Although our previous discussion focused on total heritability, our analysis also allowed us to examine the effects of X-linked loci. We found a substantial X-linked component influencing G6PD levels—a result that was expected, because mutations in the G6PD gene (Xq28) are relatively common in Sardinia where they reduce G6PD levels and protect against malaria [48]. G6PD levels also exhibited a substantial autosomal component, supporting the presence of other regulatory loci that may influence malaria susceptibility and resistance through regulation of G6PD levels. In addition, we found a substantial X-linked variance component for three other quantitative traits (cholesterol, LDL, and ESR). In some of these cases, an X-linked component has been mapped (for an example of an X-linked gene influencing cholesterol levels, see [49]), and in other cases, our results may facilitate gene mapping and identification.
