The correct answer is beceayse a mule does not have a even number of chromosomes. They have 63 chromosomes.
A mule is the product of two different species (a horse and a donkey) mating with each other. Mules are always sterile because horses and donkeys have different chromosome numbers.
For the mule, having parents with different chromosome numbers isn't a problem. During mitotic cell division, each of the chromosomes copies itself and then distributes these two copies to the two daughter cells. In contrast, when the mule is producing sperm or egg cells during meiosis, each pair of chromosomes (one from Mom and one from Dad) need to pair up with each other. Since the mule doesn't have an even number of homologous pairs (his parents had different chromosome numbers), meiosis is disrupted and viable sperm and eggs are not formed.
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