Super intelligence.
Invincibility and imortallity.
I'd go with something thats actually helpful to mankind such as Cancer and / or immunodeficiency immunity.
That would be terrible, far too much overpopulation would result, and people wouldnt be able to kill each other for more land. Not that the second part of what I said is a good thing.
Generically engineer out the chav gene
Generically engineer out the chav gene