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So, where does the excess frequency of lefthandedness in twins-and-the-siblings-of-twins come from? The best answer seems to be that they inherit it from their parents with about 70% heritability. That means about 70% of the variation in handedness in this population can be explained by genetic relationships. In those 773 twin families, each lefthanded parent increased the probability of lefthandedness in the children by about 50%. With one lefthanded parent, we see about half-again the frequency of lefthandedness seen when both parents are righthanded. With both parents lefthanded, about double the frequency. Again, no difference between the families of monozygotic twins and the families of dizygotics. Mothers of twins are almost twice as often lefthanded as their own sisters. Fathers of twins are almost twice as often lefthanded as their brothers. Twins and their siblings have an excess of lefhandedness compared to the general population. They inherit this from their parents, who are much more often lefthanded than the aunts and uncles of twins who are not themselves parents of twins. The mechanism of this apparent inheritance is not simple. There are families in this sample whose handedness distribution fits the expectations of a single recessive genotype which is expressed about half the time. There are other families where the distribution fits that of a simple dominant genotype, again with about 50% penetrance. That sort of dominant genotype pattern is almost impossible to distinguish from a multifactorial model. Over the sample as a whole, the best fit is a multifactorial model with about 70% heritability. That's technical, and it's jargon, and it is not simple to understand or to teach it in detail. I believe, however, that I can give you a useful idea without too much trouble. Multifactorial means 'involving many factors'. Not one gene. Many genes. Plus nongenetic 'environmental' factors. How many is many genes? Maybe quite many; maybe only three or four. We just barely know how to make predictions for two interacting genes. We cannot usually analyze even a two-gene situation effectively because that requires that we should somehow realize and measure a separate component of the growth or behavior representing each of the two genes. In all the world of living things, we know of only a handful of situations where the behavior of a genetic system yields a clear and simple fit to a multifactorial or polygenic model. But we more or less insist on using it as an approximate fit for all the situations in human development where we know the process is genetically controlled, but we can not make sense of any single gene model. Some of that is about certain features of growth, a great deal of it about features of behavioral development such as handedness. Twinning and nonrighthandedness are somehow genetically related. The exact mechanisms involved are not clear. But it is a good example of something very important that most people don't understand about human developmental genetics. Many of the most important genes do not show up every time they are present. Most twin conceptions result in no births at all, and for every twin pair born, about 10-12 people who grew from twin embryos are born single. It is actually possible that every lefthander in the world is a twin. I'm not saying I believe that, but it is not impossible and there is no evidence against it. There's probably more here than you ever wanted to know or think about. Simplicity is great. It really helps to understand and to explain things. But false simplicity is, well, ... false ! Most of the simple stores about the biology of twinning are false. Mirror-imaging is real. It does happen. Handedness is very much the wrong place to look for it. There are other asymmetries which are sometimes mirrored - a wrinkle in the right ear of one and the left ear of the other, the order in which the various teeth appear, a difference in size of the palpebral fissures (the opening of the eye). Play with those. Look at your twin in a mirror - that reverses any asymmetry and a difference you are accustomed to and never saw before can really show up when it is reversed. For more information, please email: Charles Boklage, East Carolina University |