Quantitative Genetics

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There are many discussions on this site about a variety of phenotypes. Most of these phenotypes are “quantitative traits.” There seems to be a common conflation in the minds of many about how these traits work, and an attempt to apply the simpler mendelian genetics that applies to discrete traits (a small number of expressions of the trait, for instance, blue, green or brown eyes, etc.). Here is a short & simple primer on Quantitative Traits and another on Mendelian Genetics. Of course, Mendelian genetics is the bedrock from which quantitative traits, or continuous traits (height, IQ, etc.), emerge, but one must use different methods to analyze these traits because they are polygenic (multiple genes influence their expression) & often environmentally sensitive (the environment has a strong influence on their expression). The normal distribution, regression to the mean and narrow-sense heritability are all terms associated with quantitative traits [1] (though note that since many of these terms, for instance, the first two, come out of the world of statistics, they are often found outside of genetics).

[1] To illustrate the differences between discrete and continuous traits, compare height & eye color. Imagine trying to graph the distribution of blue, brown and mixed color eyes, since there are only three points, it is very discrete. On the other hand, height is a continuous trait, and displays the common “Bell Curve” distribution. Regression to the mean occurs with height because of environment, but obviously the same does not occur with eye color, as there is a deterministic relationship genotype and phenotype (in other words, the color of one’s eyes is determined solely by genetics).

2 Comments

  1. Now these are the kinds posts that I absolutely love!!!

  2. Agreed. The explanation of regression on the mean was more attractive to me than any of the young ladies, whatever their lineage.

    But then I’m a 70 year old white guy. What do I know?

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