At conception we all inherit a set of genes from our parents that will, in time instruct our bodies to make us male or female. But for the first few weeks of our lives human embryos are neither. Instead we have a protrusion called a tubercle. If you’re genetically male the Y chromosome instructs the gonads to become testicles. They also send testosterone to the tubercle, where it is converted into a potent hormone called dihydro-testosterone This transforms the tubercle into a penis. If you’re female and don’t make dihydro-testosterone then your tubercle becomes a clitoris.