Y Chromosome                                         XY *
        -  is much smaller than its homolog, the X chromosome
        -  200 to 300 million years ago Y held genes equal to the X chromosome
        -  today X & Y share just 78 genes. The Y chromosome holds a core of 12 genes        
                                      involved in  vital cell functions in the heart, blood, and lungs.
        -  also involved in male fertility (codes for testes proteins - many are repeat amplified)
        -  holds some "housekeeping genes" (†)  &  genes with homology to the "X" (‡)   (fig)
        -  holds lots of "junk" DNA   &   STR's   (Short Tandem Repeats)
        -  holds a gene - SRY  (sex determining region)  encodes a protein that triggers
                         formation of testes by activating genes on several of the autosomes.

X & Y evolved from a matching pair of autosomes some 240 to 320 millions years ago
        tips of X & Y can engage in recombination, but rest of Y can not
             the degeneration of Y chromosome occurred after evolution of SRY gene, because
             of a loss of ability to recombine during meiosis... without recombination genes on
             Y mutated, stopped working, & degenerated      (fig)  
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