You’re correct in your belief that you can’t change what Mother Nature has given your horse the genes for. Some horses just never develop great manes and tails. That being said, there are a number of things you can do to protect what you do have and encourage faster growth. Somebody above mentioned no combing/brushing unless you have to, and generally, I agree with that. Don’t ever let your horses’ mane or tail become tangled, but comb/brush sparingly and carefully. I also only comb or brush a mane or tail when it has conditioner in it (I’m not a fan of MTG at all, and never leave detangler in a mane or tail). A good quality balanced diet is helpful and adding a good quality hoof supplement to your horses diet will help with mane and tail growth as well, as the same nutrients are needed to grow hair as to grow hooves. Also, just like with supplementing to improve hoof quality or growth, it takes some time to really see results, so be patient.