Several sources that I’ve read claim the the famous Olsen twins (now 15-year-old girls controling a multi-million dollar empire) are fraternal, not identical, twins. But they look exactly alike! They even took turns playing the same character on TV for years. How do they know that they are fraternal? Why do they look identical if they aren’t genetically?
That’s two separate questions.
I too am curious how they know that they’re fraternal. Maybe they took some kind of DNA test?
But looking extremely similar is not impossible even among siblings born years apart. My guess is that the older they get, the more different they’ll look, and that they’ve been using makeup to disguise whatever differences have already appeared.
I do not know how in this case they know they are fraternal twins. But I have met siblings with a very strong resemblance, it’s not uncommon. And if siblings are actually exactly the same age, the resemblance could be remarkable. You don’t have to be identical twins to share a lot of DNA.
The genetic relationship between fraternal twins is exactly the same as between two siblings. I’m sure everyone has met a pair of siblings that by chance look extremely similar. There’s no reason the same thing can’t happen with fraternal twins.
Whether or not the Olsons are, in fact, identical or fraternal, I dunno.
Interesting, from all the cites that I found on Google, it does appear that they are fraternal twins instead of identical (one is even left handed while the other right handed). However, we still don’t know everthing that there is to know about twinning. Because of cases like this some scientists and physicians speculate that they may be a type of twin called a “polar twin”. If the speculation is correct, that would mean that they were formed by one egg that split before fertilization and then was fertilized by two different sperm. That would mean that they would be half-identical twins I guess you could say.
I’ve heard of the polar twin theory before.
My sister and I are fraternal and yet we look VERY similar. There’s a noticable height difference, but that’s the biggest difference between us.
You can generally distinguish fraternal from identical twins by whether or not the twins are in the same sac in utero. But even as I type this, I’m remembering that identical twins can be born in different sacs too. So now I’m confused.
Polar twins (I thought their existence was generally accepted, no longer merely theoretical, but what do I know?) would share on average 75% of their genetic material – that’s a heck of a lot, so you’d expect they might pass for identical. Heck, people who don’t know them well often confuse my dad and his brother, and they were born five years apart.
–Cliffy
My grandfather and his non-twin brother, born a year apart, were close to identical in appearance and remained so throughout their lives. I mistook Uncle Jim for Papa a couple of times.
Nonny
As a father of twins, I can relate that even OB/GYNs are highly underinformed on this topic. In fact, I caught two midwives and two doctors (each separately) making shiat up about twin genetics and formation.
In any case, it’s very easy to tell if children are NOT fraternal, since fraternal twins (to my knowledge) cannot share a placenta (and identicals, of course, cannot be different genders). However, in our case, the girls (and no, I am not Rubicon Olsen, although that would be a cool name) had separate sacs and placentas. This did NOT mean they were fraternal–identicals can split off early and develop separate sacs, etc.
I believe the docs (the good neonatal guys, not the OB/GYN clowns) said that separate sacs but same sexes meant that we had a 25% chance of them being identical, 75% fraternal, but that we could only tell for sure if we did DNA tests.
In any event, our girls look significantly different, so we go under the assumption they are not identical (although different developmental tracks can account for some variation).
My SWAG hypothesis is that the Olson girls think they are fraternal (separate sacs?) but are actually identical.
For what it is worth, mothers of natural fraternal twins (as opposed to those under fertility treatments) have a 10% chance of retwinning if they get repregnant. This percentage declines over time, I think, but our next child (due in a week) is a singleton…I made the doctor check A LOT.
IIRC, the right-handed/left-handed thing may not in itself be an indicator of fraternity. I recall reading that this is fairly common with identical twins, that they’re mirror images of each other. I knew a pair of twins like this, but I didn’t have the priveledge of counting sacs or placentas so for all I know they could’ve been as fraternal as the Olsen girls apparently are. 
Don’t we share over 95% with chimps?
Yes, and a sizable chunk even with bacteria. When you see numbers for how similar relatives are, they’re usually referring to the portion of the genome which can vary from person to person. So two (fraternal) siblings will be almost guaranteed to have the same number of limbs, but a chance somewhere in the neighborhood of 50% of having the same hair color (for example).
According to Cecil, by the way, twins can be determined to be identical or fraternal by similarity of fingerprints, no messy DNA tests required.
I think you can tell whether you are identical twins based on DNA testing. The Research in Molecular Biology class at my school does an experiment on this every year (they find a pair of twins and run gel electrophoresis on samples of their DNA) (I hope that link explains it, I personally find it rather complicated - and I have a test on it next week! aag) I’m not sure exactly where it goes from there, since I’m not actually in the Research class. But they can tell. I don’t know if the Olsen twins have had this done or if they know otherwise.
Well, it is possible for fraternal twins (or any siblings, or
even people with absolutely no relation) to be identical.
It’s a lot easier if the mother and father share a number
of identical chromasomes. But worst case (all
chromasomes differ), the two eggs would have to have
the same half of the mother’s chromasomes, and the
two sperm would have to have the same half of the
father’s. Any goers on what the chances are?
Of course, it helps that their parents are brother and sister.
Once the ravages of heroin addiction set in, I’m sure it’ll be easy enough to tell them apart. Well, that and the inevitable post-career photospread in Maxim magazine, highlighting the butterfly tattoo one of them will inevitably get on a drunken Mexican binge.
I’m not really much of a fan.
Say what?!?
2 to the 26th, right? Which is somewhere north of 67 million. So for every 67 million children your parents have, chances are two of them will be genetically identical.
–Cliffy
I hope your doctor is a lot smarter than my ex’s doctor was. He did ultra sound’s 3 times. He listened and listened and tested and tested. My ex was about as wide as she was tall. She was HUGH! But, he said there was only one baby. Christmas Eve, 1981, our twin daughters were born. :eek: He said, “Merry Christmas, it’s twins!” Of course, we said “HUH?” Talk about a surprise. This very same doctor insists that they are fraternal twins because there were 2 placentas. While I have no problem telling them apart, everybody else is always asking me, “How do you tell them apart?” Of course, I tell them, “They don’t even look alike. They aren’t related.” 
Having twins is a blast.
Hmm, another side-effect of pregnancy brought to light.