I was reading the twins thread in IMHO, and I realized I’ve been wondering this ever since I first saw Patty Duke on Nick at Nite about 15 years ago. Is it possible to have identical cousins? I know not genetically identical, since they’re not from the same embryo, but are there any cases of cousins who are so alike they are extremely hard to tell apart when together? Curious.
Jman
The nearest thing would be double first cousins (each parent of one cousin is a sibling of a parent of the other, like when two brothers marry two sisters). I can’t back it up, but I’ve heard it described that double first cousins are as “closely related” as siblings. I would think in this case you could have cousins that are very much alike in appearance, anyway. As for how likely that is, I don’t know.
Depends on how strict you are with the definition of “extremely hard to tell apart.” Close enough to be mistaken for siblings? Sure, probably. Close enough to be mistaken for twins? Unlikely.
Actually, there were two completely unrelated women in my freshman unit whom I mistook for twins. I always thought The Prisoner of Zenda was a crock up until that point…
Seriously, though, they were eerily similar enough that I was truly surprised to learn that they were not identical twins.
So, what is the probability of two such people ending up in the same freshman unit, having never met before?
-Ben
Actually, although virtually MATHEMATICALLY impossible, I guess it would be possible for cousins whose parents are double related (ie brother brother marry sister sister) could have, between each pair, one copy of the same DNA…so, athough the odds are beyond astronomical, it seems POSSIBLE to me.
Can anyone rule this out?
I have a first cousin who is near in age to me (male) and until we both hit puberty, everyone would always stop us in public to ask my grandmother (we were usually with her) if we were identical or fraternal twins. Now, aside from the obvious flaw in their logic, it does say quite a bit about how similar we looked. (My father and his mother are siblings) So, I would assume that if there were two cousins of the same sex it is feasible that they could look almost identical.
OK. Let’s say two brothers marry two sisters and they each have a son. We can calculate the odds that the two sons have the same allele of a given gene.
[thinking out loud here]
So the two sons would have to have gotten…no, wait. Hold on here.
OK. The two brothers would have to have the same allele for that gene - I think that’s a 50% chance; if I get my maternal A allele, my brother would have to get the maternal A or B allele - 50%. Also 50% for the sons’ maternal alleles. Then you have the odds that both brothers passed on the same allele to their sons. 50% again, methinks. If I pass on my A, my brother will pass on A or B. Same if I pass on my B. OK. So for a given gene, there’s a 50% x 50% = 25% chance that the two cousins share the same paternal allele. Similarly, there’s a 25% chance that they share the same maternal allele. Therefore, there’s a 25% x 25% = 6.25% chance that both of their alleles are shared. (Of course, dominance issues screw things up here, but let’s ignore those.)
[/thinking out loud]
So, if two brothers marry two sisters, there is a 6.25% chance that the cousins will be identical at any given locus. Since there are thousands, if not millions, of loci in the human genome, it should be pretty clear that genetically identical cousins are pretty damn unlikely.
Whew.
Double first cousins are generally as closely related as half-siblings. But if identical twin brothers marry identical twin sisters, then their offspring will be as closely related as full siblings. There was a mailbag article about a while back.
But Smeghead, you can’t just take it on a per-gene basis. Many genes are at linked loci, and so a block of several hundred genes might be passed along intact. The genome crosses over about 30 times total during meiosis, and there are 23 chromosomes. The permutations of those crossover points are going to be nearly infinite, or close enough to infinite that we don’t need to worry about this happening any time in the history of the universe.
LL
But am I right in saying that although it is nearly infinitely unlikely, it’s technically possible?
Or am I just wrong altogether?
How about if two identical twin girls marry two identical twin boys.
They may not be identical but wouldn’t they be genetically as close as brothers and sisters
It’s technically possible. So is being dealt all thirteen hearts while playing bridge.
I suggest you invest in a good Finite Improbability Drive[sup]TM[/sup] and have at the problem 
LL
For the record, in the “Patty Duke Show”, Patty and Cathy Lane are daughters of identical twin brothers who married unrelated women (at least they weren’t described as such in the show). Cathy’s mother is dead and her father is a foreign correspondent for a newspaper. Cathy’s father showed up in one episode. Not surprisingly, he was played by William Schallert.
I can’t tell you about the genes however. But cousins, identical cousins, are two of a kind.
I had often wondered about this, and now here’s the answer just appearing. Is the crossing over more or less random on a per-unit-length-of-chromosome basis, or does every pair (except X and Y) generally cross-over, and the larger ones sometimes twice? What about X and Y?
All this (nevertheless interesting) talk about how genetically similar two cousins could be is completely irrelevant. The OP was asking about phenotype, not genotype, to wit:
Can two cousins resemble each other as much as Patty and Cathy Lane did?
The answer is of course yes. Ben mentioned two unrelated women he knew in high school or college (not clear from his post) who were often mistaken for twins. When I was in high school, my classmates John Baker and David Frost played identical twin brothers in a play, even thought they weren’t related.
So there’s two anecdotal examples of unrelated people who look enough alike to be twins. If it can happen with unrelated people, certainly it could happen with cousins.
It needn’t even blow your mind.
I’ve found two people that seem to look just like me.
There was a book written about a cousin of mine, whom I’d never met. There was a picture of his family, and I discovered that his sister looks just like me (except a little heavier). So, I do have a cousin that I would say looks so much like me it’s spooky. I would like to meet her someday.
About 10 years ago, there was a person who went to the same bars I did, and looked so much like me that my best friends mistook us for each other. I only ran into her once, but since I (honest to god) thought I was glancing into a mirror at the time (it didn’t occur to me that there was no mirror there and that the clothes were different for a couple of minutes), I missed my chance to talk to her.
[TV trivia]
Besides the Patty Duke show, what other popular (but short-lived) series featured identical cousins?
Twin Peaks. [OK, so one of the cousins was dead before the pilot started. But she appeared in a lot of flashbacks and dream sequences.]
http://straightdope.com/mailbag/mtwins.html
Somewhat relevant.
Sorry if somebody posted this before, but, like I said, Web TV sucks.
Now, I’m off to see the New Bomb Turks…
The phenotype/genotype distinction was a good thing to bring up.
Also, you have to consider that not all genes are for appearance. Minor differences even in appearance-related genes won’t matter that much if the rest is alike enough. So I definitely think it’s possible. Even if they’re not identical, there are some who can sure as hell pass for it.
Everyone has brought up good points, but after the headache that last post gave me, I’m not even going to try again.
Oh, and ZenBeam - crossing over happens a LOT between every pair of chromosomes, including X/Y. It’s not entirely even, though. There are “hot spots” where it happens more often.