Our working hypothesis is that the Q-darker human chromosome segments have higher gene densities than the bright regions. Especially prominent in thisrespect are six. hot spots, the short Q-dark regions in 3p, 6p, 1 lq, 12q, 17q, and 19 (p or q), which have been chosen because their density of mitotic chiasmata is above 5. Chromosomes with gene-rich segments would act as trisomy lethals in very early embryos, whose spontaneous abortions would not be recognized. Containing active genes, the regions would be looped out in interphase and thus be more easily available for mitotic pairing and crossing-over.
To test this hypothesis, correlations and partial correlations of the following parameters have been determined: the density of mitotic chiasmata, the number and density of localized genes, the incidence of trisomic abortions, the length of chromosomes, and their Q-brightness. Overall, the correlations and partial correlations agree with, but do not prove, the working hypothesis. Far stronger evidence for our hypothesis comes from the highly significant negative effect of hot spots on trisomic abortions which would act as a kind of trisomy lethal. The gene numbers on the hot-spot chromosomes as compared with the controls, on the other hand, are in the right direction, but the difference is not significant.