PLEIOTROPY- MEANING, CLASSES & EXAMPLES

PLEIOTROPY- MEANING, CLASSES & EXAMPLES

Introduction

• Meaning: The word “Pleiotropy” is derived from Greek  pleion, means “more”, and tropos, means “way” and occurs when one gene affects two or more characters.
• Definition: Pleiotropy refers to the phenomenon of a single gene affecting multiple traits.
• Contrasts with Mendelian genetics, where one gene = one trait.
• Key features: A single gene product (protein/enzyme) may- Participate in multiple biochemical pathways and affect different organs or tissues.
• Explains why genetic disorders often have multiple symptoms
• Pleiotropic gene– Gene that exhibits multiple traits.

Examples:

• Phenylketonuria (PKU): mutation in gene for enzyme phenylalanine hydroxylase. It leads to mental retardation, skin pigmentation issues, and neurological problems.
• Sickle Cell Anemia: mutation in hemoglobin gene. Causes anemia, resistance to malaria, organ damage.
• Mendel’s pea plants: seed coat color gene also influenced flower color and axil pigmentation

Classes (Arise of Pleiotropy)

Pleiotropy entails a mapping from one thing at the genetic level to multiple things at a phenotypic level. The natures of the things vary in different contexts.

Pleiotropy can arise from several distinct but potentially overlapping mechanisms, such as gene pleiotropy, developmental pleiotropy, and selectional pleiotropy.

1. Gene pleiotropy

This concept implies the number of functions a molecular gene has. These functions can be defined either genetically through the number of measured characters affected by a knockout, or biochemically, for example, by the number of protein-protein interactors a gene has or the number of reactions it catalyzes.
Gene pleiotropy occurs when a gene product interacts with multiple other proteins or catalyzes multiple reactions.

2. Developmental pleiotropy

Gene influences multiple traits during growth. Here mutations are relevant unit but not molecular gene. Developmental pleiotropy is a feature of the genotype-phenotype map that defines the genetic and evolutionary autonomy aspects of phenotype, independent of fitness. Simply, it occurs when mutations have multiple effects on the resulting phenotype.

3. Selectional pleiotropy

A gene affects traits that impact evolutionary fitness differently. Selectional pleiotropy occurs, when the resulting phenotype has many effects on fitness based on factors such as age and gender. A key feature of selectional pleiotropy is that traits are defined by the action of selection and not by the inherent attributes of the organism.

Evolution

Pleiotropy can have an effect on the evolutionary rate of genes and allele frequencies.

In the context of life history evolution, pleiotropy means that a single gene affects the fitness of the organism at two or more ages. If a new mutation improves fitness in both young and old animals, then it is likely to be favored by natural selection, and will increase in the population.

Conversely, a gene that decreases fitness in both young and old organisms will be eliminated by natural selection. The more interesting cases are those in which the fitness effects on young and old organisms are negatively correlated, a condition referred to as ‘negative pleiotropy’ or ‘antagonistic pleiotropy.’

Medawar’s principle suggests that mutations that improve early fitness at the expense of late fitness will be favored by natural selection, while those with the converse effects will be eliminated.

As far as human molecular genetics concerned, Pleiotropy refers to disorders where multiple, seemingly unrelated organ systems are affected. For example, one individual in a pedigree may exhibit cardiac arrhythmia, whereas another individual with the same disorder in either the same or different pedigree shows muscle weakness and deafness. Since the manifestations of disease are so vastly and usually inexplicably different, disorders that show a high degree of pleiotropy are often difficult to diagnose.

As a group, mitochondrial disorders typically show a high degree of pleiotropy, as any organ system can be affected, to almost any degree, with any age of onset.

MCQs

Q1. Pleiotropy refers to:

1. One gene → One trait
2. One gene → Multiple traits
3. Multiple genes → One trait
4. Multiple genes → Multiple traits

Answer: B) One gene → Multiple traits

Q2. Which of the following is a classic example of pleiotropy?

1. Colour of pea flower
2. Phenylketonuria (PKU)
3. ABO blood group inheritance
4. Eye colour in humans

Answer: B) Phenylketonuria (PKU)

Q3. Marfan syndrome is caused by mutation in:

1. Hemoglobin gene
2. Fibrillin-1 gene
3. Phenylalanine hydroxylase gene
4. Collagen gene

Answer: B) Fibrillin-1 gene

Q4. Which of the following best explains pleiotropy in evolution?

1. A gene mutation affects only one trait.
2. A gene mutation affects multiple traits, influencing fitness.
3. Traits are inherited independently.
4. Genes are always linked.

Answer: B) A gene mutation affects multiple traits, influencing fitness.

Q5. Mendel’s pea plant experiment showed pleiotropy when:

1. Seed coat colour gene also influenced flower colour and axil pigmentation
2. Tallness gene influenced only stem height
3. Round seed gene influenced only seed shape
4. Pod color gene influenced only pod color

Answer: A) Seed coat color gene also influenced flower color and axil pigmentation.

Read also…
CHROMOSOMAL THEORY OF INHERITANCE
TYPES OF CHROMOSOME-STRUCTURE&DEFINITION
LINKAGE AND CROSSING OVER-AN OVERVIEW
HOMOLOGOUS CHROMOSOMES-FEATURE & FUNCTION