Scientists have discovered (to date) that humans have approximately 23,000 gene "slots" into which we receive gene variants (also called "alleles" or "polymorphisms") from our parents. Think of it as a set of 23,000 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.
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?