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Parents pass on 23 chromosomes to their children, with a 50/50 chance for each chromosome. Considering that people of a families lived over millennia in the same region the abundance of some chromosomes in this region should be quite high. So if we for a moment neglect the occurrence of mutation the reincarnation of a genetic similar individual should just be a question of time. Therefore I asked myself how to actually calculate the probability and time span of this occurrence. (with and without mutation rates because of the complexity of the model)

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closed as unclear what you're asking by AliceD, rg255, Remi.b, fileunderwater, Chris Feb 14 '16 at 20:24

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  • $\begingroup$ And how would you define similar? $\endgroup$ – Mithoron Feb 13 '16 at 13:39
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    $\begingroup$ Looks more like a question for math. $\endgroup$ – Gyro Gearloose Feb 13 '16 at 14:52
  • $\begingroup$ Assume there is no mutation, leave enough time and the there will be no single living individual that will differ genetically from any other. If you really want to answer how much time it takes to have a probability of say $\frac{1}{2}$ that two individuals are exactly clones of each other in a population, we need to know the population size, the number of loci, the mating system, the population structure and the level of genetic diversity to start with. $\endgroup$ – Remi.b Feb 13 '16 at 16:01
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Genetic clones are not "reincarnations" – there is much more to a person than their genes. For starters there are epigenetic and environmental factors.

Also, chromosomes are not passed on whole from one generation to the next. Genetic recombination occurs which means, for example, that a mother's egg contains chromosomes that are a mixture of her parents' chromosomes. As a result, chromosomes are generally not passed on identically from generation to generation. This means that it is therefore not true to say "Considering that people of a families lived over millennia in the same region the abundance of some chromosomes in this region should be quite high". The Y chromosome in males is an obvious exception to this rule (it is passed on down the paternal line mostly unchanged).

These points aside, the chances of two people having identical genomes is so infinitesimally small and genetic inheritance is so complex that I do not think there is a sensible model to calculate it.

If you really want a number you could simplistically say that the human genome is about 3 billion base pairs. Humans share about 99.5% of their DNA with everyone else, so let's just say that to be clones two people would have to match at that other 0.5% of the genome – 15,000,000 base pairs. For each base pair there are four options: A, T, C or G. The probability that someone would match on every single of these base pairs could thus be 1 in $15,000,000^4 = 5.1 \times 10^{28}$.

That's 1 in 51,000,000,000,000,000,000,000,000,000.

To put this in perspective, since time began there has been about 108,000,000,000 people who have lived, so humanity would have to repeat it's history several billion billion times to make this remotely likely.

Calculating this number make more assumptions than I care to count. You might take off a few zeros if you are considering a highly inbred population.

It is theoretically possible that an accidental genetic clone of a person could occur, but the point is it's just not going to happen.

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