Evidence for Sister-Strand Crossing over in Maize.

ROSSING OVER, the exchange of chromatin between homologous chroC mosomes, is of utmost importance in genetic studies. Although little is known about the mechanism which is responsible for this exchange, a number of facts on this subject have been fairly well established: (1) Crossing over is produced by breakage and reunion of broken end's (even on BELLING'S hypothesis, breakage and reunion have to be postulated to explain 3-strand and 4-strand doubles). (2) The position of the exchange is exaotly between homologous regions of the chromosomes. (3) Crossing over occurs at prophase of the first meiotic division. (4) Only 2 of the 4 chromatids are involved in any one crossover. (5 ) The occurrence of one crossover decreases the probability of another occurring in its vicinity-the phenomenon of interference. It has generally been assumed that there is little or no sister-strand crossing over. This