Y Chromosome                                                 XY *
        -  is much smaller than its homolog, the X chromosome
        -  holds only a dozen or so genes    (X holds 2,000 to 3,000 genes)    
                     involved in male fertility (code 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*