From Creatures Wikia
Having haploid or monoploid genetics means an organism has only one version of each gene in their DNA, one from the mother OR one from the father. This is in contrast to diploidy, triploidy etc. Grendels, Ettins and Norns are haploid, having only one version of each gene in their dDNA. This means that, save for use of the variant and sex switches in a gene header, genes are always expressed, and a recessive/dominant pattern of inheritance is not possible. This in turn eliminates most of the dangers posed by inbreeding.