That said, the grandparent's figure are rather wrong, for two reasons. First is that the heritability of IQ is typically estimated to be around 0.8 instead of 0.5, which means that the expected IQ of parents with IQ of 150 is 140, instead of 125. Second is that this is only the expected IQ. If they have multiple children, some will typically be above the expectation, and some below. More specifically, given standard deviation of 15, around a quarter of children of parents of 150 IQ will have IQ of 150 or higher.