Yes having 6 fingers on your hand is a dominant trait. Most people who have this trait get the extra finger cut off after birth. There is a small group in the world where the people there have 6 fingers.
When fingers are very sore and it is hard to open and close your hand, it could signify something as simple as over working your hand. This could also signify a break in the hand.
Cramps in the fingers of the left hand could result from simple muscle fatigue. It could also be a deficiency in an electrolyte such as sodium or potassium.
If your hand and fingers are numb and painful, you may have something pinched in your shoulder or back. If this lasts for more than a few days you should seek the attention of a doctor.
Petting
its that thing with fingers on it on the end of your arm!