Most mammals, like humans have an XY genetic system for determining sex where XX produces females and XY produces males. However, sometimes when sex chromosomes are dividing during meiosis, non-disjunction occurs and embryos are formed with numbers of sex chromosomes different from those that are typical. For example on occasion males have the sex chromosomes XXY or XYY. There are also sometimes females with a single X chromosome or an XXX combination of sex chromosomes. There are even sometimes individuals who are genetically XY, but due to male hormone insensitivity, develop some or nearly all female characteristics. Research some of these cases in humans. What do these cases teach us about the way that sex chromosomes function to determine sex in mammals?All of the evidence that we have points to a common origin of animal life on earth. Throughout the tree of life we often see the same patterns repeated over and over again. This shows us that evolution does not build things from scratch, but rather it tinkers with existing structures and physiologies. The HOX genes, which are the instructions for building bodies, are a great example of this. These genes, found in animals as different as fruit flys and mice, tell developing embryos to build structures like limbs and wings in the right places. This use of the same genes, for similar purposes, in distantly related organisms is called conservation of mechanism. However, sex determination systems, as varied as they are, can seem like an exception to conservation of mechanism. Is that really the whole story though? Read this peer reviewed article from PLoS Biology and explain. What are the hidden similarities between some of the vastly different systems of sex determination?Sex Determination across Evolution: Connecting the Dots http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0030021 Nova Online “How Is Sex Determined?” http://www.pbs.org/wgbh/nova/miracle/determined.html Nova Online “Sex: Unknown” http://www.pbs.org/wgbh/nova/gender/ Scientific American “How is the gender of some reptiles determined by temperature?” http://www.scientificamerican.com/article.cfm?id=experts-temperature-sex-determination-reptiles Scitable “Genetic Mechanisms of Sex Determination” http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314 Nature Podcast January 24th,2008- an interview with evolutionary biologist Dan Warner on his breakthrough in understanding the adaptive value of temperature dependent sex determination.(segment begins at 24:28) http://media.nature.com/download/nature/nature/podcast/v451/n7177/nature-2008-01-24.mp3 transcript http://www.nature.com/nature/podcast/v451/n7177/nature-2008-01-24.html Aaron Reedy: Wide World Science http://wideworldscience.blogspot.com/