The presence of a small number of discrete forms - as you find in a classic Mendelian trait like eye color - suggests that the phenotype is controlled by a very small number of genes. In contrast, a complex trait such as body size is influenced by multiple genes as well as environmental factors, giving rise to a continuous spectrum of phenotypes. This causal complexity makes discovery of the genetic determinants of the trait - so-called quantitative trait loci (QTLs) - very difficult. In the open-access journal PLoS Biology, Fiona Oliver, Julian Christians, and colleagues extend their work on a single QTL with a large effect on body size variation in mice, and show that the responsible gene is one already linked to a Mendelian growth disorder in humans.
In this study, the authors further refined the map of a QTL on the X chromosome associated with a 20% difference in growth rate of mice, and found it contains only four genes: glypican-3 and glypican-4 (Gpc3 and Gpc4), and two other genes of unknown function. The glypicans are membrane-bound growth regulators; loss of function of Gpc3 in humans causes a rare syndrome of overgrowth, skeletal and other abnormalities, and neonatal death. In Gpc3, but not the others, size correlated with a significant difference in gene expression: larger mice had low levels of the messenger RNA, and smaller mice had high levels - the same pattern seen in the human loss-of-function disorder. The authors identified several non-coding polymorphisms in Gpc3 that differed between the two forms, although it remains to be seen whether the differential effect on growth is due to these or other DNA differences nearby.
The results from this study point out an important feature of inheritance, namely, that a gene implicated in a Mendelian trait can also contribute to quantitative variation. While catastrophic expression failure, such as a loss of function, can cause disease, smaller changes in expression of the same
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