A genotype is the composition of the set of genes that make up an organism. These genes are called "allele." The genotype determines the hereditary character and limitations of an organism, cell or individual. In humans, and other creatures that reproduce sexually, the genotype comprises the genes that are inherited from both parents.
Like fingerprints, the genotype of each person is unique, except in the case of identical twins. Although in the Feb. 14, 2008, issue of the American Journal of Human Genetics, a study was published by American, Swedish and Dutch scientists that shows DNA of identical twins is not exactly identical as previously thought. Some segments of the DNA studied were either missing or had duplicates, which can explain the reason why one identical twin may get an inherited disease while the other does not.
A genotype is not the same thing as a phenotype, which is the observable behavior and development of an organism. To put it another way, the genotype is the code and the phenotype is the manner in which the code manifests.
Genotyping, then, is generally used in the investigation of inherited genetic traits and which combinations of genotypes produce the same results. The best way of demonstrating how this works is by considering blood groups.
There are four major human blood groups, A, B, AB and O. A and B are dominant over O, which is recessive. Also, A and B are co-dominant, which means that a child from parents with A and B type will almost always have type AB blood. Thus, for example, a child with type A blood could come from 12 separate combinations of genotype: AA x AA, AO x AA, AO x AO, AB x AA, OA x OB, AA x AO, AA x AB, OB x OA, AA x OB, OB x AA, AB x OA and OA x AB.