One common misconception about heritability is that it can be applied to a single individual. It can' t. It is meaningful to say that individual dif ferences in height are 90 percent heritable, but it makes absolutely no sense to say , "Meredith' s height is 90 percent heritable." You cannot say , for example, that the first 63 inches of he height are due to genes and the other 7 inches are due to the environment. For an individual, genes and environment are inextricably intertwined. Both play a role in determining height, and they cannot be separated. Thus, heritability refers only to differences in a sample or population, not to an individual.
Another common misconception about heritability is that it is constant. In fact, it is nothing of the sort. Heritability is a statistic that applies only to a population at one point in time and in a particular array of environments. If the environments change, then heritability can change. For example, in principle, heritability can be high in one population (e.g., among Swedes) but low in another (e.g., among Nigerians). And heritability can be low at one time and high at another time. Heritability always depends on both the range of genetic dif ferences in the population and the environmental differences in that population. To draw on a concept from Chapter 2, heritability does not always generalize across persons and places.
A final common misconception is that heritability is an absolutely precis statistic (Plomin et al., 2001). Nothing could be further from the truth. Error or unreliability of measurement, for example, can distort heritability statistics. And, because heritability statistics are typically computed using correlations, which themselves fluctuate from sample to sample, further imprecision creeps in. In sum heritability is best regarded as merely an estimate of the percentage of phenotypic differences due to genetic dif ferences. It is not precise. It does not refer to an individual. And it is not eternally fixed
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