This is mathematically convenient, as the standard exponential distribution has both the expected value and the standard deviation equal to 2"Ne". Therefore, although the "expected" time to coalescence is 2"Ne", actual coalescence times have a wide range of variation. Note that coalescent time is the number of preceding generations where the coalescence took place and not calendar time, though an estimation of the latter can be made multiplying 2"Ne" with the average time between generations. The above calculations apply equally to a diploid population of effective size "Ne" (in other words, for a non-recombining segment of DNA, each chromosome can be treated as equivalent to an independent haploid individual; in the absence of inbreeding, sister chromosomes in a single individual are no more closely related than two chromosomes randomly sampled from the population). Some effectively haploid DNA elements, such as mitochondrial DNA, however, are only passed on by one sex, and therefore have one quarter the effective size of the equivalent diploid population ("Ne"/2)