that also has its origins in the 1970s (Willadsen and Polge, 1981; Willadsen, 1989). What distinguishes it from somatic cell nuclear transfer, the technology that led to the creation of Dolly and much of the controversy over human cloning, is the stage of development at which the nuclei are transferred (Wilmut et al., 1998). In the older procedure, the cells or blastomeres used were from the so-called morula stage of cell development (although some were from the cleavage stage and others from the blastocyst stage) when the embryo still is an undifferentiated mass and its cells presumed still capable of forming all tissues of a fetus.
The cloning technologies of embryos splitting (EMS) and embryonic nuclear transfer (NT) were introduced into dairy cattle breeding in the 1980s. The Animal Improvement Programs Laboratory of the USDA’s Agricultural Research Service (ARS) is responsible for tracking the performance of dairy cattle throughout the U.S. Recently, working with the Holstein Association, they evaluated the performance of cloned Holsteins produced by EMS and NT (H.D. Norman, USDA–ARS, personal communication). The numbers of EMS and NT clones were documented by gender and birth year. All NTs were from embryos rather than adult cells. Through 2001, there were a total of 2,226 EMS (754 males and 1,472 females) and 187 NT (61 males and 126 females) Holstein clones registered. Of female EMS clones, 921 had yield records, and 551 had noncloned full siblings with yield records. Of the 126 female NT clones, 74 had yield records, but only 11 had noncloned full siblings. These familial relationships were used to compare the performance of cloned and noncloned full siblings for standardized traits and genetic evaluations as part of the national evaluation program. These standardized traits included total milk yield, fat content (by weight and pecent), protein content (by weight and percent), somatic cell score, and productive life (in months). Also calculated were yield from contemporaries and predicted transmitting ability. Norman and his colleagues concluded that the numbers of clones have decreased for EMS males and for all NT clones over the past decade. Animals that were selected for cloning were slightly superior genetically to the contemporary population mean for yield traits; the yields of NT clones were similar to, and those of EMS clones were slightly less than, those of their noncloned full siblings.
“Modern” cloning involves taking an unfertilized egg, removing its chromosomes, and introducing the nucleus from a differentiated cell of the animal to be cloned, which is frequently an adult (Box 1.1; Wilmut et al., 1997; Polejaeva et al., 2000; Kuhholzer and Prather, 2000). The introduced nucleus is reprogrammed by the cytoplasm of the egg and directs the development of a new embryo, which is then transferred to a recipient mother to allow it to develop to term. The offspring formed will be identical to their siblings and to the original donor animal in terms of their nuclear DNA, but will differ in their mitochondrial genes and possibly also in the manner their nuclear genes are expressed or biochemically engineered (see Box 1.1 and Chapter 2). Cloning