One reason they think so: there are cases in which one identical twin is autistic and the other is not; the two have identical genes, so environmental factors must contribute.

得出这一结论的原因如下：有的病例中，同卵双胞胎之一得了自闭症而另一个却没有，他们有相同的基因，因此环境因素应该对自闭症起了作用。

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This couldhappen because a chimeric twin that grew from an egg which his sperm did notfertilise would nonetheless have some skin cells containing his genes.

发生此类现象是因为一个嵌合双胞胎会产生于没有得到雄性动物施精的卵, 但它会在某些皮肤细胞中含有该雄性动物的遗传基因.

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Married siblings are more likely to pass on twin copies of harmful genes, leaving their children vulnerable to a variety of genetic defects. Tutankhamun's malformed foot may have been one such flaw.

兄妹之间通婚会将缺陷基因两份拷贝都传递给下一代，他们所生育的后代容易产生各种遗传缺陷，例如图坦卡蒙的畸形足。

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The Twin Research Unit is looking to extend their work to identify the exact genes responsible in this and other disease areas and would like to hear from Twin Volunteers aged over 18, either identical or particularly non-identical who can call the twin hotline on 0990-770099.

BBC: Identical twins

The twin unit was set up in 1992 to look at the role that genes play in the development of rheumatic diseases in older women and has now expanded to lok at most common diseases, behaviours and traits.

BBC: NEWS | Health | Genes to blame for restless sleep

But while twin studies are useful for establishing the extent to which a characteristic is heritable, they do not finger the particular genes at work.

ECONOMIST: The genetics of happiness