Crossing-over modifiers and other genetical changes in the crossing-over value

Abstract

This is probably one of the most fascinating chapters in analytic genetics, and one of the most important chapters in cyto-genetics. The fact is, as Rasmusson (93) puts it, “... the linkage values are inherited as any quantitative character of any organism propagating in a sexual way .... We must also conclude that it is most probable that the variations in linkage values represent a quite common and ordinary trait of heredity”. It is however absolutely necessary not to underrate the modifications caused by environmental conditions. This Rasmusson unfortunately did. He stated that all results indicate that modifications caused by environment are almost unimportant. How Rasmusson possibly could have said this even in 1927—’28 when so many experiments have convinced geneticists of the very great importance of environmental influences on crossing-over, cannot be. understood. It might be that due attention to Sturtevant’s (114) article: „A case of rearrangement of genes in Drosophila“ would have mitigated an extreme opinion as Rasmusson’s. This will be mentioned in due course. In the meantime the great importance of crossing-over modifiers and related phenomena e.g. of genetically changed linkage values is admitted, and we may pass on to consider the facts so far obtained.