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When a couple are expecting a baby, the big question is: girl or boy? Adam Rutherford explores the many ways Nature decides that question. If you’re a human, a kangaroo or a komodo dragon, it’s in the sex chromosomes. If you’re a crocodile, it’s the temperature of your egg. And if you’re a fish, it can be one sex first and, later in the life, the other.

Adam’s investigation includes conversation with Professor Jennifer Graves, a leading authority on sex determination, at La Trobe University in Australia. She explains what the weird nature of the platypus’ sex chromosomes tells us about how human gender is decided.

Adam also meets one of London Zoo’s Komodo Dragons, the world’s largest and fiercest lizard. Female dragons can produce young without mating with males, but all their babies are males. How so?