The Y chromosome, the smallest chromosome in the karyotype, is one of the two sex chromosomes. In 1905, Nettie Stevens identified that the Y chromosome is a sex-determining chromosome, while conducting a study on the mealworm Tenebrio molitor. He further proposed that chromosomes had always existed in pairs. In 1890 Hermann Henking discovered that the Y chromosome was the pair of the X chromosome. All chromosomes normally appear to take on a well-defined shape during mitosis when viewed under a microscope. This shape is vaguely X-shaped for all chromosomes. Interestingly, the Y chromosome resembles the English alphabet Y during mitosis, due to the fusion of the two very short branches (Bainbridge, 2003). Each species of organism is made up of a different set of chromosomes with a set of sex-determining chromosomes. The X and Y chromosomes are believed to have evolved from a pair of identical chromosomes, known as autosomes. Male mammals are heterogametic (produce X and Y chromosomes containing sperm in equal proportions) and females are heterogametic (all female gametes carry X). The Y chromosome has been found to have a high content of repetitive DNA sequences consisting of pseudogenes and has no function (Delbridge et al., 1999). In males, the Y chromosome consists of the SRY gene which triggers the development of the embryo as a male. The Y chromosome is the smallest chromosome comprising 2-3% of the haploid genome and contains between 70 and 200 genes (Quintana-Murci et al., 2001). The Y chromosome consists of 2 arms: short arm (Yp) and long arm (Yq). These arms are made up of 2 pseudoautosomal regions PAR1 and PAR2 that recombine with their homologous regions on the X chromosome. The absence of recombination makes genetic mapping of the Y-specific region impossible and the... half of the document... frequency in the UTY gene of the human Y chromosome. OMICS A Journal of Integrative Biology Volume 15, Number 3, 201127. Greenfield, A., Carrel, L., Pennisi, D., Philippe, C., Quaderi, N., Siggers, P., et al.. (1998 ). The UTX gene escapes X inactivation in mice and humans. Hum Mol Genet 7, 737–742.28. SATORU TOYOSAWA, COLM O'HUIGIN, FELIPE FIGUEROA, HERBERT TICHY AND JAN KLEIN. Identification and characterization of amelogenin genes in monotremes, reptiles and amphibians. Proc. Natl. It happens. Sci. USA vol. 95, pp. 13056–13061, October 199829. Anette Mayer, Georgia Lahr, Dick F. Swaab, Christof Pilgrim, Ingrid Reisert. The Y-chromosome genes SRY and ZFY are transcribed in the adult human brain. Neurogenetics (1998) 1:281–28830. Koopman P, Gubbay J, Vivian N, Goodfellow P, Lovell-Badge R. Male development of chromosomally female mice transgenic for Sry. Nature. 1991;351:117–121