Complex multicellular organisms, such as mammals, express two complete sets of chromosomes per nucleus, combining the genetic material of both parents. However, epigenetic studies have demonstrated violations to this rule that are necessary for mammalian physiology; the most notable parental allele expression phenomenon is genomic imprinting. With the identification of endogenous imprinted genes, genomic imprinting became well-established as an epigenetic mechanism in which the expression pattern of a parental allele influences phenotypic expression. The expanding study of genomic imprinting is revealing a significant impact on brain functions and associated diseases. Here, we review key milestones in the field of imprinting and discuss mechanisms and systems in which imprinted genes exert a significant role.

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Maternal stress is commonly cited as an important risk factor for spontaneous abortion. For humans, however, there is little physiological evidence linking miscarriage to stress. This lack of evidence may be attributable to a paucity of research on maternal stress during the earliest gestational stages. Most human studies have focused on ''clinical'' pregnancy (>6 weeks after the last menstrual period). The majority of miscarriages, however, occur earlier, within the first 3 weeks after conception (Ϸ5 weeks after the last menstrual period). Studies focused on clinical pregnancy thus miss the most critical period for pregnancy continuance. We examined the association between miscarriage and levels of maternal urinary cortisol during the first 3 weeks after conception. Pregnancies characterized by increased maternal cortisol during this period (within participant analyses) were more likely to result in spontaneous abortion (P < 0.05). This evidence links increased levels in this stress marker with a higher risk of early pregnancy loss in humans.stress ͉ miscarriage ͉ placentation ͉ fetomaternal conflict ͉ evolutionary theory

To investigate associations between genetic, linguistic, and geographic variation in Africa, we type 50 Y chromosome SNPs in 1122 individuals from 40 populations representing African geographic and linguistic diversity. We compare these patterns of variation with those that emerge from a similar analysis of published mtDNA HVS1 sequences from 1918 individuals from 39 African populations. For the Y chromosome, Mantel tests reveal a strong partial correlation between genetic and linguistic distances (r ¼ 0.33, P ¼ 0.001) and no correlation between genetic and geographic distances (r ¼ À0.08, P40.10). In contrast, mtDNA variation is weakly correlated with both language (r ¼ 0.16, P ¼ 0.046) and geography (r ¼ 0.17, P ¼ 0.035). AMOVA indicates that the amount of paternal among-group variation is much higher when populations are grouped by linguistics (U CT ¼ 0.21) than by geography (U CT ¼ 0.06). Levels of maternal genetic among-group variation are low for both linguistics and geography (U CT ¼ 0.03 and 0.04, respectively). When Bantu speakers are removed from these analyses, the correlation with linguistic variation disappears for the Y chromosome and strengthens for mtDNA. These data suggest that patterns of differentiation and gene flow in Africa have differed for men and women in the recent evolutionary past. We infer that sex-biased rates of admixture and/or language borrowing between expanding Bantu farmers and local hunter-gatherers played an important role in influencing patterns of genetic variation during the spread of African agriculture in the last 4000 years.

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