Effects of genomic imprinting on quantitative traits Hamish G. Spencer Received: 13 July 2008 / Accepted: 16 July 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Standard Mendelian genetic processes incor- porate several symmetries, one of which is that the level of expression of a gene inherited from an organism’s mother is identical to the level should that gene have been inher- ited paternally. For a small number of loci in a variety of taxa, this symmetry does not hold; such genes are said to be ‘‘genomically imprinted’’ (or simply ‘‘imprinted’’). The best known examples of imprinted loci come from mam- mals and angiosperms, although there are also cases from several insects and some data suggesting that imprinting exists in zebra fish. Imprinting means that reciprocal het- erozygotes need not be, on average, phenotypically identical. When this difference is incorporated into the standard quantitative-genetic model for two alleles at a single locus, a number of standard expressions are altered in fundamental ways. Most importantly, in contrast to the case with euMendelian expression, the additive and dom- inance deviations are correlated. It would clearly be of interest to be able to separate imprinting effects from maternal genetic effects, but when the latter are added to the model, the well-known generalized least-squares approach to deriving breeding values cannot be applied. Distinguishing these two types of parent-of-origin effects is not a simple problem and requires further research. Keywords Genetic correlation  Genetic variance  Genomic imprinting  Heritability  Population genetic theory Introduction One of the several symmetries of standard Mendelian genetics—what Normark (2006) calls the ‘‘euMendelian symmetries’’—is the symmetry of expression: a gene’s expression is independent of its parental origin. For a number of both animal and plant genes, however, this symmetry is broken, with the maternal and paternal copies expressed at different levels, at different stages of devel- opment or in different tissues (or some combination thereof). Such genes are said to be ‘‘imprinted’’ because the differential expression implies they have been marked in some way by the transmitting parent so that they retain a memory of their parental origin. The archetypal imprinted mammalian gene is insulin- like growth-factor 2 (Igf2), which is imprinted in all eutherian and marsupial species examined to date: human, domestic mouse, two species of deer mouse, rat, sheep, cow, pig, opossum and tamar wallaby (Giannoukakis et al. 1993; DeChiara et al. 1991; Vrana et al. 1998; Pedone et al. 1994; McLaren and Montgomery 1999; Dindot et al. 2004; Nezer et al. 1999; O’Neill et al. 2000; Suzuki et al. 2005). In most fetal tissues of these species, the maternal copy of Igf2 is inactive, only the paternal copy being transcribed. Neither monotreme (echidna and platypus; Killian et al. 2001a) nor avian (chicken; Koski et al. 2000) Igf2 appears to be imprinted, although intriguingly, some chicken embryos transcribe both copies of igf2, whereas in other individuals it is apparently expressed monoallelically, with different individuals showing either paternal expression or maternal expression (Koski et al. 2000). Indeed, outside the theria (eutherians and marsupials), there are no unambiguous cases of endogenous vertebrate genes being imprinted (Morison et al. 2005). Nevertheless, in zebrafish, some transgenes have been found to be H. G. Spencer (&) National Research Centre for Growth & Development and Allan Wilson Centre for Molecular Ecology & Evolution, Department of Zoology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand e-mail: h.spencer@otago.ac.nz 123 Genetica DOI 10.1007/s10709-008-9300-8