For example, that's what the American Chestnut Foundation is doing through backcrossing: http://www.acf.org/r_r.php
Although the Chinese genes for resistance are only incompletely dominant, they nonetheless usually express themselves clearly when present in seedlings purposely inoculated with a virulent form of the blight fungus. And that is how each backcross generation is tested - by inoculation with blight. Only those seedlings that show the greatest resistance are used for further backcrossing to an American parent.
But every backcross, although necessary to recover desirable American traits, also reintroduces the genes for blight susceptibility from the American parent. In order to remove those genes, the next steps at TACF are intercrosses. In the first intercross, the most blight-resistant 15/16ths American trees are crossed with other blight-resistant 15/16ths American trees. Again, only resistant seedlings are saved.
At the first intercross, it may prove difficult to select inoculated seedlings which have only inherited genes for blight resistance from their Chinese ancestor and no genes for blight susceptibility from their American ancestors. Testing in subsequent generations or a test cross back to an American parent will confirm that first intercross trees contain only the Chinese genes for resistance. Most or all the progeny of parents containing only genes for blight resistance should show blight resistance, whereas some progeny of parents with genes for susceptibility should show susceptibility to blight.
When crossed with each other, these highly blight-resistant parents will breed true for resistance, since they will have no American genes for susceptibility to blight. This second intercross will yield nuts for restoration.
These seedlings are even trickier, since their parents were already multi-species hybrids of American, Chinese, Japanese, and Korean chestnuts that all cross-pollinated in the orchard of the nursery I bought their seed from. So, technically they're hybrids of hybrids of hybrids! Since I'm more interested in producing trees for nuts under cultivation, I don't need as much American genetics in my trees. It's really only needed up here for cold hardiness; Asian chestnut species have a hard time coping with -30F winters that American chestnuts shrug off. I'll just weed out any that are runty or get cut back by winter cold, and keep the most vigorous 6-7 for my yard. They should fruit on their own within 5-8 years, at which point I'll grow out those nuts, and so on and so forth. If chestnut blight finds my little patch, all the better because it would help weed out the ones that aren't resistant enough.
It sounds crazy to most people, but I think it's great fun!
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