Sex determination in mammalian embryos is the process by which an embryo is determined, at the cellular level, to become a male or a female. Upon fertilization, a zygote will receive an X chromosome or a Y chromosome from the father's sperm to accompany the X chromosomes provided by the mother's egg. It is at this point that a zygote can be said to be male or female. However, the development of male or female sexual organs is determined only later, between the sixth and seventh weeks of development. Genetics plays a significant role in sex determination, providing many genetic loci that assist the process of cell 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 ovaries that form eggs or testes that form sperm. This type of sex determination is influenced exclusively 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, directed 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 dimensions, muscle development, and vocal cartilage (Gilbert, 2014, p. G-22). This article will focus on events that take place during both primary and secondary development. Discussions will first focus on when in embryonic development this determination begins to take hold. Once we place the onset of this sex determination at a specific stage of overall embryonic development, we will take a look at the specific roles of the X and Y chromosomes and their individuals...... middle of paper..... . Biological Chemistry, 272(11), 7167-7172.Rives, N., Joly, G., Machy, A., Simeon, N., Leclerc, P., & Mace, B. (2000). Evaluation of left chromosome aneuploidy 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. (2001, July 3). Eleven years of sexual discovery. Genome Biology, 2(7). Singh, N. D., & Petrov, D. A. (2007). Evolution of gene function on the X chromosome versus the autosome. Evolution of genes and proteins, 3, 101-118.