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.