Whether or not your child is going to have a penis is largely the province of the Y chromosome, but studies suggest the size of that penis is a joint genetic effort between mothers and fathers. Although most of the genes responsible for penis size live along the X chromosome, “there are some genes in the Y chromosome that have links to penile lengths and size,” Dr. Jamin Brahmbhatt, a Florida-based urologist, told Fatherly.
“So you can’t blame your mom for your small penis.”
READ MORE: The Fatherly Guide to Genetics
For the first seven weeks of development in the womb, nobody has a penis. After that, a fetus will either start developing male or female genitalia. For those given a Y chromosome, that means a