We don't have 20,500 genes; We have 20,500 gene variants

Scientists have discovered (to date) that humans have approximately 20,500 gene "slots" into which we receive gene variants (also called "alleles" or "polymorphisms") from our parents.  Think of it as a set of 20,500 mailboxes in a long row.  Into each box is deposited one gene package (variant) from our mother, and one from our father.  The gene packages deposited in a particular box (say, mailbox number "GRM3") are fairly similar, but not identical; they are unique gene variants which explain human diversity.

Geneticists identify each gene location by a Hugo name, and typically refer to the specific contents of that gene (i.e. the gene variant) by letter (e.g. variant A, variant B, variant C..) or some other name.

For example, everyone has a variant of the GRM3 gene - but if you receive the 'A' variant you have a greater chance of developing Schizophrenia.

Do gene variants serve a purpose when they are responsible for causing diseases?  It's hard to speculate.  Do schizophrenics contribute something essential to the survival of society, perhaps once every 1,000 years, that encourages the variants to remain in the collective genome of society, even though it's a terrible price to pay for those afflicted?