Why does the child of a blonde haired woman and a dark
haired man usually have dark hair. Also, why does the dark
characteristics seem to be dominant over the light (skin, hair etc.)?
Hair color is believed to be a polygenic trait which means that more than one
gene controls the phenotype. These alleles are additive, which means that
for every dark allele added the hair color gets darker. So if there are
three genes for hair color and you get two copies of each, one from mom and
one from dad, then you have 6 possibilities. To get the lightest hair all
six alleles must be light. To get the darkest hair all six must be dark. If
mom has all light alleles and dad has half dark and half light, it depends on
which six the child gets at random. So the child could have brown hair, if
it has half dark and half light. Or it could still have blond hair if it
gets all of dad's light genes.
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Update: June 2012