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About 1 in 20,000 men has no Y chromosome, instead having 2 Xs. This means that in the United States there are about 7,500 men without a Y chromosome. The equivalent situation - females who have XY instead of XX chromosomes - can occur for a variety of reasons and overall is similar in frequency.
A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly in response to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the "ratcheting" of evolutionary speed as one clade competes with another for a limited resource.
Whenever I hear this question asked the answer always involves the advantages of mixing up the genes of two parents, as opposed to some sort of asexual reproduction which produces clones. A good answer - but not to the question I am asking!
....we have genes on the Y that can turn females with XX chromosomes into males and genes on the X that can turn males with XY chromosomes into females… wow! Maleness and femaleness are NOT determined by having an X or a Y, since switching a couple of genes around can turn things upside down.