Pleiotropy: Definition, Overview, Meaning, Examples, Topics

Pleiotropy refers to the phenomenon where a single gene influences multiple, unrelated phenotypic traits. The word “pleiotropy” is derived from the Greek words pleion (more) and tropos (way). It is a fundamental concept in genetics. In Mendelian genetics, one gene corresponds to a single trait. However, pleiotropy reveals a more complex gene expression. Many genes are not limited to only one specific trait.

This Story also Contains

1. What is Pleiotropy?
2. Gene Pleiotropy
3. Examples of Pleiotropy
4. Human Genetic Disorders
5. MCQs on Pleiotropy

The principle of pleiotropy plays a crucial role in understanding how genes function, how traits develop and how genetic disorders can show in complex ways. It is important for fields like genetics, medicine, and evolution. In humans, Marfan syndrome is an example of pleiotropy where one gene affects the heart, eyes, and bones.

What is Pleiotropy?

Pleiotropy is the phenomenon by which one gene, termed as pleiotropic gene, influences several unrelated phenotypic traits. This involves higher complexity in the gene interaction and their impacts on organisms. It can occur because a gene may be involved in multiple biochemical pathways or its product may function in different tissues. For example, a gene that codes for a hormone may affect growth, metabolism, and reproduction simultaneously.

It provides applications in evolutionary biology, medicine, and genetic engineering. A classic example of pleiotropy is the gene causing sickle cell anaemia. It is related to the shape of the blood cells and resistance to malaria.

Gene Pleiotropy

Pleiotropy is the phenomenon where the pleiotropic gene affects several phenotypic traits. This happens because the product of the gene (usually a protein) can affect various biological pathways. For example, an allele of a single gene may produce effects on different tissues or organs. An example of pleiotropy is the gene associated with sickle cell anaemia. The single mutation causes sickle-shape of RBCs as well as provides resistance against malaria .

On the other hand, polygenic traits are influenced by several genes working together. Each gene contributes a little effect to the overall phenotype. Human height and skin colour are some of the examples. While pleiotropy emphasizes how one gene can influence multiple traits, polygenic inheritance explains how many genes work together to influence a single trait. It is important to understand the difference between these two to explain complex genetic interactions and a wide variety of phenotypes in living organisms.

Examples of Pleiotropy

Pleiotropy can be observed in both humans and other organisms where a single gene impacts multiple traits. This happens because the products of genes may function in different cells. Here are a few examples of pleiotropy:

Phenylketonuria (PKU)

Phenylketonuria (PKU) is an autosomal recessive disease. It is caused by a mutation in the PAH gene. This inhibits the production of phenylalanine hydroxylase, an enzyme necessary to break down amino acid phenylalanine. This leads to intellectual disability, behavioural issues, skin problems, and a strange smell. It describes how one gene mutation has the potential to cause multiple different traits.

Sickle Cell Anemia

Sickle cell anemia is caused by a point mutation in the HBB gene coding for hemoglobin. This affects the shape of erythrocytes, oxygen carrying capacity, pain crises, prone to infection, and resistance to malaria. It is an excellent example of pleiotropy.

Marfan Syndrome

Marfan syndrome is caused by a mutation in the FBN1 gene. This condition affects the structure and function of connective tissues, leading to long limbs, flexible joints, vision problems, and skeletal deformities.

Human Genetic Disorders

Pleiotropy is seen in a variety of human genetic disorders. It occurs when a mutation in one gene causes changes in different parts of the body. For example, the FBN1 gene causes Marfan syndrome which affects the bones, heart, and eyes. These effects come from single gene mutation, not chromosomal mutations. Another example is the NF1. It is caused by the loss of function of the gene NF1. This leads to both skin and nerve tumours as well as cognitive difficulties.

These conditions explain the phenomenon of pleiotropism, where one pleiotropic gene causes multiple, often unrelated, phenotypic effects. Such disorders are often studied using pedigree analysis which helps to trace the inheritance pattern in a family. This puts clearly that genetic influences within the body can sometimes be remarkable, and that mutation of a single gene can have major effects on different body systems.

MCQs on Pleiotropy

Q1. The best example for pleiotrophy is:

1. Phenylketnoneuria

2. Colour blindness

3. ABO blood group

4. Skin colour

Correct Answer: 1) Phenylketonuria

Explanation:

Pleiotropy refers to the genetic effect of a single gene influencing multiple phenotypic traits. Disorders like phenylketonuria, sickle cell anemia, and cystic fibrosis are examples of pleiotropy, where a single genetic mutation leads to various health issues. In phenylketonuria, a mutation in the gene for phenylalanine hydroxylase causes intellectual disabilities and other neurological issues. Sickle cell anaemia results from a haemoglobin gene mutation, leading to anaemia, pain, and organ damage. Cystic fibrosis, caused by a mutation in the CFTR gene, affects multiple organs, leading to respiratory and digestive problems. In all these conditions, one gene mutation impacts several bodily functions.

Hence, the correct answer is option 1) Phenylketoneuria

Q2. Pleiotropic gene:

1. Expressed only in primitive parts of plants

2. Evolved during the Pliocene era

3. Controls a trait only in combination with another gene

4. Controls multiple traits in an individual.

Correct Answer: 4) Controls multiple traits in an individual

Explanation:

As we already studied in Pleiotropy -

• There are instances where a single gene can exhibit multiple phenotypic expressions. Such a gene is called a pleiotropic gene.

• The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways that contribute towards different phenotypes.

Hence, the correct answer is option (4) Controls multiple traits in an individual.

Also Read:

• MCQs on Pleiotropy

• Incomplete Dominance and Mendel’s Experiment

• Mendelian Disorders