Connie has black hair. Walt has black (or dark brown) hair. Why is Jeremy blond?
Blond hair is a recessive trait. A very common one which is why if you do a pedigree of your family it might appear dominant but the truth is that neither parent has to have blond hair to have a blond child.
It's also worth noting that blond hair sometimes darkens to brown as people get older. It is entirely possible that either Walt or Connie was blond as a child, and that Jeremy may be dark-haired by the time he is an adult.
What I want to know is why non-family members look more like Jeremy's parents than Jeremy himself. I mean, Hector looks more passable as Walt's son, and Sara has been noted to have the same body shape as Connie.
I couldn't tell Hector and Walt apart when I first started reading the strip.
In an early strip Connie is shown to be a psychologist, so why is she so clueless about how to raise Jeremy?
It's easier to diagnose other's problems than your own.