The topic of this video module is genetic basis for variation among humans. The main learning objective is that students will learn the genetic mechanisms that cause variation among humans (parents and children, brothers and sisters) and how to calculate the probability that two individuals will have an identical genetic makeup. This module does not require many prerequisites, only a general knowledge of DNA as the genetic material, as well as a knowledge of meiosis. However, these topics are covered briefly in the lesson. In addition to class discussions, the types of in-class activities for this lesson include: predicting the phenotypes and genotypes according to the blending and the particulate hypotheses of inheritance; determining the results of a single crossing over between non-sister chromatids; calculating the number of possible combinations that result from independent assortment, crossing over and random fertilization; and estimating the probability that two individuals have the same genetic material. This lesson can be completed in 50-60 minutes, with the students working on in-class activities during the 20-25 minutes of the lesson. Materials needed include colored pencils or pens and a blackboard or equivalent.