This chapter provides an outline of Mendelian and molecular genetics, with a particular emphasis on how DNA provides evidence of a single origin for all living things. It discusses the role of genetics in serial homology, and how that demonstrates genetic relatedness among vertebrates especially, and it shows how homologous HOX genes provide the head–tail orientation and body plan segmentation for animals as diverse as the house fly and human, and that we see this same pattern of segmentation as far back as the Cambrian. It also explains how the independent DNA of mitochondria and chloroplasts provide evidence for a deep history of symbiosis between the host and parasite for both plants and animals. This chapter provides a support for later chapters reviewing principles of evolution and the evolutionary history of complex organisms.