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Modern Genetic Analysis Chapter 1. Genetics and the Organism Genetics and Human Affairs
Genetics In Process 1-1: Making genetically identical mammals

Genetically identical mammalsespecially humansare a topic of considerable interest to both geneticists and nonscientists. Identical twins, which occur naturally by the splitting of a fertilized egg to create two individuals, have always been the subject of curiosity, and have even formed the basis of scientific attempts to calculate the genetic component of variation in human phenotypes, such as certain types of behavior.

More than 20 years ago it was found that individual cells of a plant could be grown in culture to produce a group of genetically identical plants, called a clone. At the same time, it was also possible to clone the African clawed toad Xenopus laevis by using a micropipette to remove nuclei from intestinal cells of a tadpole and injecting them into enucleated toad eggs. Clearly the developmentally advanced tadpole cells contain all the genetic information that can support normal development of an adult animal, a condition known as totipotency. However, later developmental stages were not totipotent.

In February 1997, front pages of newspapers worldwide carried the picture of a pleasant-looking sheep named Dolly, who had been produced by implanting the nucleus from a cell of the udder of a Finn Dorset ewe into the enucleated egg of a Scottish Blackface ewe. The simple- sounding procedure was successful only if the donor cells were starved in culture in order to induce the arrest of mitotic cell division, a ploy that seemed to make the donor nuclei compatible with the recipient egg, which was in a late stage of meiosis. This surprising experimental result showed that the adult mammalian nucleus is still totipotent.

The story made headlines, not because of totipotency but because it was clear that extensions of the technology that produced Dolly also might be used to clone genetically identical human beings. This touched off a heated debate on the ethics of such a procedure and some of its possible misuses. Indeed several countries subsequently passed laws or statutes that regulated or even banned this type of experimentation in humans. Are there legitimate uses for human cloning? One suggestion is that if an infant or small child dies by accident, then this individual's short window of existence could be extended by cloning. However, at present the issue is moot because the efficiency of the mammalian cloning process is still lowmany defective embryos were produced before Dolly.

© 1999 by W. H. Freeman and Company. All rights reserved.