As Hamilton & Zuk pointed out, some loci may be of special importance for sexual selection because they play a crucial role in the co-evolution between parasites and hosts. In previous work I have tried to extend Hamilton & Zuk's parasite hypothesis for sexual selection, partly by including findings of immunologists and endocrinologists: in some species, handicapping signals may specifically reveal the current needs of the immune system which depends on the host's susceptibilities to different parasites. In other species, depending on the constellation of some key variables, non-handicapping signals could directly reveal the identity of resistance genes. Despite the general conflict of interests between the sexes, sexual selection may, in these cases, lead to signallers (i.e. mostly the males) focusing on improving their offspring's survival chances instead of trying to maximize their number. Males achieve this by allowing choosy females to optimize costs and benefits of each resistance. Both parts of the extended parasite hypothesis suggest that female choice for specific heritable matequalities aim to optimize the resistance genetics of the unfertilized eggs. However, intersexual selection could go further than just choosing a mate. Here, I list the possible selection levels at which the mother and/or her ova could select for specific sperm haplotypes before, during and after the formation of the zygote. For many of these possible selection levels, evidence suggests that selection after mating might favour heterozygosity or even certain specific allele combinations at loci which are involved in the parasite-host co-evolution (e.g. the major histocompatibility complex or the transferrin locus).