David and Goliath or Y versus X.

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Chromosomes are active machines which remain nested in the cell nucleus (its' core).   They contain “clocks” which are preprogrammed (by evolution) and stimulated by the changing internal cell environment to trigger the creation of partial replications of themselves (pieces of ribonucleic acid) which in turn travel outside of the cell nucleus.  One of the actions of the gonosomes is to create a maturational cascade which leads to the creation of the gender-specific reproductive organs.   The gonad starts secreting sex-specific hormones around the seventh or eighth week of gestation.  This secretion into the blood stream affects in turn the development of body features,  including subtle details of brain morphology.   In short, a single gene on the Y chromosome decides whether a human being will be born with a penis or a vagina. Also,  this gene determines to a great extent (as we shall see in detail) whether that person will later be sexually attracted to men or women (sexual orientation),  and whether that person will consider himself or herself to be psychologically male or female (sexual identity).  The latter two eventualities depend on fine tuning of the brain occurring before birth.   At around age twelve for females and around age thirteen or fourteen for males,  the chromosomes release new sets of partial copies of themselves which have the effect of further sexualizing the body and the mind.  This developmental milestone is called puberty.   What actually happens is that the fully formed gonads secrete a surge of steroid (sex-specific) hormones,  most importantly progesterone and estrogen for females and testosterone for males.   We will see in other chapters some of the extent to which the X chromosome is biologically so much more important than the Y chromosome.  One indicator of the importance of the X chromosome is the large number of diseases caused by bad genes located on it.  There are several dozens of major diseases which are caused by mutated genes located on the X chromosome (daltonism, hemophilia, Tay-Sachs disease, Lesch-Nyhan's disease).  We know of none to be due to genes located on the Y chromosome.   In fact,  twenty years ago, the only trait known to be transmitted exclusively from father to son via the Y chromosome was ear hair.