Does anyone know where the hereditary trait for having only 28 teeth comes from?

Hypodontia is the lack of some teeth. Usually, hypodontia refers to the lack of development of one or more teeth, and oligodontia may be used to describe the absence of 6 or more teeth. Some systemic disorders which may result in hyperdontia include Apert syndrome, Cleidocranial dysostosis, Crouzon syndrome, Ehlers-Danlos syndrome, Gardner syndrome, and Sturge-Weber syndrome.[67] Some systemic disorders which may result in hypodontia include Crouzon syndrome, Ectodermal dysplasia, Ehlers-Danlos syndrome, and Gorlin syndrome.[68]

well I am in my mid-30's and do not have my wisdom teeth yet..ok I lie I have 1 that has not erupted, sitting low and not moved since my mid 20's when it started to come up for a week. My sister does not have them either, although my younger brother and both parents had them!!
I am not sure which teeth you don't have...is it these teeth??

I personally feel its an evolutionarly advanced trait as humans have been evolving a smaller jaw for a while and the wisdom teeth now cause problems for many people or do not even come up at all........if you don't have your wisdom teeth considrer yourselves more "advanced" than other people....in the past people with problem teeth probalby would have died from infections and hence those people with good/ok teeth would have survived and pass on the good teeth (or no  wisdom teeth etc) onto their children, who would have passed that on etc etc etc

I actually feel that its good for you. Its really very painful when the wisdom teeth are coming. Also there is no problem if you don't have them, so it's good that you are saved from the pain. 

I made some research on it and found good information here. You will definitely get some help from it.