The conventional Mendelian model for diploid organisms assumes that the expression of an autosomal allele within an individual should be invariant of its sex of origin, that is, whether it is inherited from the father or the mother. This model is incorrect for a subset of alleles across many taxa, in particular mammals. In 1989 David Haig and Mark Westoby outlined the evolutionary rationale for parent specific gene expression.^1 In 1992 paternal and maternal specific imprinting of the insulin growth factor 2 and insulin growth factor 2 repressor loci (Igf2, Igf2r) in mice confirmed their predictions.^2 Over the last 15 years as many as 200 loci have been identified as being subject to parental specific genomic imprinting.^3 The mechanisms involved are clear at the most basic level, the repression of gene expression via epigenetic mechanisms such as methylation, but the details of the molecular genetic processes are still relatively obscure and mysterious.^4 But perhaps the larger question is why parental specific gene expression, operationally the transformation of a locus to a haploid or monoallelic expressive state, should occur on a particular subset of loci. Fundamentally this is an evolutionary question, and concurrently with the proximate exploration of the molecular mechanisms there has grown a theoretical body of work which explores the ultimate rationale of the emergence of these peculiar molecular systems.
What is the relevance of theory to biology? The examination of biological questions has traditionally occurred on various methodological and organizational levels. Darwin's theory of evolution via natural selection upon heritable variation was proposed without a viable genetic theory. It was 40 years between the publication of The Origin of Species and the emergence of modern genetics, and another 20 years before evolutionary biology integrated the Mendelian framework.^5 Of course, this preceded the work of Francis Crick and James Watson on DNA, so for a generation the Neo-Darwinian Modern Synthesis of evolutionary biology and genetics existed without any understanding of the biochemical nature of genes. These historical precedents show that despite a lack of understanding of mechanistic details, an evolutionary theoretical program can be fruitful in laying the groundwork for future understanding of a whole system, and a cogent set of predictions may serve as the blueprint for a detailed line of work to elucidate proximate causes.
