Dr. snider, a geneticist, is looking for a slight variation of a particular gene that would cause a certain abnormality. he is looking for an:

I believe he is looking for an allele
An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosomes. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent.