They don't check for DNA when they do a hair test or any other drug test. Any test they run on a sample from your body they have to tell you what they are testing for and actually get your consent. If you don't give the consent for drug testing then you don't get the job. If your hair is too short then they will look for body hair or they can even use fingernails. I think most collectors like at least 1/2-3/4 of an inch or they will go elsewhere for the sample. They can go back about 3 months which is 1 & 1/2 inches of hair. How much have you been smoking in the last 3 months? What kind of hair do you have?