Students are always fascinated by their heredity, but what they take from their previous generations and express today in the way they look. Naturally curious, every child I've ever met has inventoried the many ways they look like -- or unlike -- their relatives. It's part of a human's natural development to figure out his or her place in the world, and kids often use their powerful skills of observation to do this.
So teaching genetics and heredity makes sense. Sometimes an understanding of the topic comes more easily to a child than it does to adults, simply because they are paying attention more than we typically do.
Often, when people start to consider phenotype (i.e. the outward expression of our genetic traits), they look first to the obvious -- eye color, hair color, skin color, inherited disease, etc. But I think it's fun to look at somewhat odd traits that we may not even realize are genetic! It puts in perspective the amazing amount of information encoded on our DNA!
For this I really like the free activities from The University of Utah as part of their Teach.Genetics resources for teachers. Start with the Inherited Human Traits: A Quick Reference Guide. Here you'll learn that things like left and right handedness, the ability to roll your tongue, the ways you ears attach to your head and the way to fold your hands are all genetically linked. Students get such a kick looking for these traits in themselves and classmates.
The website provides a survey activity, where you can tally the results and create a bar graph based on the results. (Always fun! Literally, one of my students today shouted, "A survey? I love survives ") Or you can continue the research -- asking friends and family about the traits -- and either graph the data or create a fun family tree based on it. (The family tree is easily adapted to homeschool use, and my daughter and I plan to complete this activity at home.) I also love playing Traits Bingo using the research.
There is also Generations of Traits which is a really fun hands-on activity the shows the passing of genetic trait from grandparents down. I like it because ti makes it clear that though may share traits with others in your family, the actual genes always come from your parents. It also shows how completely random your genetic mix is. This would be a fun one to extend with a study of the probability involved with each generation.