In the last section, evidences for actual exchange of chromosome segments leading to crossing over were presented. One would, however, like to know whether exchange of segments takes place between entire chromosomes or between chromatids at four-strand stage. There is enough evidence which suggests that crossing over takes place at pachytene stage after the chromosomes have already duplicated giving rise to four strands, two strands belonging to each paired chromosome. The theoretical consequences of two strand and four strand crossing over are given in Figure 10.19. While two strand crossing over would lead to four recombinants, four strand crossing over will give two parental combinations and two recombinants among four gametes produced from a mother cell. By using attached X-chromosomes in Drosophila
and also by studying linear order in which recombinants and parental combinations are found in an ascus of fungus Neurospora,
it could be proved that crossing over takes place at four strand stage rather than at two strand stage.
Fig. 10.19. Difference in consequences between crossing over at the two strand and four strand stage (redrawn from Strickberger's "Genetics")