3 Within Family Differences

At a purely anecdotal level, when parents hear about the possibility that genes create differences between people, they will sometimes respond ``Well, that's pretty obvious. I've raised three sons the same way and they've all turned out differently." At issue here is not whether their conclusions are soundly based on their data, so much as to indicate that not all variation is due to factors that family members share in common. No matter how much parents contribute genetically to their children and, it seems, no matter how much effort they put into parenting, a large part of the outcome appears beyond their immediate control. That is, there are large differences even within a family. Some of these differences are doubtless due to the environment since even identical twins are not perfectly alike.

Figure 1.5: Bar chart of absolute differences in weight within MZ and DZ twin pairs.

Figure 1.5 is a bar chart of the (absolute) weight differences within pairs of twins. The darker, left-hand column of each pair gives the percentage of the DZ sample falling in the indicated range of differences, and the lighter, right-hand column shows the corresponding percentages for MZ pairs. For MZ twins, these differences must be due to factors in the environment that differ even within pairs of genetically identical individuals raised in the same home. Obviously the differences within DZ pairs are much greater on average. The known mechanisms of Mendelian inheritance can account for this finding since, unlike MZ twins, DZ twins are not genetically identical although they are genetically related. DZ twins represent a separate sample from the genetic pool ``cornered" by the parents. Thus, DZ twins will be correlated because their particular parents have their own particular selection of genes from the total gene pool in the population, but they will not be genetically identical because each DZ twin, like every other sibling in the same family, represents the result of separate, random, meioses in the parental germlines.