Genetics is one of the hardest things for some middle school students to understand. Here are a couple videos I have used or made to help introduce genetics and punnett squares.
https://www.youtube.com/watch?v=B_PQ8qYtUL0https://www.youtube.com/watch?v=07Jmdaz6NFA&list=PL7e7WAyFoRAsZb8Oqys5V5lGH1uCwPp95&index=1&t=19s