In a boring, homogeneous world, guys everywhere would have the same hair, the same body, the same mjulus. And every woman would have the same shape, and their v…would be the same size and color. On the one hand, it’d be pretty utilitarian to know exactly what you’re getting into when someone’s (anyone’s) pants come off. On the other, there’d be no mystery and where’s the fun in that?
Luckily, we don’t live in this predictable world where when you’ve seen one penis or vagina you’ve seen them all far from it. What our genitals look like is determined by a variety of factors, the biggest being human genetics, which can produce incredibly varied, sometimes curious results.
But figuring out who in your family tree you can thank or scream at for the size of your penis is far trickier. Of course, you inherit your genes from your mom and dad, but their genes are also a mix of their parents’ genes, which are a mix of their parents’ parents’ genes, and so on, stretching all the way back to when the first proto-humans crawled out of the primordial ooze.
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.
That said, most men in a family do tend to share a similar dick length and shape. If a father has a bigger penis, the son’s will probably be similar in length. Kind of like how if your father is tall, you will more than likely be tall as well. But it’s not so much a guarantee as it is an educated guess.