Biodiversity is usually regarded as an asset or resource, the stock of which is partly natural and partly determined by humans. Humans both subtract from and add to this stock and consequently, the change in the stock is heterogeneous. This heterogeneity is not taken account of by some authors who focus only on the loss aspect. Frequently, the conservation of this stock is seen as important for the achievement of sustainable development; sustainable development being defined (but not always acceptably) as a situation on which the welfare of each future generation is no less than that of its preceding generation. Definitions of biodiversity are quite wide but here its focus is restricted to genetic diversity. Humans alter the stock of genetic diversity by eliminating some species or varieties of these and also add to this stock by selective breeding and genetic engineering. Both direct and indirect human impacts on diversity occur. The types of possible changes in the genetic stock are classified in a simple manner. It is pointed out that not all the genetic stock has positive consequences for human welfare because some of the genetic material has negative consequences (e.g. pests) for humankind or for some groups of human beings. This can make its evaluation of the genetic stock difficult. Implications of additions to the genetic stock by human manipulation of it (e.g. by the development of GMOs and selective breeding) are given particular attention. This raises the question of how many future generations should be taken into account in making choices about biodiversity and the manner in which their welfare should be allowed for. For example, should discounting be applied? Also how much precaution is needed to allow for uncertainty, for example, is a safety first rule advisable? These issues are discussed.