A polygenic trait is a trait controlled by many (poly) genes. Human skin color is an example; it thought to be the controlled by about 12 genes.
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I know all about this, but could you specify what you mean?
A. Natural Selection. This is because the weeds that are not resistant to it die out before they can reproduce, but the resistant ones grow to maturity and then reproduce, passing down the resistance gene. Over time, the resistance gene would become a normal thing in weeds, while the non-resistant gene would not exist anyone, since the gene could not be passed on.
