国家地理:In the Womb: Multiples 孕育日记: 多胞胎
时间：2018-12-05 作者：英语课 分类：国家地理2007年
英语课
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 1 in the mother's eggs sit outside the nucleus 2, known as mitochondrial DNA 3. These genes can mutate after the egg has split, subtly modifying the way each embryo 4 grows. Small differences in height, build and even personality will become more and more apparent as the twins grow up and reach maturity 5.
This is why some scientists are now wary 6 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 8 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 7 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
This is why some scientists are now wary 6 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 8 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 7 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
1 genes
n.基因( gene的名词复数 )
- You have good genes from your parents, so you should live a long time. 你从父母那儿获得优良的基因，所以能够活得很长。 来自《简明英汉词典》
- Differences will help to reveal the functions of the genes. 它们间的差异将会帮助我们揭开基因多种功能。 来自英汉非文学 - 生命科学 - 生物技术的世纪
2 nucleus
n.核，核心，原子核
- These young people formed the nucleus of the club.这些年轻人成了俱乐部的核心。
- These councils would form the nucleus of a future regime.这些委员会将成为一个未来政权的核心。
3 DNA
(缩)deoxyribonucleic acid 脱氧核糖核酸
- DNA is stored in the nucleus of a cell.脱氧核糖核酸储存于细胞的细胞核里。
- Gene mutations are alterations in the DNA code.基因突变是指DNA密码的改变。
4 embryo
n.胚胎，萌芽的事物
- They are engaging in an embryo research.他们正在进行一项胚胎研究。
- The project was barely in embryo.该计划只是个雏形。
5 maturity
n.成熟；完成；(支票、债券等)到期
- These plants ought to reach maturity after five years.这些植物五年后就该长成了。
- This is the period at which the body attains maturity.这是身体发育成熟的时期。
6 wary
adj.谨慎的，机警的，小心的
- He is wary of telling secrets to others.他谨防向他人泄露秘密。
- Paula frowned,suddenly wary.宝拉皱了皱眉头，突然警惕起来。
7 chromosome
n.染色体
- Chromosome material with exhibits of such behaviour is called heterochromatin.表现这种现象的染色体物质叫做异染色质。
- A segment of the chromosome may become lost,resulting in a deletion.染色体的一个片段可能会丢失，结果产生染色体的缺失。
8 chromosomes
n.染色体( chromosome的名词复数 )
- Chromosomes also determine the sex of animals. 染色体也决定动物的性别。 来自《简明英汉词典》
- Each of four chromosomes divide longitudinally. 四种染色体的每一种都沿着纵向分裂。 来自《现代汉英综合大词典》