If crossing over did not occur during there would be less genetic variation within a .
With crossing over potentially every chromosome could consist of a combination of both maternal and paternal alleles which along with random segregation of chromosomes during anaphase II would result in a nearly limitless number of genetic variations between related individuals and within a species.
Without crossing over each chromosome would be either maternal or paternal greatly reducing the number of possible genetic combinations which would greatly reduce the amount of genetic variation between related individuals and within a species.