Sexually reproducing species widely rely on sex chromosomes to determine sexual fate. The alligator is one such reptile. The temperature an alligator is exposed to during a critical window of development determines whether it will wind up a male or a female. More commonly, however, heredity determines sex, which seems a more reliable way of producing even proportions of the sexes. Like sexual reproduction, sex chromosome systems have evolved independently but in common ways.
Some themes guide the evolution and logic of sex chromosome systems.
One such these is that chromosomes come in sister pairs, one inherited from the mother and one from the father. Sex chromosomes, for example the X and the Y in mammals, often look starkly different, yet are thought to derive from once-identical sister chromosomes. For example, the human Y may house only two dozen genes, whereas an average human chromosome houses one hundred times that number. A general characteristic of sex chromosomes is that they do not recombine as other pairs of chromosomes do, except at their tips. Recombination is a process where sister chromosomes align, intermingle, and trade fragments of genetic material before segregating into egg or sperm cells.