A plant's seed color is determined by two genes, A and B, each with two alleles. The A gene has three genotypes (AA, Aa, and aa) with phenotypic effects of 8, 6, and 4 units of color, respectively, and the B gene has similar effects. If seed color is determined by a threshold system, where at least 10 units of color are needed for purple seeds, what proportion of the F₂ progeny will be purple?