Genomic imprinting effects on complex traits: a phenotype-based perspective.

Parent-of-origin dependent gene expression associated with genomic imprinting is expected to result in the phenotypic signature of a difference between the average phenotypes of the reciprocal heterozygotes. Quantitative genetic mapping approaches have exploited this phenotypic manifestation to identify putatively imprinted loci affecting the expression of 'complex' traits. By focusing on the phenotypic consequences of imprinting, quantitative mapping techniques enable us to rapidly scan the genome for imprinted loci. Such genome-wide scans can be used to identify the array of pleiotropic effects of imprinted loci and to establish associated imprinting patterns including partial imprinting and more complex imprinting patterns, such as polar overdominance. Moreover, quantitative mapping makes no assumptions about underlying mechanisms of parent-of-origin-dependent effects and can thus help identifying alternative mechanisms causing imprinted expression. Thus, we suggest that phenotype-based mapping approaches can significantly advance our understanding of various aspects of genomic imprinting, especially when integrated with other molecular or bioinformatic approaches.