Recombination frequencies in this region are 20 fold higher than on autosomes . The small fourth chromosome of Drosophila melanogaster remains achiasmate and is normally segregated. Pseudoautosomal Regions Recombination in the pseudoautosomal region guarantees disjunction of X-Y chromosome pairs during male gametogenesis. The high pseudoautosomal recombination frequency compensates for the diminished region of homology between X and Y chromosomes. Some chromosomes, like the fourth chromosome, are segregated faithfully by a system that does not involve recombination.
Recombination is sufficient for normal disjunction.
Current concepts of recombination are a synthesis of conclusions from morphological , genetic and molecular investigations.