GENOTYPE AND PHENOTYPE
The genetic or hereditary constitution of an individual, which is the whole complement of genes present, forms the genotype. The term can also be applied to any particular pair of alleles that an individual possesses at a specific locus on a chromosome. In contrast, the visible or measurable characteristics of an individual constitute the phenotype. A phenotype includes biochemical, physiological, morphological and behavioral characteristics or, indeed, any observable biological trait that is apparent throughout life, such as the total physical appearance and constitution of an individual or any specific trait, such as size, weight or eye color and, of course, includes characteristics of clinical importance and the presence of a disease. Some phenotypic traits, for example eye color, are directly observable but others, such as the blood group of a patient , may only become apparent following specific tests. Phenotypic traits do not necessarily occur merely following the expression of the genotype of an individual; some, such as the blood groups, are completely determined by heredity but many others, for example weight and height, result from interactions between the genotype and the environment.