Looking at our human population we know that about half of us are female and half of us are male. The way this 1:1 sex ratio is determined is by genetic means, you get an X from your mum and either an X or Y from your dad; meaning there is an equal chance to be a female (XX) or male (XY). This seems to be a good way to achieve an even sex ratio. However quite a few animals go about a far more daring way to determine the sex of their offspring. They let the environment do it — in particular by using the temperature.