In a diploid yeast population which is heterozygous for a given marker
, A(1)A(2) mitotic crossover (mit. c.o.) between the centromere and th
e marker will give rise to homozygous daughter cells, A(1)A(1) and A(2
)A(2). Since this causes a decrease in the frequency of A(1)A(2) cells
, mit. c.o. is an important population genetic process in vegetatively
propagated yeast cultures. The effect of mit. c.o. is counteracted by
mutations and, in the case of heterosis, by selection. We present a m
athematical analysis of these interactions.