Sister and non-sister chromatid U-type exchange in rye meiosis

Aberrant meiotic chromosome configurations in an experimental population of rye lines are known to result from spontaneous U-type exchanges during meiotic prophase. Both sister chromatid and non-sister chromatid exchanges occur and this study is concerned with the relative frequencies of sister and non-sister exchanges. The anaphase I observations reveal a marked excess of non-sister U-type exchange configurations and it is argued that this reflects an original excess of prophase non-sister U-type exchanges. This conclusion is discussed with special reference to the origin of meiotic U-type exchanges and their relation to regular crossover exchanges.