All animals have a set of DNA coding for genes present on chromosomes. In humans, most mammals, and some other species, two of the chromosomes, called the X chromosome and Y chromosome, code for sex. In these species, one or more genes are present on their Y chromosome that determine maleness. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located on the Y chromosome that code for maleness. Offspring have two sex chromosomes: an offspring with two X chromosomes will develop female characteristics, and an offspring with an X and a Y chromosome will develop male characteristics. Mammals In most mammals, sex is determined by presence of the Y chromosome. This makes individuals with XXY and XYY karyotypes males, and individuals with X and XXX karyotypes females. In the 1930s, Alfred Jost determined that the presence of testosterone was required for Wolffian duct development in the male rabbit. SRY is a sex-determining gene on the Y chromosome in the therians (placental mammals and marsupials). Non-human mammals use several genes on the Y chromosome. Not all male-specific genes are located on the Y chromosome. Platypus, a monotreme, use five pairs of different XY chromosomes with six groups of male-linked genes, AMH being the master switch. Humans Human male XY chromosomes after G-banding A single gene (SRY) present on the Y chromosome acts as a signal to set the developmental pathway towards maleness. Presence of this gene starts off the process of virilization. This and other factors result in the sex differences in humans. The cells in females, with two X chromosomes, undergo X-inactivation, in which one of the two X chromosomes is inactivated. The inactivated X chromosome remains within a cell as a Barr body. Other animals Some species of turtles have convergently evolved<unk> s, specifically those in Chelidae and Staurotypinae. Other species (including most Drosophila species) use the presence of two X chromosomes to determine femaleness: one X chromosome gives putative maleness, but the presence of Y chromosome genes is required for normal male development. In the fruit fly individuals with XY are male and individuals with XX are female; however, individuals with XXY or XXX can also be female, and individuals with X can be males. In Abraxas XX is male while XY is female. Plants Very few dioecious angiosperm species have XY sex<unk> , such as the Silene latifolia. In these species sex<unk>  is similar to mammals where male is XY and female is XX. Other<unk> s Birds and many insects have a similar<unk>  of sex<unk>  (ZW sex-determination<unk> , in which it is the females that are heterogametic (ZW), while males are homogametic (ZZ). Many insects of the order Hymenoptera instead have a haplo-diploid<unk> , where the males are haploid (having just one chromosome of each type) while the females are diploid (with chromosomes appearing in pairs). Some other insects have the<unk> , where just one chromosome type appears in pairs for the female but alone in the males, while all other chromosomes appear in pairs in both sexes.
XY sex-determination system
Sex-chromosome dosage compensation
XY gonadal dysgenesis
Marker chromosome
Nuclear sexing
Germline mosaicism
45,X/46,XY mosaicism
XX gonadal dysgenesis