Clones and Cloning (Genetics & Inherited Conditions)
Asexual reproduction occurs in numerous bacteria, fungi, and plants, as well as some animals, leading to genetically identical offspring or clones. In addition, humans can assist in such reproduction. For instance, cuttings from plants generate thousands of replicates. Dividing some animals, such as earthworms or flatworms, allows them to regenerate. However, most vertebrates, including all mammals, reproduce sexually, requiring fertilization of an ovum by sperm. In such species, clones occur, as in the case of identical twins, when an embryo splits into two early in development. This process can be instigated artificially using microsurgical techniques to divide a harvested early-stage embryo and reimplanting the halves into surrogate dams (mothers). While this can be considered animal cloning, the term should be reserved for cloning from nonembryonic cells.
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Cloning Procedure (Genetics & Inherited Conditions)
Animal cloning typically refers to mammals or other higher vertebrates and involves creating a duplicate animal starting from a differentiated cell. Although such a cell only has the ability to perform its specialized function, its nucleus retains all genetic information for the organism’s development. Animal cloning requires that such information be reprogrammed into an undifferentiated cell that can reinitiate the developmental process from embryo to birth and beyond.
In theory, the process is straightforward. It consists of taking a differentiated cell from an adult animal, inserting its diploid nucleus into a donor ovum whose own haploid nucleus has been removed, initiating embryonic development of this ovum, inserting the resultant embryonic mass into a receptive surrogate dam and allowing it to proceed to term. In practice, the technique is difficult and was thought to be impossible until 1997. It also appears fraught with species specificity. Various differentiated cells have been used as the starting source; mammary cells were used in the first case, while skin fibroblasts and cumulus cells are now often used. The preparation of the anucleate ovum is an important step. A limitation to cloning dogs appears to be the difficulty in obtaining ova suitable for nuclear transfer. The technique for inserting the nucleus is crucial, as is the conversion to the undifferentiated embryonic state. Transfer of the embryonic...
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Identicalness (Genetics & Inherited Conditions)
Such a clone is not absolutely identical, because of mitochondrial differences and environmental effects. While the nuclear genome must be identical to its progenitor, the mitochondrial genome of the clone will invariably be different, because it comes from the ovum used. While mitochondria make a minor contribution to the total genetic makeup, they can influence phenotypic expression. In addition, the prenatal environment can affect some traits. Coat color and color pattern are characteristics that can be developmentally influenced; the first cloned cat was not an exact duplicate of its progenitor in coloration. Some behavioral features are also impacted during intrauterine development.
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Cloned Animals (Genetics & Inherited Conditions)
The first cloned animal was a sheep named Dolly. While she was the only live offspring generated from 277 attempts, her birth showed that animal cloning was possible. Shortly thereafter, mice and cattle were cloned. Reproducible cloning of mice is more difficult than imagined, whereas more cattle were cloned in the first five years after Dolly’s birth than any other species. Goats, pigs, and a cat were among the animals that were subsequently cloned.
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Problems and Potential Benefits (Genetics & Inherited Conditions)
Prominent among the problems with animal cloning is its inefficiency. Although this may not be surprising as the technology is still under development, no more than 2 percent of embryos generated lead to viable offspring. Additionally, most cloned animals are larger than normal at birth, often requiring cesarian delivery, and some have increased morbidity and mortality. Some have had smaller telomeres and shorter lives. Dolly exhibited this trait and lived for only six years (although she was euthanized, she clearly would not have lived much longer)—half of the average life span. Conversely, some cloned mice do not exhibit shortened telomeres or premature aging, even through six consecutive cloned generations. Further research will establish whether these problems are inherent to cloning, are consequences of some aspect of the current procedure, or are attributable to the small numbers of cloned animals studied.
The benefits of animal cloning would involve duplicating particularly valuable animals. Livestock with highly valued production characteristics could be targets for cloning. However, the technique is likely to be most beneficial in connection with transgenesis, to replicate animals that yield a therapeutic agent in high quantities or organs suitable for transplantation into humans. If animal cloning can be made efficient and trouble-free, its potential benefits could be fully developed.
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Further Reading (Genetics & Inherited Conditions)
Houdebine, Louis-Marie. Animal Transgenesis and Cloning. Translated by Louis-Marie Houdebine et al. Hoboken, N.J.: John Wiley & Sons, 2003. Describes the molecular biological techniques used to clone animals and create transgenic animals and the limits and risks of cloning, gene therapy, and transgenesis.
Panno, Joseph. Animal Cloning: The Science of Nuclear Transfer. New York: Facts On File, 2005. An overview designed for the general reader. Provides the history and basic facts of animal cloning, describes the cloning of Dolly the sheep, and examines the ethical and legal issues surrounding the creation of cloned animals.
Patterson, Lesley, William Richie, and Ian Wilmut. “Nuclear Transfer Technology in Cattle, Sheep and Swine.” In Transgenic Animal Technology, A Laboratory Handbook, edited by Carl A. Pinkert. 2d ed. London: Academic Press, 2002. Describes the detailed protocol needed to clone three livestock species, as well as the limitations to increased efficiency.
Pennisi, Elizabeth, Gretchen Vogel, and Dennis Normile. “Clones: A Hard Act to Follow.” Science 288, no. 5472 (2000): 1722-1727. Reviews the status of animal cloning, three years after the announcement of Dolly. The problems, questions, and concerns are presented in a highly readable text.
Wilmut, Ian, Keith Campbell, and Colin Tudge. The Second Creation: The Age of Biological Control by...
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Web Sites of Interest (Genetics & Inherited Conditions)
ActionBioScience.org. http://www.actionbioscience.org/biotech/pecorino.html. Features the article “Animal Cloning: Old MacDonald’s Farm Is Not What It Used To Be” and several useful links to the animal cloning debate.
Human Genome Project, Cloning Fact Sheet. http://www.ornl.gov/sci/techresources/Human _Genome/elsi/cloning.shtml. A basic overview of the subject, including a description of the technologies of DNA, reproductive, and therapeutic cloning.
Roslin Institute. http://www.roslin.ac.uk. The site of the oldest cloning group in the world, founded in 1919, which cloned Dolly the sheep. Includes information on genomics and animal breeding.
U.S. Food and Drug Administration (FDA), Animal Cloning. http://www.fda.gov/AnimalVeterinary/SafetyHealth/AnimalCloning/default.htm. In 2001, the FDA began examining the safety of food from cloned animals and their offspring. This page provides access to the agency’s findings, released in 2008, that these foods were safe for human consumption.
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