Pleiotropy: Pleiotropy is described as a single gene’s expression of several traits. The term “pleiotropy” comes from a Greek word that means “various ways.” Gregor Johann Mendel, the father of genetics, made a number of intriguing discoveries about the colour of various plant components. The plants with colourful seed coats also have colourful blossoms, according to Mendel. He also noticed that the pea plants had white-coloured blooms and colourless seed coats with no colouration on their axils. The colour of the seed coat had traditionally been linked to the flower’s individual axil and colour. Mendel’s observations were based on the outcome of Pleiotropy, it was determined. Where a single gene is involved in a variety of phenotypic features.

Scroll down to learn more about the definition, process, some of the examples and much more about Pleiotropy.

Pleiotropy Definition

Pleiotropy is defined as a phenomenon in which a single gene controls or affects many phenotypic traits. Such genes are called pleiotropic genes, which may be lethal for the body. The term “pleiotropie” was first coined by Ludwig Plate in \(1910.\) Pleiotropy is a deviation from Mendel’s law of unit factors.

Pleiotropy Diagram

Fig: Pleiotropy

Pleiotropy Process and Function

1. Pleiotropy explains the genetic effect of a single gene on multiple phenotypic traits.
2. The mechanism is that the genes that code for a particular protein are either used by various cells or have a cascade-like signalling function that affects various targets.
3. Pleiotropy arises due to several distinct but overlapping mechanisms, such as gene pleiotropy, developmental pleiotropy and selectional pleiotropy.
4. Gene Pleiotropy takes place when a gene product interacts with multiple other proteins or catalyses multiple reactions. It is also referred to as molecular gene pleiotropy. Pea plants bearing white coloured flowers consist of colourless axils and seed coats, whereas plants with purple flowers have brown-grey coloured seed coats with reddish axils. Thus, instead of affecting only one characteristic, the colour gene affects three.
5. Developmental Pleiotropy takes place when mutations have multiple effects on the resulting phenotype.
6. Selectional Pleiotropy takes place when the resulting phenotype has many effects on fitness (depending on factors such as age and gender).

Examples of Pleiotropy

The genes responsible for causing genetic disorders in humans are mainly pleiotropic. Following are few examples of human genetic disorders and their description:

1. Phenylketonuria (PKU)

1. Phenylketonuria is an inborn error of metabolic and autosomal recessive genetic disorder, which is caused due to lack of an enzyme called phenylalanine hydroxylase (PAH).
2. The gene for the PAH enzyme is present on chromosome number \(12,\) and the abnormal gene in the homozygous recessive condition.
3. The affected people have heterozygous parents, which are carriers of the gene and are apparently normal but the activity of the enzyme is less than normal.
4. PAH enzyme is necessary to convert amino acid phenylalanine into tyrosine.
5. Due to the lack of the enzyme, not only the amount of phenylalanine increases, but the amount of an alternate by-product called phenylpyruvate (ketone derivative of phenylalanine) also increases considerably.
6. High amounts of phenylalanine in the blood affects many vital processes of the body. It gets deposited in the CNS and affects the development of the brain, and may cause severe brain damage and mental retardation.
7. Phenylpyruvate cannot be stored in our body and is excreted out through urine and hence the disease is called phenylketonuria.
8. Decreased availability of tyrosine leads to reduced melanin formation and hence reduced colouration of hair and skin. This may cause eczema of the skin.
9. Also, tyrosine is a component of various hormones like thyroxine, adrenaline, nor-adrenaline, and thus affects the body’s metabolic activities due to its deficiency.
10. The disease can be kept under control by restricting the foods containing phenylalanine.

Fig: Cause of Phenylketonuria

Fig: Pattern of PKU Inheritance

2. Sickle-cell Anaemia

1. Sickle-cell anaemia is an autosomal recessive hereditary disease caused by a pleiotropic gene with a lethal effect in the homozygous recessive condition.
2. The abnormal mutated gene is present on chromosome number \(11.\)
3. The abnormal mutated gene causes the production of an abnormal \({\rm{\beta }}\)-haemoglobin chain.
4. This disease is controlled by a gene having alleles HbA (normal) and HbS (mutant gene). This disease is shown by the homozygous recessive individuals, i.e., HbSHbS, while HbAHbS shows sickle-cell trait.
5. Sickle-cell anaemia is also an example of incomplete dominance, as the dominant allele (HbA) is not completely able to mask the expression of the recessive allele (HbS).
6. HbS is caused by a single base substitution at the \({6^{{\text{th}}}}\) codon of the mRNA of the \({\rm{\beta }}\)-globin gene from GAG (codes for glutamic acid) to GUG (codes for valine).

Fig: Mutation

7. Thus, HbS arises due to point mutation or substitution mutation or transversion of the HbA gene.
8. This disease is caused by the substitution of glutamic acid (Glu) by valine (Val) in the \({6^{{\text{th}}}}\) position of the \({\rm{\beta }}\)-globin chain of the haemoglobin molecule.
9. Glutamic acid is a negatively charged amino acid and is involved in forming electrostatic force of attraction with positive amino acid. Such bonds are not formed by valine as it is a neutral amino acid.
10. The mutant haemoglobin molecule undergoes abnormal polymerisation and folding, which leads to a change in the shape of RBCs from biconcave shape to elongated sickle-shaped RBC.
11. Such sickle-shaped RBC has low elasticity and cannot pass through small vessels. When such RBCs come in contact with small vessels or capillaries, they break down, leading to blockage and anaemia. Also, the oxygen-carrying capacity of the RBCs is highly reduced.
12. In extreme cases, sickle-cell anaemia can also lead to permanent blindness, liver obstruction and disorders of the heart.

Fig: Sickle Cell Anemia

13. The homozygous recessive individual has high death rate chances because of severe anaemia caused by premature destruction of the sickle-shaped red blood cells.
14. But the heterozygous individuals (also referred to as sickle-cell trait) survive but show mild anaemia because they have both normal and abnormal haemoglobin, and only some of the red blood cells are sickle-shaped.
15. Such heterozygous individuals are highly resistant to malaria because malarial parasites cannot live within these abnormal cells. Malarial parasites survive in normal red blood cells, and so affect the normal homozygous dominant individuals.

Fig: Pattern of Sickle Cell Anaemia Inheritance

Did you know?

With natural selection favouring the heterozygotes, the severely stricken malarial regions (part of Africa) may have up to \(40\% \) of the population as carriers of sickle-cell anaemia.

Summary

Pleiotropy is a phenomenon that occurs when a single gene affects many features in living creatures. Pleiotropy can be caused by a gene mutation. Marfan syndrome, a human genetic condition affecting the connective tissues, is an example of pleiotropy. The eyes, heart, blood vessels, and bone are all typically affected by this condition. Pleiotropy is produced by a mutation in a human gene that causes Marfan Syndrome.

Pleiotropy is defined as a single gene that controls multiple traits, which is a deviation from Mendel’s law of unit factors. It arises due to several distinct but overlapping mechanisms, such as gene pleiotropy, developmental pleiotropy and selectional pleiotropy. Pleiotropy causes many human genetic disorders that may lead to the death of an individual. Moreover, the most common examples of genetic disorder caused due to pleiotropy is phenylketonuria and sickle-cell anaemia.

Frequently Asked Questions (FAQs) on Pleiotropy

Q.1. What is the most common example of pleiotropy in humans?
Ans: Phenylketonuria (PKU) and sickle-cell anaemia is the most common example of pleiotropy in humans. PKU is caused due to the deficiency of the enzyme phenylalanine hydroxylase due to the presence of an abnormal gene. Sickle-cell anaemia is caused due to point or substitution mutation of the HbA gene, resulting in a mutated HbS gene.

Q.2. Why is pleiotropy so common?
Ans: Pleiotropy is so common because, in a complex organism, a protein formed by a single gene can be expressed in more than one way. Also, it is due to the overlapping of multiple metabolic pathways in an organism.

Q.3. What is pleiotropy?
Ans: Pleiotropy is defined as a phenomenon in which a single gene controls or affects many phenotypic traits. It is a deviation from Mendel’s law of unit factors.

Q.4. What is the difference between pleiotropy and polygenic inheritance?
Ans: Pleiotropy is when one gene affects multiple characteristics (e.g., Phenylketonuria) and Polygenic Inheritance is when one character is controlled by multiple genes (e.g., skin pigmentation).

Q.5. What are the consequences of pleiotropy in the human body?
Ans: Some of the consequences of pleiotropy in the human body include unusually tall height, thin fingers and toes, dislocation of the lens of the eye and many heart problems.

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