Haploid-Diploid Sex Determination
In ants, bees, wasps, some mites, sex is determined by the number of SETS of chromosomes. Males are haploid, and descend from unfertilized eggs. Females are diploid, they result from the genetic contribution of a mother and a father.
Females of these species can either fertilize each egg, or let it remain unfertilized, thus controlling the sex of their offspring.
This sometimes results in sex ratios that are skewed in favor of females, because mothers ensure that their sons will not compete with each other for mates by laying fewer male eggs.