Question

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Write a note on pleiotropy.

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Answer 1

Explanation:

Pleiotropy:

i. When a single gene controls two (or more) different traits, it is called pleiotropic gene and this phenomenon is called pleiotropy or pleiotropism. The ratio is 2:1 instead of 3:1.

ii. According to Mendel’s principle of unit character, one gene (factor) controls one character (trait), but sometimes single gene produces two related or unrelated phenotypic expressions.

iii. For example, the disease, sickle cell anaemia is caused by a gene Hbs. Normal or healthy gene is HbA and is dominant.

iv. The carriers (heterozygotes – HbA/HbS) show signs of mild anaemia as their RBCs become sickle-shaped (half-moon shaped) in oxygen deficiency. They are said to have sickle-cell trait and are normal in normal conditions.

v. The homozygotes with recessive gene Hbs however, die of fatal anaemia.

vi. Thus, the gene for sickle-cell anaemia is lethal in homozygous condition and produces sickle cell trait in heterozygous carrier.

vii. Two different expressions are produced by a single gene.

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Answer 2

Answer:

Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Therefore, a mutation in a pleiotropic gene may have an effect on several traits simultaneously due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.

Pleiotropy can arise from several distinct but potentially overlapping mechanisms, such as gene pleiotropy, developmental pleiotropy, and selectional pleiotropy. Gene pleiotropy occurs when a gene product interacts with multiple other proteins or catalyzes multiple reactions. Developmental pleiotropy occurs when mutations have multiple effects on the resulting phenotype. Selectional pleiotropy occurs when the resulting phenotype has many effects on fitness (depending on factors such as age and gender).

An example of pleiotropy is phenylketonuria, an inherited disorder that affects the level of phenylalanine in the body. Phenylalanine is an amino acid that can be obtained from food. Phenylketonuria causes this amino acid to increase in amount in the body, which can be very dangerous. The human disease is caused by a defect in a single gene on chromosome 12 that affects multiple systems, such as the nervous and integumentary system. Other examples of pleiotropy are albinism, sickle cell anemia, and certain forms of autism and schizophrenia. Pleiotropy not only affects humans, but also animals, such as chickens and laboratory house mice, where the mice have the "mini-muscle" allele.

Pleiotropic gene action can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favors one allele, while selection on other traits favors a different allele. Some gene evolution is harmful to an organism. Genetic correlations and responses to selection most often exemplify pleiotropy.