system: the Y chromosome carries factors responsible for triggering male development, making XY sex determination mostly based on the presence or absence of the Y chromosome. It is the male gamete that determines the sex of the offspring. In this system XX mammals typically are female and XY typically are male. However, individuals with XXY or XYY are males, while individuals with X and XXX are females. Unusually, the platypus, a monotreme mammal, has ten sex chromosomes; females have ten X chromosomes, and males have five X chromosomes and five Y chromosomes. Platypus egg cells all have five X chromosomes, whereas sperm cells can either have five X chromosomes or five Y chromosomes. XY sex determination is found in other organisms, including insects like the common fruit fly, and some plants. In some cases, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome. 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. ZW sex determination In birds, which have a ZW sex-determination system, the W chromosome carries factors responsible for female development, and default development is male. In this case, ZZ individuals are male and ZW are female. It is the female gamete that determines the sex of the offspring. This system is used by birds, some fish, and some crustaceans. The majority of butterflies and moths also have a ZW sex-determination system. Females can have Z, ZZW, and even ZZWW. XO sex determination In the X0 sex-determination system, males have one X chromosome (X0) while females have two (XX). All other chromosomes in these diploid organisms are paired, but organisms may inherit one or two X chromosomes. This system is found in most arachnids, insects such as silverfish (Apterygota), dragonflies (Paleoptera) and grasshoppers (Exopterygota), and some nematodes, crustaceans, and gastropods. In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female. In the nematode Caenorhabditis elegans, most worms are self-fertilizing hermaphrodites with an XX karyotype, but occasional abnormalities in chromosome inheritance can give rise to individuals with only one X chromosome—these X0 individuals are fertile males (and half their offspring are male). ZO sex determination In the Z0 sex-determination system, males have two Z chromosomes whereas females