By creating herds of cloned animals that have the same genetic composition, either entirely or with respect to some particular traits, the diversity of those animals is significantly reduced. As a consequence, cloned animals and herds may be more susceptible to disease and less able to withstand an outbreak. This could have serious repercussions for the population as a whole and for the agriculture industry.
The range of potential environmental impacts associated with the proliferation of cloned animals has not been thoroughly investigated and is largely unknown and uncertain.
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References
1. Turner, J. (2002). The Gene and the Stable Door: A Report for the Compassion in World Farming Trust.