"Each allele has a slightly different code and may make a slightly different product, or control a process in a slightly different way. It is rather like models of a particular make of car, in that each model is essentially similar with just some minor differences. (Hopefully!) each model of car will transport you around wherever you want to go, but some will do it faster, some will be more comfortable, or look sleeker, while others will have greater fuel efficiency, and yet others will be better for particular types of terrain"