Of the four traits studied, the largest proportion of phenotypic variance explained by the SNPs is for height and the smallest is for BMI. Why are the results for height and BMI so different? Heritability of height is approximately 80% and we estimate that more than half of this variation (45/80 = 0.56) is tagged by common SNPs. Estimates of the narrow sense heritability of BMI appear to be more variable, ranging from 42% to 58% when estimated from the correlation of full brothers and fathers and sons30 to 60–80% from twin studies31. Nevertheless, even if we assume that the narrow sense heritability for BMI is 50% then only 17/50 = 0.34 of additive genetic variation is explained by common SNPs. Given these assumptions and the standard errors in Table 1, the standard error of the difference in the proportion of genetic variance explained for height and BMI is approximately 0.07, so that the observed difference of 0.22 appears statistically significant. These results are consistent with the proportion of phenotypic variation for height and BMI explained by genome-wide significant SNPs,
