"This is an area that isn’t completely understood, but many genes involved in the growth of penis and limbs come from the X chromosome," Darius Paduch , director of sexual health and medicine at Weil Cornell, said in an interview with Mel Magazine . "Men have one X chromosome, and women have two. Since boys always inherit the X chromosome from the mother and the selection of that X chromosome is random, this can explain why one brother may have inherited genes for a large penis from one of the mother’s X chromosomes, but another brother inherited an average-size penis from the other."