This chapter discusses the role of fertilization in human reproduction and development, describes the barriers to fertilization, and presents the consequences of fertilization. Two cells on the verge of death are the participants in fertilization, one of the most thought-provoking events in biology. The human egg plays a passive role in sex determination because it normally contains one X chromosome. The genes on the Y chromosome are expendable for the individual because no normal human female has them. The chapter explores the chromosomal basis of human sex determination, and explains sex-linked inheritance and provides examples of characteristics inherited in this way. Sex-linked inheritance refers to those traits that are determined, and therefore inherited, by genes located on the X chromosome. The type of inheritance most people are familiar with is genetic inheritance through the genes on the chromosomes that we receive from our parents.