A line selected for high bristle number in Drosophila has a mean of 25 sternopleural bristles, whereas a low-selected line has a mean of only 2 bristles. Marker stocks for the two large autosomes II and III are used to create stocks with various mixtures of chromosomes from the high (h) and low (l) lines. The mean number of bristles for each chromosomal combination is as follows:
(Chrom II)(Chrom III) -> Mean Bristle #
(h/h)(h/h) -> 25.1
(h/l) (h/h) -> 22.2
(l/l)(h/h) -> 19.0
(h/h)(h/l) -> 23.0
(h/l) (h/l) -> 19.9
(l/l)(h/l) -> 14.7
(h/h)(l/l) -> 11.8
(h/l) (l/l) -> 9.1
(l/l)(l/l) -> 2.3

What conclusion can you reach about the distribution of genetic factors and their actions from these data?