Drosophila sex-chromosomes The<unk>  is a sex-determination<unk>  used to classify many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants (Ginkgo tree). In this<unk> , the sex of an individual is determined by a pair of sex chromosomes. Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two different kinds of sex chromosomes <unk> , and are called the heterogametic sex. In humans, the presence of the Y chromosome is responsible for triggering male development; in the absence of the Y chromosome, the fetus will undergo female development. In most species with<unk>  sex<unk> , an organism must have at least one X chromosome in order to survive. The<unk><unk>  contrasts in several ways with the ZW sex-determination<unk>  found in birds, some insects, many reptiles, and various other animals, in which the heterogametic sex is female. It had been thought for several decades that in all snakes sex was determined by the ZW<unk> , but there had been observations of unexpected effects in the genetics of species in the families Boidae and Pythonidae; for example, parthenogenic reproduction produced only females rather than males, which is the opposite of what is to be expected in the ZW<unk> . In the early years of the 21st century such observations prompted research that demonstrated that all pythons and boas so far investigated definitely have the<unk><unk>  of sex<unk> . A temperature-dependent sex<unk><unk>  is found in some reptiles and fish.
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