Some genes exclusively express only their maternal or paternal copy.
Mammals inherit one copy (allele) of each gene from their mother and another copy from their father. Yet for many genes, only one of these alleles is always expressed in a cell. The choice of which allele is expressed is random in some cell types – notably those of the olfac¬tory and immune systems; for others, such as those of the developing placenta and brain, certain genes are ‘imprinted”. The hallmark of imprinted genes is that some are expressed only when inherited from the mother and others only when inherited from the father.
Imprinted genes were thought to be fewer than 100 in number.
Studies published in the Journal Science by Gregg and colleagues identify 1,308 candidate imprinted genomic regions in the mouse brain, encompassing 824 annotated genes as well as the entire X chromosome.
It has already been shown that, in the mouse placenta, the X chromosome is imprinted, with genes from the maternal X being exclusively expressed, thereby avoiding immunological rejection of foreign fetal proteins that might be encoded by the paternal X chromosome”,

Many of the genes on the X chromosome are also expressed in the brain. In males (XY), the single X copy always originate from the mother, but in females ( XX) either the maternal or the paternal copy of the X chromosome is inactivated early in embryonic development, and this occurs at random. Gregg et al report preferential expression of the maternal X chromosome in two brain regions. Compared with the paternal X, the expression of the maternal X chromosome was 11 % higher in glutamate-secreting neurons of the cortex, and 19% higher in the preoptic region of the basal forebrain.
In general, the expression of imprinted genes is exclusively either maternal or pater¬nal, with loss of exclusivity usually leading to expression of both alleles. This suggests that the biased gene expression described by Gregg et al. may be due to selection of cells express¬ing the maternal X chromosome, rather than imprinting. DNA replication errors increase with the number of cell divisions, which are an order of magnitude higher in the production of sperm than female oocytes
Keverne 2010 A mine of imprinted genes Nature vol 466 pp 823-4

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