POPULATION GENETIC MODELS OF GENOMIC IMPRINTING

The phenomenon of genomic imprinting has recently excited much interes t among experimental biologists. The population genetic consequences o f imprinting, however, have remained largely unexplored. Several popul ation genetic models are presented and the following conclusions drawn : (i) systems with genomic imprinting need not behave similarly to oth erwise identical systems without imprinting; (ii) nevertheless, many o f the models investigated can be shown to be formally equivalent to mo dels without imprinting; (iii) consequently, imprinting often cannot b e discovered by following allele frequency changes or examining equili brium values; (iv) the formal equivalences fail to preserve some well known properties. For example, for populations incorporating genomic i mprinting, parameter values exist that cause these populations to beha ve like populations without imprinting, but with heterozygote advantag e, even though no such advantage is present in these imprinting popula tions. We call this last phenomenon "pseudoheterosis." The imprinting systems that fail to be formally equivalent to nonimprinting systems a re those in which males and females are not equivalent, i.e., two-sex viability systems and sex-chromosome inactivation.