国家地理:In the Womb: Multiples 孕育日记: 多胞胎

（单词翻译）

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 genes¹ in the mother's eggs sit outside the nucleus², known as mitochondrial DNA³. These genes can mutate after the egg has split, subtly modifying the way each embryo⁴ grows. Small differences in height, build and even personality will become more and more apparent as the twins grow up and reach maturity⁵.

This is why some scientists are now wary⁶ 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 chromosomes⁸ 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 chromosome⁷ 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