Females have two X chromosomes while males have one X and one Y chromosome
Basically his experiments, which scientists came to know only after he was dead. His experiments, and of course, the result of this experiments that served as a proof of what Mendel afirmed, where one of the most importnat factors that made his ideas accepted.
They where also accepted because he discovered DNA, the basic laws of inheritance and chromossomes.
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It's the definition of a SPECIES.
Those animals are from the same species.