Agrobiodiversity loss leads to genetic erosion, the loss of genetic diversity, including the loss of individual genes, and the loss of particular combinations of genes (or gene complexes) such as those manifested in locally adapted landraces or breeds. Genetic vulnerability occurs when there is little genetic diversity within a population of plants. This lack of diversity makes the population as a whole particularly vulnerable to disease, pests, or other factors. The problem of genetic vulnerability often arises with modern crop varieties, which are uniform by design. An example of the consequences of genetic vulnerability occurred in 1970 when corn blight struck the US corn belt, destroying 15% of the harvest. A particular plant cell characteristic known as Texas male sterile cytoplasm conferred vulnerability to the blight - a subsequent study by the National Academy of Sciences found that 90% of American maize plants carried this trait.