I'm a fraternal twin who looks A LOT like my sister. Pretty much all the identical twins I've known have looked at least a slightly different, as much so as the Olsen twins, I would say. I've just concluded that it goes to show that genetics doesn't completely dictate the way we look, that things that happen after you're born have an effect as well. (This is supported by the fact that twins seem to look more and more different the older they get--e.g. I can't always tell in a baby picture which is me, but by 4 or 5 some noticeable differences cropped up between my sister and I.) But people often ask me if I'm sure my sister and I are fraternal, because of our really strong resemblance. So I often wonder if we're "half-identical". We're definitely at least half fraternal, though--when we were born we had separate placentas, which is the usual indication. I'm not sure if my information is correct, but I was under the impression that half-identical twins have separate placentas like fraternal twins. So yes, that could be the case with the Olsens. But even if they were identical, I don't think the degree of difference between them would be that remarkable.

Mary-Kate and Ashley are yes, Fraternal Twins.
And yes, they look very similar to each other- which is extremly rare. Because normally, when you see Fraternal Twins- they are most likely boy/girl twins. But that doesn't mean that they always a different gender to the other. Like MK&A, girl/girl or boy/boy. I dont think that ashley was breast-fed longer than Mary-Kate, I just think that it is normal to have slight differences between twins. Even mirror images twins are little bit different to one another
I have a lot to say about twins- and guess what? Im only 12. Getting to know MK&A's info is one of my hobbies

Did the Olsen twins ever have a DNA test? If not, then my guess is they are identical. Refer to the webpage http://encarta.msn.com/encnet/features/columns/?article=twinsmorethan

which you cite yourself.

Jason

In bygone days, doctors often used the number of placentas -one for identical, two for fraternal- to determine zygosity (monozygotic/identical and dizygotic/fraternal). It is highly unreliable, however, as monozygotic twins may have one OR two placentas, depending on when the 'egg' split. While dizygotic twins usually have the one placenta PER baby, the placentas have been known to fuse. DNA testing is the only way to know for certain if twins are 'identical'. Unless the Olsen twins have had the test performed and it shows that they do indeed have different DNA patterns...then I'd definitely put money on them being monozygotic/identical. It only makes sense.;)

Marykate and ashley are NOT NOT NOT identical, i can easily tell them apart!

Just because you can easily tell them apart doesn't mean they are not identicle. My mom is an identical twin, and needless to say I can tell her and my aunt apart without question. To me they look very different, even when they were younger in baby pics etc I have no problem picking my Mom out, but people who don't know them find them impossible to tell apart. This is because even identical twins have some physical differences, since not all physical traits are entirely dictated by genetics; environment influences physical traits as well (incidentally, height is known to be a trait heavily influenced by environment, like diet while growing etc.) When you become more familiar with a pair of twins, you can easily tell them apart because you get to know their small differences. I had acquaintances in high school who were also identical twins, and they were also easy to tell apart once you'd seen them a bunch of times. Anyway, my point is: it is common and normal to be able to tell identical twins apart, and the fact that you can tell the Olsen twins apart does not mean they are fraternal.

That being said, I am not trying to claim that the Olsen twins are not fraternal. I know they say they are fraternal and they probably know what kind of twins they are. However, unless they have had DNA testing there is no way for them to know. Without DNA evidence, I agree with another poster that I'd put my money on them being identical. Maybe they have had genetic testing though, I'd be interested to know.

I would also wager that Mary Kate and Ashley are actually identical twins, but just don't know it. It is probable that the doctor who delievered them identified them as fraternal by seeing two seperate placentas. It is now common knowledge that the "MOST identical" form of twins happens in the first few days after conception when the egg splits early enough for each of the babies to develop their own placentas. It would defintately be interesting if they decided to have their DNA tested (and release the reults). Lol...I wonder if they know that they have such a (weird) fan base of genetics enthusiasts hoping that they will get their dna tested?

-ApacheRose

P.S. As for the so called "half identical" theory...or polar body twinning, it is still only a theory. There is, so far, no DEFINITIVE evidence that it has ever occured. It is my personal opinion as a genetic enthusiast ONLY that if half identical twinning were possible, that the twin formed from the polar body would be deformed. In summary, you could rule out the theory (which is gaining popularity) that the Olsen's are "half identical" twins.

Identical twins can most certainly have two placentas. The most common presentation is called monochorionic/diamniotic which means there is one outer sac and two inner sacs and sharing one placenta. But, identical twins can also be dichorionic/diamniotic which means there are two outer sacs, two inner sacs, and two placentae. If this is the presentation the only way for certain to tell if they are identical (other than sex or blood type of course) is to have a DNA test to determine zygosity. So, the Olsen twins can indeed be identical and the only way to prove it one way or the other is for them to get tested (and share the results). And the first poster (Susan) should get a DNA test as well!

Ok, about the "half-identical" thing...

Twins develope when a) a an egg is fertilzed by one sperm and then splits into two (identical twins) or b) when two separate eggs are fertilized at the same time (fraternal). How, genetically, can you then get "half-identical" twins?

I am looking right now at my Anatomy & Physiology text book, and I see nothing about half-identical. Furthermore, the chances of two egg or sperm cells being GENETICALLY IDENTICAL are astronomical.

"Polar Body Twins" - Human cells have 46 chromosomes each - 23 from mom, 23 from dad. The human cells responsible for baby making have 23 each to make this possible. So, when an oocyte (premature egg cell with 46 chromosomes) is developing into an ovum (egg cell), it must divide in half to end up with 23 chromosomes. This division (meiosis), produces ONE viable cell and one polar body, which are "non-functional and later disintegrate". Given this, it is unlikely that the theory of "polar body" twins is likely.

Even identical twins can look a bit different, especially as they grow older. Genetics determines the basics, but environmental factors are at work, as well. Diet, exercise, events in utero, during birth, and life can all have an effect on the way we look.

On the same note, fraternal twins can look just as similar or just as dissimilar as any two siblings born at any time. There are many siblings born years apart who look very much a like and could be mistaken for twins. And very often fraternal twins are g/g or b/b - no more or less often than b/g twins.

It all comes down to DNA testing. They cannot difinitively say they are identical or fraternal judging by looks or number of placentas. The only way to positively know is DNA testing.

Okay, I'm going to tackle the Turner's syndrome thing. The chromosomal anomaly present in Turner's syndrome is that a girl ends up with XO genotype instead of the usual XX...

This happens when (as I was writing above) a germinal cell (sperm/egg) is developing. They split in two (meiosis) so that each has half the chromosomes (23). The chromosomes in other cells are lined up in pairs, and then the pairs split apart during meiosis. When the sex chromosomes DON'T split, you end up with one cell having XX or XY and one cell having O.

Following so far? Okay, so when say the egg with O meets the sperm with X, you get Turner's syndrome of XO. How, then, do you end up with an identical twin BROTHER? If the zygote were to split into identical twins, they would BOTH have Turner's, since the anomoly occurs BEFORE conception.

This is the same type of anomoly that occurs in Down's syndrome. Chromosome 21 fails to devide and you get one cell with two 21's and one without one. When the cell with 21-21 is fertilized, you get Down's syndrome (three chromosome 21's, also known as Trisomy 21). If identical twins developed here they would also both have Down's syndrome.

Therefore, in my opinion, the Turner's thing doesn't make sense.

What I think is crazy is that (other than the barely noticeable height difference, or when they have their hair different colors or lengths) I can rarely tell them apart. I look back and forth from one to the other in pictures of them side by side and I am amazed by their similarities.

However, Tia and Tamera Mowry ARE identical twins, and I can tell them apart every single time.