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# If it's true that everyone has a double somewhere in the world, (let's say mine is in Australia), will my double's parents in Oz be replicas of my parents here in UK?

Question #17851. Asked by Puzzled. (Apr 02 02 7:47 AM)

Jeeves

Because of the random way that genes combine when a child is created the parents of your double would not look like yours. If a couple produce many children each one has its own physical characteristics (with the exception of twins) and there may well be large physical differences.

 Apr 02 02, 8:52 AM
eliasen

I'll assume 'double' means 'has identical genetic makeup,' and not just 'sorta looks like.'

There are about 4 billion base pairs in the human genome. Let's assume that only 1% of these vary from person to person, and the rest are needed to make a working human body.

That gives 40 million base pairs that can be different from individual to individual. Each base can have four possibilities (A,C,T,G). This gives 4%5E40000000 possibilities. This is a huge number -- it has 24 million digits if I were to write it out.

If these values are taken to be random, the odds of you having any particular single makeup (say, the one you have) is 1 divided by this number. Nobody else would arrive at this purely by chance.

You had to inherit any given stretch of DNA from either your father or your mother. The strands of DNA are crossed over so you get parts from each parent, (so you may get brown eye genes from mom but dad's receding hairline.) But dividing by 2 or 4 or whatever make essentially no dent on a number this size.

Normally, the genes that you'd inherit from your father or your mother can combine randomly. That's why you don't look exactly like your brother or your sister. You could imagine combinations where fragments of DNA came from your father, or your mother, but gave the same net effect.

However, if we follow your premise that everyone in the world has a double, you have to concede that your father has an exact double, and so does your mom.

Of the 4%5E40000000 ways that the varying base pairs can be arranged, the probability of getting a specific arrangement purely by chance approaches zero, even if there were as many people in the world as there are particles in the universe.

 Apr 16 02, 12:28 PM
eliasen

So, the same mystical force that ensures that 'everyone has a double somewhere in the world' would, as a matter of reducing improbability, have to force that your parents were the doubles of your double's parents. So, given the odd assumption that you postulate, the probability is a resounding YES.)

Actually, given what we know about sexual reproduction, it's easy to prove that if we assume that if everyone has a double somewhere in the world, then the probability that everyone is identical to everyone else is arbitrarily close to 1. This is left as an exercise for the reader.

(Feel free to plug in your own numbers for any of the assumptions I've made above. The fact that the number of different ways of getting a specific configuration grows so much faster than the number of individuals on the planet still makes the analysis valid.)

Tue Apr 02 19:20:57 CST 2002 (Delete Entry)

eliasen says:

In the above response, the carets were munged. Read 4 to the 40000000 power wherever you see an ugly number. I would write out 4*4*4*4*4*4*4... 40 million times but that'll have to wait until later.

Tue Apr 02 19:30:39 CST 2002 (Delete Entry)
(Reposted - Edited for content - McG)

 Apr 16 02, 12:32 PM

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