Biology majors: Question about identical twins

Factual Questions
1

So, I was reading a book a while back (a fiction book, for the record). Two of the main characters were twins - brother and sister (male and female) - but described as identical twins (IIRC, there was some explanation about it being a very rare occurence).

Now, from my biology days, I remember that fraternal twins (two eggs) can be either same-sex or different sexes. However, identical twins, because it’s a split egg with exactly the same genetic code, MUST be the same sex. So, I think I’m clear on the standard stuff we learn in science class.

My question, though: is it absolutely, positively, one hundred percent IMPOSSIBLE for identical twins (to clarify, coming from the SAME egg) to be different sexes? By this, I mean is there some sort of quirk or rare occurence that would make this a feasible occurence (even if it’s a million- or billion-to-one odds)? Are there any recorded instances of this happening, and if so, what weird anomoly would cause this? And, to add to it, I’d have to specify that there are no extreme physical/mental deformities or abnormalities - these would be twins that appeared completely normal to the average person.

The only idea my layperson brain can come up with is that (from what I think I remember) all embryos start out as female. But if they are male (XY), at some point along the development timeframe the male hormones will kick in and the fetus will begin developing the male parts. So, what if, for some reason, on one of the embryos, this failed to happen and thus the baby continued to develop as a female? Is it possible, and, more importantly, would the baby be absent any glaring physical or mental defects? (You there, in the corner - stop laughing!)

If anyone has any ideas or knowledge about this, I’d appreciate some input. I understand this was a fiction book, but it was pretty blatant about the identical twin but boy-girl issue (how that made them so special and rare, etc.)…it didn’t particularly add to the storyline (i.e. wasn’t a big scientific novel or anything), it was just something added as an extra. But it’s bugging the heck out of me!

2

I completed a zoology major in college, and this is not possible. All embryos start out looking female, but will have the appropriate male (XY) or female (XX) chromosomes.

There are various birth defects related to the development of a fetus’ genitalia - I found a link that might help you out: http://www.mayoclinic.com/findinformation/conditioncenters/invoke.cfm?objectid=5DEC63B0-846C-4634-99178B5E2F3EF19F

3

I have always wondered about this too…

The nearest situation I could come up with to explain this would be if an egg to split first, and then be fertilized by 2 seperate sperm, thus creating a 1/2 idential 1/2 fraternal set of twins.

However, I do not believe that ever happens.

4

Was this book called The Judas Child ?

I read that too. But it’s not possible to have identical twins who are not identical and that includes gender. Hence the designation ‘identical’ (Is that a record for the word identical in one sentance?)

Mine are both boys, I’ve checked :wink:

5

There is such a thing as so-called “half-identical” twins that involves the egg splitting in two, and then being fertilized by two seperate sperm. In this case, the twins could be opposite sex, and would be much more similar than fraternal twins. However, they wouldn’t be “true” identicals.

For “true” identical twins to be opposite sex, I can’t think of a way that could happen without some abnormality, specifically Turner’s syndrome. This would involve the loss of a Y chromosome by one male twin sometime after the twinning process. The resultant XO embryo would technically be a girl, but would show the symptoms of Turner’s (short stature, sterility, mental retardation)

Of course, to a genetic purist, an XY and XO twin would not be identical, as an X chromosome is about 2% of the genome. Thus, they’d only be 98% “identical”

6

There is the possiblity that there are two genetically identical XY fetuses, and then somehow one of them develops androgen insensitivity (where the body doesn’t respond to testosterone). That fetus would develop into a female, but would still be genetically identical to her twin bother. Of course she would be quite different phenotypically.

7

That is technically possible although I can’t imagine how that would happen under natural conditions. If you wanted to split hairs however, the twin with the androgen insensitivity would be a pseudo-hermaphrodite (often called just a hermaphrodite although they are not the same) rather than a regular XX female. She would be sterile and the testes would still be present in there somewhere.

8

That is correct. “Half-identical” twins are also known as “polar twins” and scientists really don’t understand that much about them except that they are not truly identical but often look it if they are they same sex. Believe it or not, the Mary-Kate and Ashley Olsen are not identical twins but are believed to be polar twins because they look so similar. I guess that it is equally likely that polar twins could be opposite sex as same sex but I don’t think that much is known about that at all. It is fairly hard to investigate the genetic contribution of the mother and father is not 50-50 but shifts simply by chance and can end up with weird results even in non-twin siblings.

9

Thanks everyone for the input, and DeniseV, for the link. I like the explanation about polar twins - sounds like the average person might consider them identical twins, since they came from the same egg…even if they’re not “truly” identical by scientific definition. Most importantly - whew - I’m glad I asked. Ever have these questions that just hit you at random times and keep bugging you? :slight_smile:

Doublemint - funny, I could remember my obsession with this question, but couldn’t remember the name of the book! But you’re right - it is The Judas Child. I browsed Amazon and I seem to remember the Uncle Mortimer character talking about the identical boy/girl twins. I don’t even remember if I liked the book or not.

Thanks again all! I knew smarter minds than mine could figure this one out.

10

The OP is referring to an old Piers Anthony Book. It was in the “Cluster Series”, but I don’t recall which one. In it, artificial identical twins were routinely made by splitting a fertilized egg. In this case, the zygote was male (XY). In order to get a female, in one of the XY zygotes the Y chromosome was removed and the X chromosome artificially doubled, thus making identical twins where one was male (XY) and one female (XX). The twins were genetically identical in every way except the sex chromosomes.

MM

11

Wait wait wait. . . can someone write a bit more about this ‘polar twin’ business? I ask because I and my sister are pretty frikkin’ well-nigh identical, although we grow more and more different physically as we grow older, but our mom said once without much explanation that the doctor said that we were in fact not identical (I think my mom took this to mean that he thought we were fraternal. . .) so this might solve a personal mystery.

12

I am sorry to say that what is said above is largely all that is known about the existence of “polar twins”. Many doctors will have never heard of them because the speculation about their formation is rather recent and based on circumstantial evidence rather than direct evidence. That being said, it is very probable that they result from a single egg that splits and gets fertilized by two different sperm.

I can imagine some ways that tests could be done to check for identical X chromosone linked traits among twins suspected of being polar twins but I am not aware of any of those studies being done so far. Maybe you and your sister could be guinea pigs and prove this phenomenon.

13

Hmm. I’m 30 so I don’t think that was ‘recent’ enough to have been a consideration. Mystery remains, I suppose. Interesting stuff, though! Thanks for more information on the twin front.

14

Sherri Tepper’s book Sideshow features “male” and "female’ conjoined twins - but she actually has a coherent explanation for them. The twins, you see, in addition to being joined are also “intersex” - if I recall they had some sort of XXY configuration of chromosomes and ambiguous genitals at birth. Later, both had surgery, one to boy and one to girl appearance, including a discussion at one point of getting the girl electrolysis to remove her beard. The boy wore lifts in his shoes to make him slightly taller. I think a situation like this - ambiguous genitals and intersex twins - is the only way you’re going to get genetically identical boy/girl twins. Except I can’t figure why you’d surgify them to different genders.

15

I had thought you might be thinking of Twelfth Night. :slight_smile:

16

IIRC The case I read about concerned twins joined at the pelvis, each had one good leg (one withered leg between them which wound up being removed) and one set of getitalia for the pair. When the parents opted for seperation when the twins were about 2 or so, they gave each one leg but only one got the genitals. They then gave the remaining twin what amounted to a sex change.

I wonder why they did that myself, after all they can to female to male sex changes, so why did they arbitrarily assign the poor kid a new gender?

17

I fail to understand why “polar twins,” ie. an egg splitting prior to being fertilized by seperate sperm, would be any different from fraternal twins, where two seperate eggs are fertilized by two seperate sperm. All eggs in the mother are geneticly identical, so a ‘split’ unfertilized egg (assuming each egg contains haploid DNA) would be no different than two seperately released eggs. The genetic recombination occurs only after fertilization.

18

No, all eggs are not genetically identical.

Each cell in your body has two copies of each chromosome, one from your mother and one from your father. Before a normal body cell (a somatic cell) divides, first all the chromosomes double and then the cell splits, and each new cell gets each pair of chromosomes. These cells are called diploid (di=2) since the chromosomes are paired.

But gametes (eggs and sperm) work differently, at least in vertebrates. When a new gamete is being formed it doesn’t undergo the chromosomal duplication process. So when it splits, each new cell gets only one of the each chromosomal pair. And the one that it gets is essentially random. Since you have 23 chromosome pairs, there are 2^23 possible combinations of chromosomes. These cells are called haploid, they contain only half the genetic material of a normal cell. So the odds of any two gametes being identical is very slim…unless something odd happens.

In this hypothesized case, the haploid egg cell would divide again, creating two identical haploid egg cells. Normal identical twins have 100% identical DNA. Normal siblings have, on average 50% identical DNA. Polar twins would be 75% identical.

Of course, for normal siblings it is theoretically possible that they don’t share ANY identical DNA. When gametes form the division of chromosome pairs is essentially random. But we can imagine a pair of gametes where every split went the opposite way. That would be as unlikely as getting 23 heads in a row and then getting 23 tails in a row.

19

If they’re not the same gender, how could anyone say they are identical? Could Cecil said to be “identical” to Cecilia?

FWIW, an episode of “Quincy” once stated the “fact” that in “many cases”, one identical (gender and appearance) twin has the placement of internal organs reversed from the other twin. This was the means Quincy used to ferret out a long-lost twin who murdered and then took the place of their well-to-do sister. BS. AFAIK, organ reversal (or whatever you’d call it) has happened very rarely in single births, but it is so rare as to be statistically not happening when combined with the much less rare (but still not everyday) occurrence of identical twins.

But I’d still say that even these ficticious twins are not truly identical. Just similar.

20

No they can’t do female to male sex changes, at least not in the way I think you’re thinking. Modern medical science cannot create a functioning set of male genitals, or even a convincing facsimilie thereof.