The Chrysalids by John Wyndham

David Strorm's father doesn't approve of Angus Morton's unusually large horses, calling them blasphemies against nature. Little does he realize that his own son, his niece Rosalind and their friends, have their own secret aberration which would label them as mutants. But as David and Rosalind grow older it becomes more difficult to conceal their differences from the village elders. Soon they face a choice: wait for eventual discovery or flee to the terrifying and mutable Badlands. "The Chrysalids" is a post-nuclear story of genetic mutation in a devastated world, which tells of the lengths the intolerant will go to keep themselves pure.