Its been a long time since I had genetics in college, so lets see if I can get this right. If the white flower gene is a recessive gene (w) and purple is dominant (P), then each seed that grows into a white flower plant has to have the white gene from both parents. The white recessive gene can be in the population but infrequently expressed. The number of plants in your area with all white flowers will ultimately depend on the frequency of the white flower gene/allele in the population. For example, maybe 10 percent of Maypop plants in your area have Pw genetics (these would produce purple flowers, but still have the white gene). In a plant with Pw genetics, half of the pollen and ova will have the white flower gene. You would still have to get 2 plants in your area with the white gene to pollenate each other--then 25% of seeds from such a pollenation would be ww, and produce a plant with all white flowers. If 10 percent of local plants are Pw, then on average you need 20 different local plants to be pollenating one another to have a chance at producing seeds that will grow into plants with all white flowers. So again, the overall prevalence of white flower plants will depend on the overall frequency of the white flower gene in the population. I'm sure this varies geographically.
If the white gene is recessive, then crosses of 2 plants with all white flowers should yield all seeds with white flower genes. Crosses of your white flower plant with other local purple flower plants will probably produce mostly purple flower plants, but this depends on how many plants in your area carry the white gene.
One interesting thing that I am finding in my experiments with P. incarnata, is that it is actually somewhat difficult to get successful pollenations. I am not sure why this is the case. You would think that every time you used a paint brush to transfer pollen between 2 different incarnata plants, it ought to be successful 100% of the time, but that is far from the case. At first I thought maybe it was a time of year thing with the earliest flowers maybe having sterile pollen. The main fruit drop in my area occurs throughout the month of September, and given the average of 8 weeks from pollenation to fruit drop, this means that July pollenations should be more successful than June pollenations (I was getting 10% success in June). But so far July hasn't been a whole lot better. I have had better success on cloudy/rainy days, so it may be a temperature thing (its been hotter than usual this year). In addition, I have found that seeds from locally collected fruit (2 different locations) have a low germination rate (in some cases zero). I have so far collected 7 different incarnata plants from 3 different states, and I'm hoping to add another 4 to 6 before the summer is over. I should get some interesting results over the next 2 years, and might possibly shed some light on why it is so difficult to germinate passie seeds.
Mark
