THE RELATION OF THE AGE OF THE FEMALE TO CROSSING OVER IN THE THIRD CHROMOSOME OF DROSOPHILA MELANOGASTER

The four methods of examining the relation of amount of multiple crossing over to age of mothers agree in showing that the "internode length" or average distance required for double crossing over has changed in a characteristic fashion, giving an M-shaped curve. These changes have not been independent of changes in total recombination but concomitant with them. However, the changes in recombination percentages were far greater than could be accounted for by change in internode length, and the larger factor must be assumed to be changes in the coefficients of crossing over. The amounts of these changes are greatest for the mid-sections of the chromosome and least for the distal sections. The changes in the two limbs are of like amount for equal distances from the center of symmetry in the distribution of simple and multiple crossing over.