We have denoted two possible models of gene order evolution as the "card" and "chess" models. The first suggests random shuffling of genes during evolution, the second--non-random gene transposition, gene order being checked by natural selection. We discuss here localization of the mating type locus in fungal genomes. In 8 of 10 genetically well studied species of ascomycetous fungi, the mating type locus is linked to a centromere; in one species, it segregates regularly during the second meiotic division, and only one species does not show any regularity in the mating type locus segregation. Centromeric linkage of the mating type locus maintains heterozygosity of all centromeric genome regions during intratetrad fertilization observed in some fungi. Non-random localization of mating type locus can be considered as the means for conservation of heterozygosity.