If my dad has the MTHFR gene mutation and my mom doesn't (I probably am heterozygous for it) will it affect me?

If your father has the gene, and your mother dosen't, it's all up to chance really. It just depends on which set of genes you get, out of all the possibilities. It also depends on your grandparents... They might have carried the gene, and therfore might highten your chances of getting it. It's hard to give a really accurate guess, without knowing more imformation to set up a punnet square. Your doctor could probably do it for you.