The employees of the shark exhibits at the Henry Doorly Zoo in Omaha, Nebraska, had just discovered that one of their bonnethead sharks had given birth to a female baby bonnethead. They weren't just excited: they were shocked. For three years, there had been only three bonnethead sharks in the tank in which the baby shark was born. They were all female.

Some female sharks, including bonnetheads, are known to store sperm for later fertilization. But none of the females in the tank had had contact with a male for three years, long before they were mature enough to reproduce. And female bonnetheads had never been known to store sperm for longer than five months. How could this explain how the shark got pregnant? When investigators analyzed the baby shark's DNA, they found that it was homozygous for all the traits they examined, including two rare traits. Throughout this chapter, look for clues that help explain how the baby bonnethead's mother got pregnant. Also, consider how sharks typically reproduce and the effect of that process on the offspring's genetic material. Then, solve the mystery.  

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