16. Crossover values in male and female flowers. Studies on the frequency of crossing over for different regions of chromosome 5 in mega- and microsporocytes have been continued. Earlier work by Emerson and Hutchison, Stadler, Eyster, Collins and Kempton, and Rhoades and Rhoades have shown no consistent difference in crossing over for chromosomes 2, 4, 9, and 10 in the male and female flowers. However, a considerable amount of data have been accumulated which show that this does not hold for chromosome 5. These data prove that in the male flowers the frequency of crossingover is greater than in the female. Because of ease in classifying most of the data are for the a2-bt region. Two different stocks have been used. In one of them a relatively high amount of recombination occurs while in the second stock a much lower value was found. The difference between the high and low stocks is not known but in both higher crossover values in the male flowers was found. Exact reciprocals were made in obtaining male and female crossover percentages.

Summary of high a2-bt line (10 pairs of reciprocals)
  A2 Bt A2 bt a2 Bt a2 bt % Recomb.
Male 1156 420 414 1103 27.0
Female 1294 256 278 1290 17.2
Summary of low a2-bt line (16 pairs of reciprocals)
  A2 Bt A2 bt a2 Bt a2 bt % Recomb.
Male 2348 373 410 2590 13.7
Female 1902 110 120 1827 5.8

In addition to the above data on the a2 bt region, data have been obtained on the a2 bm, bm Pr and bt-Pr regions. There is a consistent and highly significant increase in crossingover in the male flowers for all of these regions. The data also suggest that the greatest reduction occurs in those regions adjacent to the centromere, i.e. there is a proportionately greater reduction in the a2 bm and a2 bt regions than in the bm Pr and bt Pr regions but, owing to the difference in length of these regions, this point has not been statistically established as yet.

In order to determine if the crossover difference for the two sexes found for chromosome 5 is a cellular characteristic affecting all chromosomes indiscriminately or is peculiar to chromosome 5, tests were made involving the c wx region in 9 and the bm pr region in 5 simultaneously. No difference in crossingover in the two sexes was found for the c wx region.

M. M. Rhoades