The simian crease is one of the listed signs in Down Syndrome which would imply there's some genetic factor at play. My eldest daughter has a simian crease on one hand and the other hand is 'normal'.
Sovereign definition, a monarch; a king, queen, or other supreme ruler. See more.
What you think they mean: The same thing. What they actually mean: To reign is to hold royal office. The Queen reigns over the United Kingdom. Reins are things you use to walk a dog or harness a horse (hence the phrase free rein, which means to hold a horse's reins loosely).