Over the last 50 years, scientists have conducted cloning experiments in a wide range of animals using a variety of techniques. In 1979, researchers produced the first genetically identical mice by splitting mouse embryos in the test tube and then implanting the resulting embryos into the wombs of adult female mice. Shortly after that, researchers produced the first genetically identical cows, sheep and chickens by transferring(使转移；使调动) the nucleus of a cell taken from an early embryo into an egg that had been emptied of its nucleus.

　　It was not until 1996, however, that researchers succeeded in cloning the first mammal from a mature (somatic) cell taken from an adult animal. After 276 attempts, Scottish researchers finally produced Dolly, the lamb from the udder(牛、羊等的)乳房) cell of a 6­year­old sheep. Two years later, researchers in Japan cloned eight calves from a single cow, but only four survived.

　　Besides cattle and sheep, other mammals that have been cloned from somatic cells include: cat, deer, dog, horse, mule, ox, rabbit and rat. In addition, a rhesus monkey has been cloned by embryo splitting.

　　【诱思导学】

　　1．What is cloning?

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　　2．When was the mammal cloned?

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　　【答案】　1.The term cloning describes a number of different processes that can be used to produce genetically identical copies of a biological entity. The copied material, which has the same genetic makeup as the original, is referred to as a clone.　2.It was cloned in 1996.

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