Crossing Over 

• Crossing over is the process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. 
• Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids.
• The pairing of homologues at the beginning of meiosis, ensures that each gamete receives one member of each pair. Homologues contact each other along much of their length and are held together by a special protein structure called the synaptonemal complex.
•  This association of the homologues may persist from hours to days. 
• The association of the two chromosomes is called a bivalent, and because there are four chromatids involved it is also called a tetrad. T
• The points of attachment are called chiasmata.
• The forty-six chromosomes of the human diploid genome are composed of twenty-two pairs of autosomes, plus the X and Y chromosomes that determine sex. 
• The X and Y chromosomes are very different from each other in their genetic composition but nonetheless pair up and even cross over during meiosis. 
• These two chromosomes do have similar sequences over a small portion of their length, termed the pseudoautosomal region, at the far end of the short arm on each one.

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