Figure 1. Influence of polyandry on parental allele relatedness. Note that relatedness of maternal alleles is ‘neutral’ with respect to number of mates, whereas polyandry (different fathers) abolishes relatedness between paternal allelic components of embryo genotype.

Figure 2. Schematic of a transcriptional ‘arms race’ between maternal and paternal alleles at a locus that influences parental investment (PI). Incremental increases in expression of one parental allele are tracked by incremental decreases in expression of the other parental allele.

Figure 3. (a) Active ‘selfish’ X^p. Embryos of genotype X^mX^p (female) and X^mY (male) compete for maternal resources. Thickness of vertical arrows represents relative amount of maternal resources demanded or transferred. In the absence of preferential paternal X-inactivation, ‘selfish’ X^p-linked alleles (e.g. X-linked placental growth factors) will cause dosage imbalances between male and female embryos that may be translated into relatively increased PI in female embryos to the detriment of male sibs. (b) Inactivated X^p. On the assumption that the Y-chromosome does not contain loci encoding significant effects on PI, imprinted X^p inactivation equalizes gene dosage and PI between male and female embryos.