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2016 Elsevier Inc. All rights reserved. While sex is nearly ubiquitous among eukaryotes, there is astounding diversity in the way that an individual's sex is determined. Humans exhibit genetic sex determination where the genotype determines whether we develop as a female or male. Genetic sex determination can be either an XY or ZW system. In XY systems females have two X chromosomes and males have an X and a Y chromosome. ZW systems are the reverse of this with ZZ males and ZW females. Though common and found in many vertebrates and invertebrates genetic sex determination is one of many options. For instance, in some reptiles and fish the sex of an individual is determined by environmental cues. In most species temperature during development will determine the sex, but other cues such as social environment can also determine sex. Although absent in tetrapods haplodiploidy is a common sex determination system among invertebrates. Under haplodiploidy sex is determined by whether the embryo develops from a fertilized or unfertilized egg. This allows females to alter the sex ratio of offspring - a benefit under certain conditions. Sometimes sex determination in an individual is even manipulated by a parasite. Usually only females can transmit these parasites to the next generation, so the parasite benefits from forcing genetic males to develop as functional females. Studies in model organisms have revealed that the molecular mechanisms that control this amazing diversity in sex determination systems often use many of the same genetic pathways to produce the sexes, and that small changes at the beginning of these pathways can explain some of the diversity that we observe. Despite advancements in this field many outstanding questions remain. For example, does the type of sex determination effect speciation rate? Finally, why do some groups like amphibians show frequent changes in sex determination systems while others like birds are static?
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Encyclopedia of Evolutionary Biology