|MadSci Network: Genetics|
Before I answer your question, I'm going to address some of the implicit assumptions contained within it, so that my biases are clearly stated before I proceed. Okay, the first assumption is that races are discrete. Race is an invention of society, and is not a particularly meaningful or useful term in the context of genetics. For example, in the U.S. we have historically identified three major races, corresponding to the three old world continents: Africa, Asia and Europe. However, closer inspection reveals a great deal of variation within each "race" for virtually every trait examined, and this variation overlaps between "races". For example, in caucasians skin color ranges from pale in Scandinavians to tan in Southern Europe to very dark in some regions of India. The same is true of each of the other "races". That is, for each trait used to distinguish groups from one another there are no clear boundaries between groups, only a continuous gradient. So the boundaries between "races" in the U.S. really are simply arbitrary boundaries between continental populations. What is more important is how much variation exists within each population. Let's use height as an example. Are African Americans significantly taller than European Americans? Possibly, if we used membership in the NBA as a metric, but that's beside the point. So are Africans really taller than Europeans? The answer is that it depends on which Africans and which Europeans we look at. If we look at the Masai, who live on the savannah, the answer is yes, the Masai are pretty much taller on average than any European group. However, if we look at the bushmen they are shorter than most European groups (possibly excluding the Finns), and the pygmies of Africa are shorter on average than any other population on earth. So comparisons of Africa vs. the rest of the world don't really make sense, given that there is more variation within Africa than in the rest of the world combined. Putting it another way, there are African populations which are more different from one another than from any non-African population. This distinction holds at the molecular level. In DNA it is possible to assess the fraction of all variation which is shared between populations, and the fraction of variation which is private to a single population. Over 80% of all variation is shared between populations, leaving less than 20% as restricted within populations. The greatest amount of variation restricted to a single continent exists within Africa, so at the DNA level as well, Africans are more diverse than the rest of the planet. Okay, the second assumption I want to address is the measurement of IQ. Herrnstein and Murray ignited quite a controversy with their book "The Bell Curve", which asserts that African Americans have significantly lower IQ than European Americans, even after correcting for other factors like socio economic status. An excellent counterpoint to this book is Stephen Jay Gould's "The Mismeasure of Man", which is one of my favorites. Anyhow, my gut feeling is that Herrnstein and Murray are fine statisticians, but need a primer in history. At the beginning of the 20th century there was a movement to limit immigration by South and Eastern Europeans. This was justified by arguments that they were mentally inferior (the term back then was "feeble minded"). This coincided with the development of some of the earliest IQ tests, including the Stanford Binet test, and such tests were rapidly applied and used to "prove" that South and Eastern Europeans have inferior intelligence compared to Northern Europeans. Of course, the fact is that SE Europeans didn't speak English too well and the tests had questions including knowledge of how many cylinders a specific make of automobile had, so we look back on the analyses and recognize that the flaw was not in the SE Europeans, but in the test. My point is that not many Americans honestly believe that Greeks, Italians and Russians are inferior today, so I doubt that the supposedly lower average IQ of African Americans will hold up either. So now let's almost get back to your question. Before we talk IQ, let's talk about height. Regression to the mean is an old concept, first introduced by Francis Galton in a study of hieght. He found that the height of children tended to be between the population average and the average of their parents' heights corrected for sex (the mid-parent height). Translation, children of tall parents tend to be taller than average, but shorter than their parents. Two points need to be addressed here: first, regression to the mean is a group phenomenon. Any individual child of tall parents may be even taller than their parents. It's just that the average child of tall parents tends to be shorter than the parents. Second, regression to the mean is more dramatic for more extreme observations. That is, the average child of modestly tall parents will be a little bit shorter, while the average child of extremely tall parents will be much shorter than the parents. Ignoring environmental effects, it has been proposed that height is a multigene trait, and therefore regression to the mean reflects multigenic inheritance. Let's assume there are a number of genes which determine height, and each gene has both "tall" (t) and "short" (s) alleles. Thus, tall individuals carry a preponderance of "tall" alleles at height determining genes. Let's say there are ten height genes. If we had two tall parents, each carrying 75% "tall" alleles, what fraction of the alleles transmitted to the kid will be "tall" alleles? Well, if sampling is random, we'd expect to see the kid get 75% "tall" from each parent and to be the same height. So something else causes regression to the mean. The most plausible explanations are allelic interaction (dominance) and locus interaction (epistasis). Dominance means that the contribution of a "tall" allele to height at a given gene depends on the genotype: the effect of a "tall" allele depends on whether the other allele at the same gene is "tall" or "short". Epistasis is the same notion except involving multiple genes: the contribution of "tall" alleles at gene A is dependent on genotype at locus B. Returning to our example, the parents are probably tall not only due to carrying 75% "tall" alleles each, but also because they carry them in the right combinations. Even though the kid will get 75% "tall" alleles, the kid is unlikely to have them in the right combinations, and will therefore tend to be shorter on average, even if occasionally the kid will get just the right combination and will be taller than either parent. So now let's get back to IQ. If we assume that IQ is as easy to measure as height (it isn't), that there are no environmental effects (there are), that the amount of variation in the African American population is the same as in the European American population (it is not), and that on average African Americans have significantly lower IQ than European Americans (depends on who you ask), then your son would be right: 140 IQ parents from the less IQ endowed population would be more extreme relative to the average, and their children would therefore tend to have lower IQ than children of parents with a 140 IQ from the more IQ endowed population. As you can see, if his assumptions were correct, your son would be right. However, I think there are enough flawed assumptions that you needn't worry that he's correct. Chris A good review of regression to the mean is available at http://trochim.human.cornell.edu/kb/regrmean.htm Galton's original regression data is available at http://www.stat.ucla.edu/history/regression.gif
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