Gene with its locus on one of the sex chromosomes, X or Y. As there are few genes on the Y chromosome, in humans, most sex-linked genes are on the X chromosome. However, there are some genes on the Y chromosome, notably the gene SRy that stimulates development of the testes and subsequent development of the embryo into a male. There is also a Y STR (short tandem repeat on the Y chromosome) used in genealogy DNA testing. (Note that in some organisms it is not the presence of a Y chromosome that controls development into a male. For example in Drosophila it is the number of X chromosomes, 1 for male and 2 for female. In turtles, sex is determined by incubation temperature of the eggs.) In birds, butterflies and moths, males are XX (or ZZ) and females are XY (ZW). In grasshoppers and crickets females are XX and males are XO (just one X chromosome). In bees and wasps, diploid individuals are female and haploid individuals are male. Earthworms and some snails are hermaphrodite (have both male and female anatomies). Some organisms, such as oysters and some fish, can change sex during their life cycle.