In this module, students explore how meiosis, fertilization, and crossing over contribute to genetic variation in sexually reproducing organisms. Students compare mitosis and meiosis, model the stages of meiosis with pipe cleaners and string, and examine how diploid reproductive cells produce haploid gametes. Students investigate fertilization using mosquito chromosomes and a genetic counselor lab to show how egg and sperm cells combine to form a diploid zygote. The module concludes with an investigation of crossing over, showing how chromosomes exchange genetic material to create new combinations. Students will learn that meiosis and fertilization pass chromosomes from parents to offpring, while crossing over increases genetic variation.