What causes genetic variation? For example, what causes some people to have curly hair and others to not? What causes some people to have a genetic disease such as cystic fi brosis and others to not?

Guinea pig coat color is determined by a single gene. the allele for black coat color is dominant to brown. in a cross between twoblack-haired guinea pigs, 20 offspring are born. if both parents were heterozygous, probability would predict that approximately howmany of the 20 offspring would have brown hair?