Scientists have deciphered the make-up of the Y chromosome, the bundle of DNA that defines the male.

The genetic data will help researchers understand the causes of male infertility, as well as the evolution and function of the Y. It turns out that large chunks of the DNA sequence are multiple repeats of seemingly meaningless stretches of genetic code.

Buried within these regions are genes that are essential for male development, including the master switch that “makes” a baby boy.