Mormons settled Utah in the 1800s, and in the 200 years since they've been quite incredibly successful at attracting other people to their faith (and consequently often to their state too) from other states, Canada, and European countries, with everyone multiplying fruitfully. There's plenty of genetic diversity. You could satisfy your curiosity with almost any specific Mormon couple though by asking how closely related they are, Mormons are good at keeping track of their ancestry.