The genotype is the specific genetic makeup (the specific genome) of an individual, in the form of DNA. Together with the environmental variation that influences the individual, it codes for the phenotype of that individual.
With careful experimental design, researchers can use satistical methods to correlate differences in the genotypes of populations with differences in their observed phenotype. These association studies can be used to determine the genetic risk factors associated with a disease. They may even be able to differentiate between populations who may or may not respond favorably to a particular drug treatment. Such an approach is known as personalized medicine.