Since blight resistance is the result of the combined activities of many genes from the Chinese or Japanese chestnut parent, early breeders believed that only by the continuous crossing back to that parent could those genes be conserved. The chances of breeding a blight-resistant tree which looked and grew like an American chestnut under such circumstances were exceedingly low; on the order of one in several thousand.
At the American Chestnut Foundation (TACF), however, the primary approach is to continually cross back to the American parent, not to the Chinese and Japanese species. This method, called backcross breeding, is the standard method for transferring a single trait into an otherwise acceptable plant. It entails crossing Chinese and American trees to obtain a hybrid that is 1/2 American and 1/2 Chinese. The most resistant hybrids are then backcrossed to another American chestnut to obtain trees which are on average 3/4th American and 1/4th Chinese, then 7/8th American and 1/8th Chinese, and so on.
Since each backcross uses only those trees that show the most resistance, The American Chestnut Foundation is confident that even though the number of Chinese genes steadily falls, the genes for resistance are always conserved. The ultimate goal is to produce fully resistant trees which are 15/16th American and on 1/16th Chinese. Many of these resistant trees will be indistinguishable by experts from pure American chestnut trees.
Basic Representation of the American Chestnut Backcross Breeding process