Testicles are glands that make sperm and male sex hormone (testosterone). There are normally 2 testicles, but even one testicle is capable of making sperm that can result in either a boy or girl. It is not true that one testicle produces sperm for girls and another for boys, because the man's sperm contains X and Y chromosomes which happen naturally. If a sperm with a Y chromosome fertilizes the egg, the baby will be a boy. If the sperm with an X chromosome fertilizes the egg, the baby will be a girl.
Attribution: Straight Talk Foundation & Marie Stopes