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Traits carried on the sex chromosomesSex Linked TraitsSex Chromosomes v. AutosomesSex chromosomes determine gender; Autosomes are all other chromosomesXX are femalesXY are malesChickens and moths: males have identical sex chromosomes and females are differentMost plants and some fish lack sex chromosomesSome worms depend on the number of chromosomesTurtles, lizards, and crocodilians are temperature dependent. In mammals, the Y carries the Sex-determining Region Y (SRY). This codes for the gonads to develop into testes. All other sex linked genes are on the X chromosome.

Thomas Hunt MorganWorked with fruit flies4 pairs of chromosomes3 pairs identical in both genders; 1 pair was differentXX females and XY malesNoticed that there was one fly with white eyesCrossed a red with a white all offspring were redCross two of those red red and white all of the white were maleConcluded that the gene for eye color was on the x chromosomeCalled a sex-linked trait