Question

What prevents recombination between the x and the y chromosome?

Answer 1

There are two small pseudoautosomal regions (PAR1 and PAR2) present at tip of both X and Y chromosome. These regions are the only regions that can undergo recombination between X and Y chromosomes, due to homology between them. PAR1 spans first 2.7 Mb of the proximal arm of these chromosome shwile PAR2 spans distal 320 kb region on long arms of the chromosome. Rest of the regions on X and Y chromosome cannot cross-over. Cross-over or recombination is prevented between X and Y chromosomes except at the PAR regions due to no significant homology. Pairing of PARs is required for male meiosis. Here a XY body is formed, that is transcriptionally inactivated by meiotic sex chromosome inactivation (MISC). This pairing or recombination is allowed only during meiosis so that female offsprings that have one X and one Y can inherit alleles that are present on Y chromosome.

The middle regions of the chromosome do not recombine to prevent mutations as well as to prevent degenercy that occurs over evolution. If there was recombination allowed, then deleterious mutations would be carried over in future generations. The DNA sequence of X and Y chromosome are entirely different. Hence, homologous recombintion cannot occur, except at PARs. Chromosoal seggregation of chromosomes requires homologous regions which can pair and cross over during recombination. As X and Y chromosomes have completely different sequences, they cannot pair during recombination, except at PAR. The Y chromosome has a shorter arm than X chromosomes,which contains fewer genes than X chromosomes. Reccombination can occur in a non-homologous manner, but this is not a normal event. Recombination nodules present on X and Y chromsomes are insufficient in frequency for obligate cross over of X and Y chromosome. The recombination nodules are generally required during prophase 1 of meiosis and are associated with synaptonemal complex. These recombination nodules are also present in wrong substage of pachytene in X and Y chromosome. The non-homologous synapse between X and Y chromosome is prevented by differentiation or thickening of regions that are present in X and Y axes. At the time of crossing over, a desynapsis occurs between the differentiated portions of X and Y chromosome, before the timing of cross-over. This prevent recombination events.