What are dominant traits? Does it involve leather?

Dear Cecil: People often talk about dominant and recessive traits, but does anyone really understand what that means? I know, for example, that genetically speaking, brown hair is supposed to be dominant, blond hair recessive. But by what process does one gene (or chromosome) dominate? Does it involve leather? Whips and chains? Or is the word “dominant” just a shorthand way of expressing the statistical predominance of one of the outcomes when brown-haired people and blond-haired people mate? William H., Chicago

Cecil replies:

Dear William:

Admit it, Will, people do not “often” talk about dominant and recessive traits. I can’t remember the last time I heard a good dominant and recessive traits story. Be that as it may, I can assure you that, as a general rule, the concepts of dominance and recessiveness refer to real chemical functions, not simply statistical correlations.

To illustrate with the example you’ve offered–brown hair versus blond–let’s imagine a dominant gene, B, for brownness, and its recessive counterpart, W, for whiteness. (We are simplifying the real world a bit here, but hey, that’s what journalism is all about.) B produces an enzyme that is needed for the synthesis of melanin, the brown pigment that most people possess to one degree or another. W produces no such enzyme, or a defective brand that can’t do the trick.

Finally–this is the important part–B produces, all by itself, enough of this enzyme to supply a whole normal body with all the melanin it needs. So the gene pair BB produces sufficient melanin to color the hair brown; so does the pair BW; only the poor sap who draws a pair of W’s ends up with white (i.e., blond) hair.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.