<P> A temperature - dependent sex determination system is found in some reptiles . </P> <P> 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 . </P> <P> In humans, half of spermatozoons carry X chromosome and the other half Y chromosome . 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 . </P> <P> Humans, as well as some other organisms, can have a chromosomal arrangement that is contrary to their phenotypic sex; for example, XX males or XY females (see androgen insensitivity syndrome). Additionally, an abnormal number of sex chromosomes (aneuploidy) may be present, such as Turner's syndrome, in which a single X chromosome is present, and Klinefelter's syndrome, in which two X chromosomes and a Y chromosome are present, XYY syndrome and XXYY syndrome . Other less common chromosomal arrangements include: triple X syndrome, 48, XXXX, and 49, XXXXX . </P>

Which type of chromosome does the female parent give to its offspring