国家地理:In the Womb: Multiples 孕育日记多胞胎

Identicals share one of the closest human relationships biologically. They are likely to grow up looking and sounding extremely similar, yet not a 100% match. A small number of genes in the mother's eggs sit outside the nucleus, known as mitochondrial DNA. These genes can mutate after the egg has split, subtly modifying the way each embryo grows. Small differences in height, build and even personality will become more and more apparent as the twins grow up and reach maturity.
This is why some scientists are now wary of using the term identical, and prefer to use monozygotic or single-egg twins instead. Identical twins are almost always the same sex, but surprisingly there are a tiny number of boy-girl monozygotic twins. They resolve from an egg that contains an unusual mixture of sex chromosomes not the usual XX for a girl and XY for a boy. Occasionally an egg contains 3 sex chromosomes, 2 Xs and a Y. But if the egg divides to produce monozygotic twins, a chromosome may be lost in the process, leaving one embryo with the girl's XX combination, and her monozygotic twin as an XY boy.
mitochondrial DNA: an extranuclear(核外) double-stranded(双链) DNA found exclusively in mitochondria that in most eukaryotes is a circular molecule and is maternally inherited ― abbreviation mtDNA
monozygotic(单卵的): (of twins) derived from a single ovum(a mature female reproductive cell, which can divide to give rise to an embryo usually only after fertilization by a male cell), and so identical