When a boy goes through puberty, the steroid hormone testosterone puts hair on his chest by

In males, testis produces group of steroid hormones called androgens, the major of which is testosterone. These hormones influence growth of fascial, body, axial and pubic hair at the time of puberty.

Steroid hormones like testosterone interact with intracellular receptors and regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome.