In addition to the genes for colour, some of which I've discussed in a previous post, there are a number of genes of home horticultural importance.
Foremost are probably the genes for powdery mildew resistance. There are two genes, both of which confer resistance to Erysiphe polygoni the organism responsible for powdery.
They are known as er-1 and er-2, both are recessive. The JI database notes of er-2 "Resistance broken under controlled growth conditions so not as durable as resistance conferred by er-1".
Being recessives sahould make them easy to fix, but since the only phenotypic expression is disease resistance, it is hard to distinguish whether they are present or not until E.polygoni strikes, and then how might one distinguish between them?