What are dominant traits?

Cecil's Columns/Staff Reports
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I’m not sure exactly how red hair works, but a reasonable approximation might be that it’s dominant over blond. Then your mom could be bR, (blond-red, which would express as red), while your dad would be Bb (brown-blond, which would express as brown). You and your siblings each got one gene from dad and one gene from mom, and it just so happens that all of you got the blond gene from each.

Of course, it’s more complicated than that, since there are actually many genes which control hair color. But in general, you should almost[sup]*[/sup] never be surprised when the kids show a recessive trait, no matter what the parents are. You should only be surprised when both parents show recessive traits and the child shows a dominant trait.

  • The only exception which comes to mind is blood type: An AB parent will never have an O child, but any other combination of parents’ blood types can result in an O child.
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My old genetics textbook (J.Nagle Heredity and Human Affairs, 1974/9) gave this hypothetical model for human hair color. Two loci. One locus has at least three alleles, producing different amounts of melanin; their effect is additive, so one blond gene and one black gene give roughly the same result as two medium-brown genes. The other locus has two alleles, red/not-red, also additive; fair hair with one red gene is “strawberry”, with two red genes is vivid red.