Why is Mendelian segregation so exact

Crow, JF,  Bioessays,  13:305-312. 1991.

The precise 1:1 segregation of Mendelian heredity is ordinarily taken for granted, yet there are numerous examples of ‘cheating’ genes that perpetuate themselves in the population by biasing the Mendelian process in their favor. One example is the Segregation Distortion system of Drosophila melanogaster, in which the distorting gene causes its homologous chromosome to produce a nonfunctional sperm. This system depends on three closely linked components, whose molecular basis is beginning to be understood. The system is characterized by numerous modifiers changing the degree of distortion. Mathematical theory shows that unlinked modifiers that change the degree of distortion in the direction of Mendelism always increase in the population. This provides a mechanism for removing cheaters and preserving the honesty of the Mendelian gene-shuffle.