Note that Middle-earth Earth ('Arda') supposedly is normal Earth: LotR takes place during the Third Age, whereas we're currently in the Sixth or Seventh Age.
Modern humans are one of the most homogeneous species we know of (apart from the our crops that we have bred into homogeneity) and even humans show very dynamic inheritance patterns. A red hair can pop up after several generations of different hair colors for example.
There is no reason to assume that genetics in different (often magical) species is less dynamic then in modern humans. If anything we should have more variety and more dynamic traits as different races are interbreeding then today. If only for the reason that more diversity opens more possibilities for more creative and fun stories.
But Tolkien didn’t specify what traits inherit in which races did he? How do you know how skin tone inherits in this magical world?
Even in our universe we have species such as Chimpanzees which grow darker skinned as they age? How do you know the same doesn’t happen with wizards? In our natural world color is indeed quite dynamic. Even human skin tones, hair colors, etc. are quite dynamic in our world (even among related individuals). In a magical world there is nothing preventing the different races from being even more dynamic and diverse.