All of the genes present in an organism. The interaction between a genotype and its cell is what results in the phenotype. Natural selection operates on the genotype indirectly by way of the phenotype.
Genotype is the genetic makeup encoded in an individual's DNA. It is the primary code for the phenotype of an individual. The actual appearance or expression of the genotype can be changed dramatically by environmental factors and by the influence of other genes.
Genotype is not just the expressed gene; instead, it includes any recessive genes masked by the dominant gene of a coded pair.
Genotype can be determined by direct observation of DNA; it can also be determined by careful family studies mapping out the progress of a specific gene's expression. If the phenotype expressed is the recessive form of a diploidal pair (that is, a pair of genes that do not mix to create a trait; one either has blue eyes (recessive) or brown eyes (dominant)) then the researcher can usually determine that the genotype is two copies of the recessive gene.
Genotype and phenotype are often not directly correlated; some genes are triggered only by special environmental conditions, and some phenotypes are the result of multiple genotypes. An example of the first is cats; kittens with identical genotypes for fur color may express the color completely differently, with one kitten having gray fur and the other having orange. Researchers are not certain how this happens.
The determination of genotype was pioneered by Gregor Mendel, the father of genetics. His methodology included the use of a fairly complex grid that worked much like a multiplication table or road map distance finder. Today, more and more complex genotypes and genotype/phenotype correlations can be found using modern computer technology using advanced genetic and evolutionary algorithms.
Web Resources On Genotype
Genotype and Phenotype
The Genotype/Phenotype Distinction
Book Resources On Genotype
Genotype-by-Environment Interaction by Kang & Gauch
From Genotype to Phenotype by Sue Malcolm