PLEIOTROPY PENETRANCE AND EXPRESSIVITY PLEIOTROPY Most of the

PLEIOTROPY, PENETRANCE AND EXPRESSIVITY

PLEIOTROPY • Most of the biochemical pathways in the living organism are interconnected. Products and intermediates of one pathway may be used in several other metabolic processes. Hence, the phenotypic expression of a gene usually affects more than one character/ trait. • The term pleiotropy refers to the effect of a single gene on more than one character / trait.

• Sometimes one trait will be very evident and others will be less evident. EXAMPLE: A gene for white eyes in Drosophila also affects the shape of organs in female responsible for sperm storage as well as other structure. § Another example is frizzle-trait in chickens. The primary result of this gene is the production of defective feathers. Secondary results are both good and bad; good includes increased adaptation to warm temperatures, bad includes increased metabolic rate, decreased egglaying, changes in heart, kidney and spleen. § Similarly, sickle-cell anemia is a human genetic disease. Sicklecelled individuals suffer from a number of problems, all of which are pleiotropic effects of the sickle-cell allele. NOTE: Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is genes that code for a product that is either used by various cells or has a cascade-like signaling function that affects various targets.

More examples • Albinism is the mutation of the TYR gene, also termed tyrosinase. This mutation causes the most common form of albinism. The mutation alters the production of melanin, thereby affecting melanin-related and other dependent traits throughout the organism. Melanin is a substance made by the body that is used to absorb light and provides coloration to the skin. Indications of albinism are the absence of color in an organism's eyes, hair, and skin, due to the lack of melanin. Some forms of albinism are also known to have symptoms that manifest themselves through rapid-eye movement, light sensitivity, and strabismus .

• Marfan syndrome (MFS) is an autosomal dominant disorder which affects 1 in 5– 10, 000 people. [36] MFS arises from a mutation in the FBN 1 gene, which encodes for the glycoprotein fibrillin-1, a major constituent of extracellular microfibrils which form connective tissues. [36] Over 1, 000 different mutations in FBN 1 have been found to result in abnormal function of fibrillin, which consequently relates to connective tissues elongating progressively and weakening. Because these fibers are found in tissues throughout the body, mutations in this gene can have a widespread effect on certain systems, including the skeletal, cardiovascular, and nervous system, as well as the eyes and lungs. [36]

Sickle-cell anemia Frizzle-trait in chickens

Penetrance is used to describe whether or not there is a clinical expression of the genotype in the individual. Expressivity is the term that describes the differences observed in the clinical phenotype between two individuals with the same genotype.

PENETRANCE AND EXPRESSIVITY Ø The percentage of individuals that shows a particular phenotype among those capable of showing it, is known as penetrance. Ø There is an example of polydactyly in human, which is produced by a dominant gene. Homozygous recessive genotype does not cause polydactyly. However, some heterozygous individuals are not polydactylous. Ø If suppose 20% of heterozygous individuals do not show polydactyly, this means that the gene has a penetrance of 80%. Ø Degree of expression of a trait is controlled by a gene. Ø A particular gene may produce different degrees of expression in different individuals. This is known as expressivity.

• Different degrees of expression in different individuals may be due to variation in the allelic constitution of the rest of the genome or to environmental factors. • Thus, the terms penetrance and expressivity quantify the modification of gene expression by varying environment and genetic background; they measure respectively the percentage of cases in which the gene is expressed and the level of expression.