for centuries, plant breeders have improved the yields of major food crops such as wheat by selective breeding methods. In classical plant breeding the gene to be introduced must come from a plant that is fertile with the plant to be improved. So in wheat, for example, genetic variation can only be obtained from other wheat plants, or from very closely related ancestral grasses from which wheat has evolved. However,when wheat is crossed with these ancestral varieties, the embryos do not survive very well. New techniques have increased the rate of genetic exchange by promoting crossing-over during meiosis. This has led to the selection of embryos with genes from more distantly related plants because they are less likely to inherit harmful recessive genes from both parents.