This chapter explores the cell biology of sexual reproduction. It discusses what organisms gain from sex. The chapter examines the reproductive cells produced by males and females, and explores the specialized form of cell division, called meiosis, that generates them. It discusses how Gregor Mendel, a nineteenth-century Austrian monk, deduced the basic logic of genetic inheritance by studying the progeny of pea plants. The chapter also describes how scientists can exploit the genetics of sexual reproduction to gain insights into human biology, human origins, and the molecular underpinnings of human disease. Genetics provides a powerful way to discover what specific genes do and how variations in those genes underlie the differences between one species and another or between individuals within a species. Sexual reproduction involves the cyclic alternation of diploid and haploid states: diploid germ-line cells divide by meiosis to form haploid gametes, and the haploid gametes from two individuals fuse at fertilization to form a new diploid cell—the zygote.