Genetics* is entirely concept based. This makes it a lecture-intensive subject, and makes hands-on activities all but impossible. It requires the opposite of hands-on activities (that is, hands-off activities [that is, long boring talks]). This, quite understandably, makes it very hard to teach third and fourth graders.
Small children need something to play with: Give them wires and motors, and you can teach them electricity. Give them mirrors and magnifying glasses, and you can give them light. Give them worksheets and lectures, and you can't teach them genetics. Teaching an elementary school student genetics is like beating a dead horse: The horse can't play with anything, so it won't be amused by your lectures or your beatings.
However, that doesn't stop some people from trying.
*This lesson is true of pretty much all intangible subject matter. Quantum physics, psychology, and other abstract concepts will be equally as hard to teach to young students.