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Females have two X chromosomes and males have one X and one Y. At least, so goes the standard biology-class definition of sex, but now it looks like that definition needs revision. Researchers at the University of Hawaii figured out a way for male mice to reproduce without a Y chromosome.

Watch the video above to see how the mice were able to do it. (Hint: they needed some help.) And learn why it could be a big deal for mice and other mammals.