It used to be thought that those sex-determining chromosomes pictured here -- XY if you're male, XX if you're female -- merely determined whether you would be born with ovaries or testes, and it was the sex hormones that those organs produced that were responsible for the differences we see in the opposite sex.
Pensava-se que esses cromossomas determinantes do sexo, aqui representados — XY para os machos, XX para as fêmeas — apenas determinavam se nascíamos com ovários ou testículos, e que as hormonas sexuais  que esses órgãos produziam eram responsáveis pelas diferenças que vemos em sexos opostos.