Start with the following question:
How much of your parents made up your genetic code? How much from Mom, how much from Dad?
Encourage the students to throw around some ideas and start them thinking about why they look like Dad, or have Moms eyes. The idea that they have traits from both parents will allow them to connect to the idea of sexual reproduction being the way humans reproduce. They will also start to see that two parents are necessary in this process, and that the genetics in themselves was made up from both parents.