Why are diploid genomes widespread and dominant mutations rare?
  We have used the sexual Penna ageing model to show that the relation between dominance and recessiveness could be a force which optimizes the genome size. While the possibility of complementation of the damaged allele by its functional counterparts (recessiveness) leads to the redundancy of genetic information, the dominant effect of defective genes tends to diminish the number of alleles fulfilling the same function. Playing with the fraction of dominant loci in the genome it is possible to obtain the condition where the diploid state of the genome is optimal. If the status of each bit position as dominant or recessive mutations is changed for each individual randomly and rarely, then after a long time a stationary equilibrium of many recessive and few dominant loci is established in the sexual Penna model. This effect vanishes if the same changing distribution of dominant loci applies to all individuals. 
Diana Garncarz, Stanislaw Cebrat, Dietrich Stauffer and Klaus Blindert