination, although they tend to have fertility problems.
I’ve never heard of a case of this happening to one twin of an identical pair (who would test, anyway?), but neither have I heard of any reason it couldn’t.
Identical or monozygotic twins (1 ova 1 sperm) which can be further subdivided into groups depending on when the fertilized egg “splits”.
a) division within 72 hours, diamniotic, dichorionic
b) division between days 4-8, diamniotic monochorionic
c) division after 8 days, monoamniotic, monochorionic
d) even later division, conjoined twins (siamese)
Note that all other “types” of twins such as twin-twin transfusion as referred to in Cecil’s original explanation, heterotropic pregnancies, TRAP sequence pregnancy etc are all complications of one of the above types. That does not make them a separate type, just a complication.
Incidently, it is thought that monozygotic twins are not always identical due to unequal sharing of protoplasmic materials during the process of division. This can also happen within an individual, in genetic mosaicism.
(Reference, oh well, what about Williams Obtetrics, 19th edition, Chapter 39 “Multifetal Pregnancy”)
Well, I won’t discount the possiblity, but I imagine this probably due to a genetic effect that is not yet understood rather than an environmental effect. As such, both twins would be female yet have XY chromosomes. Of course the causation possibility of getting to much estrogen from your fried chicken -or some other environment factor - is not outside the realm of possibility (have you heard about the girls in South America that have mean puberties of 8yrs in some villages due to chickens raised with estrogen supplements?).
It’s a bit off the original topic but it sounds like you’re talking about androgen insensitivity, which is a genetic disorder so presumably would occur in both identical twins, unless due to incomplete penetrance (I mean genetic penetrance) the twins had different amounts of gene expression…
As to who would test it, everybody would test this little “girl” when she got to 17 without any periods!
I hadn’t heard any mention of the name or it’s being a genetic disorder. However, it’s been a few years since I heard anything about this, there’s no keeping up with progress!
However, not all people with the condition won't have periods. This originally came to light when they switched from physical inspection to genetic testing to make sure men didn't compete as women in events such as the olympics. Before then, it was known to be possible but considered very rare. When athletes started flunking, it became obvious that the problem was far more common than originally thought. Most sporting events switched back to physical inspection.
Actually, I hate to be pedantic but they won’t have periods, because althought they don’t respond to androgens, they do have anti-mullerian factor, so no female ineternal organs develop.
No uterus = no periods.
You might be thinking of XY Gonadal dysgenesis or Swyer syndrome (both very rare).
But hang on, they don’t actually do unsolicited genetic testing for athletes do they? Are you saying that athletes who have an unfortunate genetic disorder which makes them female in all respects except for a short, blind ending vagina and no internal female organs are banned from all sporting competition?
Firstly, that seems harsh. Secondly, that seems like a recipe for severe psychological trauma to be told by some sporting authority that you’re actually male. What about sports which are mostly prepubertal like female gymnastics for example?
The article I remember on this said that many of them had fertility problems. That would imply that they do have proper female apparatus, otherwise they would have said they were infertile.
As for unsolicited genetic testing, yes. Some sort of femaleness test has been used at the high levels of female athletic competition for a long time. It used to be a simple glance down during a physical exam, they switched to genetic testing as been less objectionable--then started finding women that tested as XY.
I’m an identical twin (coincidentally, so is my brother). How can I find out now, 50 years after my birth, what kind I am? After reading all these postings, it feels like it must be important to know. And I’d like to have an answer before my womb-mate finds out.
