Genes have had a role in all the mechanisms of sex determination discussed thus far, but in a number of organisms, sex is determined fully or in part by environmental factors. Environmental factors are important in determining sex in some reptiles; the sexual phenotype of many turtles, crocodiles, alligators, and a few birds is affected by temperature during embryonic development. In turtles, for example, warm incubation temperatures produce more females during certain times of the year, whereas cool temperatures produce males. In alligators, the reverse is true. In some species, sex chromosomes usually determine whether individuals are male or female, but environmental factors can sometimes override this chromosomal sex determination. For example, bearded dragon lizards are normally ZZ when male and ZW when female, but when the eggs are incubated at high temperatures, ZZ individuals develop into phenotypic females.

Now that we have surveyed some of the different ways that sex can be determined, we will examine one mechanism (the XX-XY system) in detail. Both fruit flies and humans possess XX-XY sex determination, but as we will see, the way in which the X and Y chromosomes determine sex in these two organisms is quite different. TRY PROBLEM 1