Genetic Factors Affecting the Strength of Linkage in Drosophila.

in two days; This number of eggs therefore has not received the exposure necessary to produce the change in crossing over. The culmination of the two days exposure is to be expected in those eggs so situated that 125 to 175 eggs will be laid before them. Such an interpretation makes it extremely likely that the change in the amount of crossing over is finally affected in the earliest oocytes, that is, at the beginning of the growth period. The above evidence on the time of applying the new temperature and the time when the change in crossing over occurs, suggests that the crossing over process takes place in the stage when the chromosomes of Drosophila are known to be finely drawn out threads. The decrease in the strength of linkage caused by temperature in no way weakens the chromosome interpretation of linkage. It rather adds to it considerable support, for it localizes the process of crossing over at a period in oogenesis when twisting between homologous threads seems possible. The evidence positively establishes the fact that crossing over does not take place during the early oogonial divisions, and makes it extremely improbable that it occurs at so late a stage in the growth period as the thick thread stage favored by Janssens as the chiasmatype.