Multiple Alleles Notes Pleiotropism & Pseudoalleles

Multiple Alleles

Introduction

In classical Mendelian genetics each gene is described as having two alternative forms (alleles). However, extensive studies showed that a gene may exist in more than two allelic forms within a population. This condition is known as multiple allelism.

• A gene locus can have multiple alleles present in the population, but a diploid individual can carry only two alleles (one on each homologous chromosome).
• Therefore, multiple alleles are primarily a population-level phenomenon.

Definition

Multiple alleles are defined as more than two alternative forms of a gene occupying the same locus on homologous chromosomes that control the same trait.

Characteristics of Multiple Alleles

1. Same locus: All allelic variants occupy the same genomic position (locus).
2. Population level diversity: More than two alleles exist in the population even though an individual carries only two at a time.
3. Mutational origin: Multiple alleles usually arise by different mutations of a single gene.
4. Different expression modes: Alleles can display dominance, recessiveness, co-dominance, or incomplete dominance relative to each other.
5. Hierarchy (order) of dominance: Frequently multiple alleles exhibit a dominance series where one allele is dominant to some and recessive to others.

Classical Examples

1. ABO Blood Group System (Human)

The ABO blood group is the most-commonly cited example of multiple alleles. A single gene (the I gene) has three common alleles:

• I^A — produces A antigen (via a specific glycosyltransferase)
• I^B — produces B antigen (different glycosyltransferase)
• i (I⁰) — non-functional allele; no A or B antigen produced

Genotype → Phenotype:

• I^AI^A or I^Ai → Blood group A
• I^BI^B or I^Bi → Blood group B
• I^AI^B → Blood group AB (co-dominance of I^A and I^B)
• ii → Blood group O

This example shows both multiple alleles and co-dominance in action.

2. Coat Colour in Rabbits

The coat colour gene in rabbits has several alleles that form a dominance hierarchy. A commonly cited series is:

C (full colour) > c^ch (chinchilla) > c^h (Himalayan) > c (albino)

Different allele combinations produce the range of coat phenotypes observed in domesticated rabbits.

3. Eye Colour in Drosophila

The eye-colour locus in Drosophila melanogaster has numerous alleles (red, white, apricot, cinnabar, etc.). Variations arise from mutations that affect pigment synthesis and transport, producing a spectrum of eye colours.

Molecular Basis of Multiple Alleles

• Multiple alleles originate by different point mutations, insertions, deletions or rearrangements affecting the same gene.
• Each allele may encode a variant polypeptide or enzyme with altered activity, specificity, or stability — leading to distinct phenotypes.
• For example, the I^A and I^B alleles of the ABO gene encode glycosyltransferases with different sugar-transfer specificities, while the i allele produces a non-functional product.

Genetic Interactions Observed with Multiple Alleles

Multiple alleles can interact in a variety of ways:

• Co-dominance: Both alleles are fully expressed in the heterozygote (e.g., I^AI^B → AB phenotype).
• Complete dominance: One allele masks another completely in the heterozygote.
• Incomplete dominance: Heterozygote shows an intermediate phenotype (less common with classic multiple-allele systems but possible).
• Dominance hierarchy: Alleles are ordered by dominance (A > B > C, etc.), useful to predict phenotype from genotype.

Significance of Multiple Alleles

1. Increases genetic diversity in populations — more alleles means a greater range of possible phenotypes.
2. Evolutionary potential: Provides raw material for natural selection and adaptation.
3. Medical and practical importance: The ABO system is critical in blood transfusion compatibility and forensic/paternity testing.
4. Research value: Multiple alleles help geneticists study mutation effects, gene function and biochemical pathways.

Comparison: Two Alleles vs Multiple Alleles

Feature	Two Alleles	Multiple Alleles
Number of alternative forms	2	More than 2
Level (individual vs population)	Both individual and population	Primarily population-level (individual carries 2 at most)
Example	Pea seed shape (Mendelian example)	ABO blood groups, rabbit coat colour

Conclusion

Multiple alleles expand Mendel's two-allele picture and illustrate how one gene locus can manifest many functional variants across a population. Familiar systems such as the ABO blood group and rabbit coat colour help B.Sc. students connect molecular changes (mutations) to observable phenotypes and understand modes of dominance, co-dominance and allele hierarchies.

Pleiotropism

Definition

Pleiotropism (or pleiotropy) refers to the condition where a single gene influences multiple, seemingly unrelated phenotypic traits. This occurs because the product of one gene may be involved in several biochemical or developmental pathways.

Examples

• Sickle-cell anemia in humans: Caused by a mutation in the β-globin gene. The altered hemoglobin leads to sickle-shaped RBCs (blood disorder), but also confers resistance to malaria, affects circulation, and can influence organ function.
• Phenylketonuria (PKU): Mutation in the gene coding for phenylalanine hydroxylase leads to accumulation of phenylalanine. This affects brain development, pigmentation, and mental health.
• Drosophila vestigial wing gene: Mutation affects wing size as well as fertility and life span.

Significance

• Shows that genes are not restricted to controlling single traits.
• Explains how mutations can have widespread effects on phenotype.
• Important in medical genetics for understanding genetic disorders with multiple symptoms.

Pseudoalleles

Definition

Pseudoalleles are closely linked genes that mimic the behaviour of multiple alleles but are actually distinct loci. They are located so close together on a chromosome that they often segregate as if they were allelic forms of a single gene, but recombination studies show they are separate.

Examples

• Drosophila eye colour (white locus): The white locus consists of several pseudoalleles that control different aspects of eye pigment formation. Initially thought to be one gene with multiple alleles, but later shown to be a cluster of tightly linked genes.
• Lozenge locus in Drosophila: Also exhibits pseudoallelism with genes affecting eye structure and pigmentation.

Significance

• Pseudoalleles helped in understanding fine structure of genes and the concept of gene clusters.
• They bridge the gap between classical genetics and molecular genetics, showing that what appeared as a single gene could actually be multiple tightly linked genes.
• Important for mapping studies and evolutionary biology.

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