Sex Ratio is the ratio of male members of a population to female members. It can be expressed as a permille value.

Fisher's sex ratio model

Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection outlined a model that explains the (approximately) 1:1 sex ratio commonly seen. Although attributed to Fisher, the theory is believed to have originated elsewhere.

Say the sex ratio is genetically determined, there being many possible ratios. When one sex is rare, it is advantageous to produce more of that sex. Thus alleles that code for that ratio are favoured and spread. The result is a stable equilibrium at 1:1. (this can be explained better!)

There are mechanistic constraints that might make a ratio 1:1 through meiosis, that may be difficult to alter.

Wolbachia

The bacterium wolbachia causes skewed sex ratios as it kills males through kin selection.