Sex determination in mammalian embryos is the process by which an embryo is determined, at the cellular level, to become male or female. Upon fertilization, a zygote will receive an X chromosome or a Y chromosome from the father's sperm to accompany the X chromosomes donated by the mother's egg. It is at this stage that a zygote can be considered male or female. However, the development of male or female sexual organs is not determined until later, during the sixth to seventh week of development. Genetics play an important role in sex determination, providing numerous genetic loci that facilitate the process of cellular determination and organ development. There are two ways to describe sex determination in mammals. The first is described as primary sex determination, which is when the gonads are determined to form either the ovaries forming eggs or the testes forming sperm. This type of sex determination is purely influenced by chromosomes (Gilbert, 2014, p. G-20). The second type of sex determination is described as secondary sex determination. This is defined as developmental events, driven by hormones produced in the gonads, that direct the development of phenotypes of structures outside the gonads. These structures include the ductal systems, external genitalia, and often sex-specific body size, muscle development, and vocal cartilage (Gilbert, 2014, p. G-22). This article will focus on the events that take place during primary and secondary development. Discussions will first focus on when, in embryonic development, this determination begins to take place. Once we place the onset of this sex determination at a specific stage of overall embryonic development, we will examine the specific roles of the X and Y chromosomes and their individuals...... middle of article..... . Biological Chemistry, 272(11), 7167-7172. Rives, N., Joly, G., Machy, A., Simeon, N., Leclerc, P. and Mace, B. (2000). Assessment of aneuploidy of sx chromosomes in sperm nuclei of 47.xxy and 46.xy/47.xxy males: comparison with fertile and infertile males with normal karyotype. Molecular Human Reproduction, 6(2), 107-112. Sex chromosomes. (2011). In K. Rogers (Ed.), 21st Century Science. New thinking on genetics (pp. 44-45). New York: Britannica Educational Publishing with Rosen Educational Services. Retrieved from http://go.galegroup.com/ps/i.do?id=GALE%7CCX4035100025&v=2.1&u=wood40746&it=r&p=GVRL&sw=w&asid=f30f880cc4659f7ff21a72bbea45ef39Sinclair, A. (July 3, 2001). Eleven years of sexual discovery. Genome Biology, 2 (7). Singh, ND and Petrov, DA (2007). Evolution of gene function on the X chromosome relative to the autosome. Evolution of genes and proteins, 3, 101-118.