The XY sex-determination system is a well-known sex-determination system. It is found in human beings and other mammals. In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness. A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. ... Orders Subclass Monotremata Monotremata Subclass Marsupialia Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Subclass Placentalia Xenarthra Dermoptera Desmostylia Scandentia Primates Rodentia Lagomorpha Insectivora Chiroptera Pholidota Carnivora Perissodactyla Artiodactyla Cetacea Afrosoricida Macroscelidea Tubulidentata Hyracoidea Proboscidea Sirenia The mammals are the class of vertebrate animals primarily characterized by the presence of mammary... SRY is the sex-determining Y chromosome gene in humans and other primates, linked to determining males. ... Binomial name Drosophila melanogaster Drosophila melanogaster (Black-bellied Dew-lover) a dipteran (two-winged) insect, is the species of fruit fly that is commonly used in genetic experiments; it is among the most important model organisms. ...

SRY is not the only male-determining gene in mammals, or even the most common: most non-primate mammals use a different Y-chromosome gene, UBE1, for this purpose. Also, two species of "mole voles", Ellobius tancrei and E. lutescens, have lost the Y chromosome entirely. In one species, both sexes have unpaired X chromosomes; in the other, both females and males have XX.

The XY sex determination system was first described independently by Dr. Nettie Stevens and Edmund Beecher Wilson in 1905. 1905 was a common year starting on Sunday (see link for calendar). ...

See also

• chromosome, for information on abnormalities of the XY sex-determination system
• intersexual for information on variations in human sexual forms
• sexual differentiation, (human)
• testis-determining factor
• Barr body
• Y-chromosomal Adam

Figure 1: Chromosome. ... An intersexual is a person (or individual of any unisexual species) who is born with genitalia and/or secondary sexual characteristics of indeterminate sex, or which combine features of both sexes. ... Sex differences between males and females range from chromosomes to anatomy to behavior. ... Testis-determining factor is a general term for the gene (or product thereof) that results in maleness in humans and some other species. ... In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male. ... In human genetics, Y-chromosomal Adam is the male counterpart to mitochondrial Eve: a real or hypothetical single male human ancestor from whom all male Y chromosomes are descended. ...

External links

• Sex Determination and Differentiation (http://www.biology.ucsc.edu/people/barrylab/public_html/classes/old_animal_behavior/Sex_devo.htm)
• SRY: Sex determination (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd.section.156) from the National Center for Biotechnology Information