Exceptions to Mendel's Principles > Pleiotropy

Pleiotropy is the phenomenon in which a single gene influences or controls the expression of more than one apparently unrelated phenotypic trait in an organism.

A gene of this type is termed a pleiotropic gene. The alleles of pleiotropic genes are inherited in the same manner as alleles of single-trait genes; however, their phenotypic expression manifests as a package of multiple, linked traits.

According to Mendel's principle of "one gene → one character," each gene was believed to control only a single phenotypic trait. Pleiotropy is an exception because one gene controls two or more seemingly unrelated phenotypic characters simultaneously.

The molecular reason is straightforward: a single gene may code for a protein that:

• Is expressed in multiple tissues or organs
• Participates in multiple biochemical/metabolic pathways
• Acts as an enzyme in a cascade, where disruption affects multiple downstream products

Sickle cell anaemia is a prominent example of pleiotropy in humans. Caused by a single recessive gene that produces abnormal haemoglobin, which distorts red blood cells into a sickle shape. Consequently, these malformed cells transport oxygen inefficiently - leading to anaemia and frequently clump together to obstruct small blood vessels.

Red blood cells in humans: A. normal type; B. sickled

It is caused by a single mutant allele Hb^S which substitutes glutamic acid with valine at position 6 of the beta-globin chain of haemoglobin.​

Alleles and Genotypes:

Cross Between Two Carriers:

Pleiotropic Effects of Hb^S Gene:

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a mutation in the PAH gene (Phenylalanine Hydroxylase gene), which is an example of pleiotropy.

Biochemical Pathway Disruption:

Multiple Phenotypic Effects of One Gene Mutation:

Effect	Mechanism
Intellectual disability / Mental retardation	Excess phenylalanine is toxic to the developing brain​
Seizures	Neurological damage from elevated phenylalanine​
Reduced skin and hair pigmentation	Tyrosine (needed for melanin) not produced​
Behavioural issues	Neurological impairment​
Eczema	Metabolic disturbance​

• Pleiotropy - A single gene controls two or more different non-related traits; such a gene is called a pleiotropic gene; e.g. sickle-cell anaemia gene (Hb^S).
• Example - Normal gene Hb^A is dominant; heterozygous carriers (Hbᴬ/Hbˢ) show mild anaemia with sickle-shaped RBCs under low O₂; homozygous recessive (Hb^S/Hb^A) die of total anaemia.
• Ratio - Cross between two carriers gives 1 Normal: 2 Carriers: 1 Sickle-cell anaemic; since anaemics die, the surviving ratio becomes 2:1 (carriers: normal) instead of the usual 3:1.
• The gene for sickle-cell anaemia is lethal in a homozygous condition but produces sickle-cell trait (mild anaemia) in a heterozygous condition - two different expressions from a single gene.