Terminal vs. Vellus Hair
Your beard, brows and bald spot are all the same hair fighting over hormones.
Grounded in the research on Vellus hair (Wikipedia); Terminal hair (Wikipedia)
Know the two hair types you actually have
Every follicle on you grows one of two things. Vellus = the short, fine, soft, usually unpigmented 'peach fuzz' under ~2mm that covers most of your body. Terminal = the long, thick, pigmented, deeply-rooted hair on your scalp, brows and lashes. Same follicle hardware, different output. Vellus exists basically everywhere except palms, soles, lips and a few other glabrous spots.
Understand the switch: it's androgens, not genetics-only
Follicles convert between types and the dial is hormones. At puberty, androgens (mainly DHT, made from testosterone by 5-alpha-reductase) trigger vellus-to-terminal conversion in the beard, armpits, chest and pubic region. That's why a 'beard' appears in months. Wikipedia's terminal-hair entry calls these androgenic hairs. Same hormone, opposite effect by region.