At first I didn't consider genetic drift to be much of a factor since the probability of fixation seemed unrealistic. But your comment gave me cause to think about it a bit closer. If the population group that the mutation occurred in was rather small, say <100 time? |

Actually, it's rather neat how it works out. If you have a larger population, then the probability of a neutral mutation being fixed is that much smaller. But the probability of it *happening* is that much bigger. And these effects, if you do the math, cancel out exactly, giving us the nice result that if all mutations were neutral (which is a good first approximation to the truth) the rate of fixation of mutations in the gene pool per generation is exactly equal to the rate of mutation in the individual.

So yes, a large population makes it more unlikely that any particular balanced translocation will get fixed, but it also means that there are that many more balanced translocations which will arise and so be candidates for fixation, and these two effects will exactly cancel one another out. Therefore, if we knew how often balanced translocations happen to individuals (and I'm not sure that we do) then we could also say the rate at which we'd expect them to become fixed in the population, and this would be completely independent of the size of the population.

So if you ask yourself: "What were the chances of *this* balanced translocation being fixed by genetic drift?" then population size is important; but if you ask yourself: "What were the chances of *a* balanced translocation being fixed by genetic drift?" then it isn't.

*Edited by Dr Adequate, : No reason given.*