Genotype & Phenotype Correlation

The Genotype and Phenotype Correlation Explained

Whereas the genotype of a person is the set of alleles that make up his or her genetic constitution, either collectively at all loci or, more typically, at a single locus; the phenotype is the observable expression of a genotype as a morphological, clinical, cellular, or biochemical trait.

Genotype is therefore the genetic composition that comprises an individual’s set of alleles and constitutes the genetic factors that create a phenotype. In contrast, the phenotype is the observed result of the interaction of the genotype with environmental factors; more specifically, is the visible or measurable properties resulting from a genotype, such as Duchenne muscular dystrophy Opens in new window or obesity.

Genotype also can refer to the alleles at a specific genetic locus. Phenotype can also be defined as the effect of gene action, whether caused by a single gene or the entire genotype.

The phenotype is usually thought of as the presence or absence of a disease, but phenotype can refer to any manifestation, including characteristics that can be detected only by blood or tissue testing.

In addition, the meaning of genotype and phenotype may be illustrated by a musical analogy.

The musical notation contains all of the necessary information or instructions for the notes to be played by each instrument and temporal relationships of the various parts—this is the genotype.

The phenotype is the sound we hear, which is strongly influenced by the environment, including the soloist, the conductor, the orchestra, and the hall in which it is performed, or the quality of the recording.

A phenotype may, of course, be either normal or abnormal in a given individual, but in genetic mapping studies, which emphasizes disorders of medical significance, the focus is on abnormal phenotypes—that is, genetic disorders.

Although each gene usually encodes a polypeptide chain or RNA molecule, a single abnormal gene or gene pair often produces multiple diverse phenotypic effects and determines which organ systems are involved, which particular signs and symptoms occur, and when they occur.

Under these circumstances, the expression of the gene defect is said to be pleiotropic. At present, for many pleiotropic disorders, the connection between the gene defect and the various manifestations is neither obvious nor well understood.

Penetrance and Expression

Some variations that occur between genotype and phenotype are explained through the principles of penetrance—the percentage of people with the gene who manifest the associated characteristics and expression—the degree to which the characteristics of a gene are manifest within the person.

The relationship between penetrance Opens in new window and expression Opens in new window for those at risk for transmission of an adverse genetic trait is profound. For example, some people with the autosomal dominant form of osteogenesis imperfecta Opens in new window express the disorder by having blue sclera and no other problems; other experience recurrent skeletal fractures even with mild trauma.

In addition, within the same family, parent and child may express the disorder to different degrees. An example is seen in families with a history of tuberculosis sclerosis, a disorder characterized by epilepsy, learning difficulties, behavioral problems, and skin lesions. Although a highly penetrant gene, some people have no disorder-related problems, even though they have both a parent and a child with clinical manifestations (National Center for Biotechnology Information, 2003).

See also:
    Adapted from credible resources on the Internet