GENETICS • LECTURE 2

Concept of Gene & Multiple Alleles

Complete Notes for CSIR-NET, GATE, DBT-BET, ICAR, ICMR & Biotechnology Exams

Topics: Alleles • Multiple Alleles • Conditional Mutants • Pseudo-alleles • Complementation Test

📚 Index / Table of Contents

1. Concept of Gene
2. Evolution of the Concept of Gene
3. Alleles
4. Multiple Alleles
5. Examples of Multiple Alleles
6. Conditional Mutants
7. Pseudo-alleles
8. Complementation Test
9. Important Comparisons
10. High-Yield Exam Points
11. Quick Revision Sheet
12. 10 Practice MCQs

1. Concept of Gene

A gene is a functional unit of heredity that contains information required to produce a functional product, which may be an RNA molecule or, in many cases, a protein. Genes are located at specific positions called loci on chromosomes. The concept of a gene has changed considerably as genetics developed from classical Mendelian genetics to molecular genetics.

Exam Definition: A gene is a heritable unit of genetic information that contributes to a biological function and is represented by a specific DNA sequence at a particular chromosomal locus.

Important characteristics of a gene

• A gene is a unit of hereditary information.
• In cellular organisms, genes are generally composed of DNA.
• Some viruses use RNA as their genetic material.
• A gene occupies a definite position called a locus.
• A gene can exist in different alternative forms known as alleles.
• Genes can undergo mutation.
• Genes may encode proteins or functional RNA molecules.
• Gene expression can be regulated at multiple levels.
• A single gene may influence a phenotype, but phenotype is often affected by multiple genes and environmental factors.

Gene, locus, allele and genotype

Term	Meaning	Example / Key Point
Gene	A functional hereditary unit	Gene involved in a metabolic pathway
Locus	Physical position of a gene on a chromosome	Specific chromosomal location
Allele	An alternative form of a gene	A and a
Genotype	Genetic constitution of an organism	AA, Aa or aa
Phenotype	Observable characteristic	Flower colour, blood group, enzyme activity

2. Evolution of the Concept of Gene

The meaning of the term gene has evolved with experimental evidence. Understanding this historical development is important because many competitive examination questions are based on the transition from the classical concept to the molecular concept.

Mendelian concept

• Mendel proposed the existence of discrete hereditary factors.
• These factors were responsible for inherited characters.
• Mendel did not know the molecular identity of these factors.
• His experiments established principles such as segregation and independent assortment.

Chromosomal concept

• The chromosome theory of inheritance connected Mendelian factors with chromosomes.
• Genes were proposed to be located on chromosomes.
• Chromosomal behaviour during meiosis provided a physical explanation for segregation and independent assortment.

Molecular concept

• DNA was established as the genetic material in most cellular organisms.
• Genes were understood as DNA sequences.
• The relationship between genes and proteins became a major area of molecular genetics.
• The classical one-gene-one-enzyme concept was later refined into the one-gene-one-polypeptide concept.
• Modern genetics recognizes genes that encode functional RNAs as well as protein-coding genes.

⭐ Exam Focus: Do not treat the classical statement “one gene–one enzyme” as universally applicable. Many proteins contain multiple polypeptide chains, and many genes encode functional RNAs rather than proteins.

3. Alleles

Alleles are alternative forms of the same gene that occur at the same locus on homologous chromosomes. In a diploid organism, two alleles are normally present for a particular autosomal locus, one inherited from each parent. However, a population can contain more than two allelic forms of a gene; this condition is known as multiple allelism.

Basic properties of alleles

• Alleles occupy the same corresponding locus on homologous chromosomes.
• They influence the same general hereditary character.
• They may differ because of mutations in DNA sequence.
• Alleles can show dominant, recessive, codominant or other relationships.
• A diploid individual normally carries two alleles at an autosomal locus.
• A population can contain many alleles even though each individual usually carries only two at a diploid autosomal locus.

Dominant and recessive alleles

• A dominant allele can determine the phenotype in a heterozygous condition.
• A recessive allele generally produces its characteristic phenotype when present without a dominant allele under simple dominance.
• Dominance does not necessarily mean that an allele is more common in a population.
• Recessive does not mean weak, inferior or less frequent.

Codominant alleles

In codominance, two alleles present in a heterozygote are both expressed. A classical example is the ABO blood-group system, in which the I^A and I^B alleles are codominant.

4. Multiple Alleles

Multiple allelism occurs when a population contains more than two alternative forms of a particular gene at the same locus. Multiple alleles are generated through mutation of an original allele followed by inheritance through populations.

Definition: Multiple alleles are three or more alternative forms of a gene present in a population and occupying the same locus.

Most important point

The phrase “multiple alleles” describes the condition at the population level. A diploid individual does not normally carry all the alleles simultaneously at one autosomal locus. It generally carries two alleles, which may be identical or different.

Example: ABO blood group system

The ABO blood-group system is one of the most important examples of multiple allelism. The principal alleles are commonly represented as I^A, I^B, and i.

Genotype	Blood Group	Relationship
IAIA	A	Homozygous
IAi	A	Heterozygous
IBIB	B	Homozygous
IBi	B	Heterozygous
IAIB	AB	Codominant expression
ii	O	Recessive combination

Relationship among ABO alleles

• I^A and I^B are codominant to each other.
• i is recessive to both I^A and I^B.
• I^AI^B produces AB blood group.
• I^Ai produces A blood group.
• I^Bi produces B blood group.
• ii produces O blood group.

Other examples of multiple alleles

• ABO blood group system in humans.
• Self-incompatibility alleles in many flowering plants.
• Coat-colour allelic series in some animal systems.
• Various allelic series identified in model organisms.

CSIR-NET Trick: If a question says “there are four alleles in a population,” do not assume that a diploid individual has four alleles. At a single autosomal locus, an individual normally has two copies/alleles, while the population may contain many alternative alleles.

5. Conditional Mutants

A conditional mutant is a mutant whose phenotype depends on environmental conditions. Under one condition, the mutant may behave approximately like the normal or wild-type phenotype, whereas under another condition the mutant phenotype becomes apparent.

Key definition: A conditional mutant expresses its mutant phenotype only under a particular environmental condition, while it may show a near-normal phenotype under another permissive condition.

Permissive and restrictive conditions

• Permissive condition: The condition under which the mutant protein or pathway retains sufficient function for survival or normal growth.
• Restrictive condition: The condition under which mutant function is severely impaired and the mutant phenotype becomes evident.

Temperature-sensitive mutants

A major class of conditional mutants is the temperature-sensitive mutant. Such mutants can show different phenotypes at different temperatures.

• At a permissive temperature, the altered protein may function sufficiently.
• At a restrictive temperature, the altered protein may lose functional activity.
• Temperature-sensitive mutations are particularly useful for studying essential genes.
• Researchers can change the environmental condition to switch the phenotype on or off.

Why are conditional mutants useful?

• They allow study of genes that are essential for survival.
• A complete knockout of an essential gene may cause death before the biological function can be studied.
• A conditional mutant permits normal growth under one condition and functional analysis under another.
• They are useful in studying cell division, DNA replication, transcription, metabolism and protein function.

Remember: Conditional mutation does not mean that the DNA mutation itself appears and disappears with the environment. The mutation is present genetically; the environment determines whether its phenotypic effect becomes apparent.

6. Pseudo-alleles

Pseudo-alleles are genes or mutant sites that produce similar phenotypic effects and are so closely linked that they can initially appear to behave like alleles, although molecular or genetic analysis shows that they occupy different but very closely linked loci.

Why are they called “pseudo” alleles?

• True alleles occupy the same locus.
• Pseudo-allelic loci are different loci.
• However, the loci can be extremely close to one another.
• Because of close linkage, recombination between them can be rare.
• This can make them appear similar to alternative alleles of one gene.

Classical significance

Pseudo-alleles were important in the development of the modern gene concept. Genetic studies showed that what appeared to be one gene could sometimes contain functionally related regions or closely linked genetic units that could recombine.

Feature	True Alleles	Pseudo-alleles
Location	Same locus	Different but closely linked loci
Relationship	Alternative forms of same gene	Closely linked genetic units
Recombination	Not applicable between alleles at the same locus	Possible, though often at low frequency because of close linkage
Phenotype	May influence the same character	Can produce similar or related phenotypes

⭐ High-Yield: The key distinction is same locus versus different loci. True alleles occupy the same locus, whereas pseudo-alleles are closely linked but genetically distinct loci.

7. Complementation Test

The complementation test is a classical genetic test used to determine whether two recessive mutations that produce similar phenotypes are in the same gene or in different genes.

Basic principle: If two recessive mutations are combined in a diploid organism and the resulting organism shows a wild-type phenotype, the mutations generally complement each other and are interpreted as affecting different genes.

How the test works

1. Obtain two mutant strains with similar phenotypes.
2. Assume the mutations are recessive.
3. Cross the two mutant strains.
4. Observe the phenotype of the resulting heterozygous combination.
5. If the offspring shows wild-type phenotype, complementation has occurred.
6. If the offspring remains mutant, the mutations generally fail to complement.

Example

Consider two recessive mutations, mutation m1 and mutation m2, producing the same visible phenotype.

• If m1 and m2 are mutations in different genes, each mutant chromosome can supply the functional product missing from the other.
• The resulting organism can therefore display a wild-type phenotype.
• This is called complementation.

If both mutations affect the same gene, neither chromosome provides a functional version of that gene. Therefore, the mutant phenotype remains. This is called failure to complement.

Complementation versus recombination

• Complementation is a functional genetic test.
• Recombination involves exchange of genetic material between homologous DNA molecules.
• Complementation can help determine whether mutations affect the same functional gene.
• Do not confuse complementation with crossing-over.

Important limitation: Complementation tests are most straightforward for recessive mutations. Interpretation can be complicated by dominant mutations, unusual allelic interactions, haploinsufficiency, intragenic complementation and other genetic effects.

8. Intragenic Complementation

Intragenic complementation is a special phenomenon in which two mutations within the same gene can sometimes complement each other and produce substantial functional activity when present together in the same cell. This phenomenon demonstrates that some proteins can contain distinct functional regions or subunits whose mutant products can interact.

• It can occur within the same gene.
• It is different from the simple rule that “same gene always fails to complement.”
• Protein structure and subunit interactions can influence complementation.
• Therefore, complementation results should be interpreted with knowledge of the biological system.

9. Important Comparisons

Alleles vs Multiple Alleles

Feature	Alleles	Multiple Alleles
Meaning	Alternative forms of a gene	More than two alternative forms present in a population
Number	Usually discussed as two alternative forms	Three or more forms
Level	Can describe an individual's allelic pair	Population-level phenomenon
Example	A/a	Iᴬ, Iᴮ, i

Multiple alleles vs polygenic inheritance

• Multiple alleles: many alternative alleles of one gene occur in a population.
• Polygenic inheritance: a trait is influenced by multiple genes.
• These concepts are not interchangeable.
• ABO blood groups illustrate multiple allelism, not classical polygenic inheritance.

Multiple alleles vs codominance

• Multiple allelism refers to the number of alternative alleles in a population.
• Codominance refers to the relationship between two alleles in a heterozygote.
• The ABO system demonstrates both concepts.

10. High-Yield Exam Points

Gene Allele Locus Multiple Alleles ABO Codominance Conditional Mutant Temperature Sensitive Pseudo-allele Complementation

• A gene is a functional unit of hereditary information.
• Alleles are alternative forms of the same gene.
• Alleles at an autosomal locus occupy corresponding positions on homologous chromosomes.
• A locus is the physical position of a gene or genetic marker.
• Multiple alleles are three or more alternative forms of a gene in a population.
• A diploid individual generally carries only two alleles at one autosomal locus.
• ABO blood group is a classic example of multiple allelism.
• Iᴬ and Iᴮ show codominance.
• i is recessive to Iᴬ and Iᴮ.
• Conditional mutants show phenotype depending on environmental conditions.
• Temperature-sensitive mutants are important conditional mutants.
• Permissive conditions allow sufficient mutant function.
• Restrictive conditions reveal the mutant phenotype.
• Pseudo-alleles are closely linked genetic units at different loci.
• True alleles occupy the same locus.
• Complementation is commonly used to determine whether recessive mutations affect the same or different genes.
• Wild-type phenotype in a complementation test generally indicates mutations are in different genes.
• Mutant phenotype after crossing two recessive mutants generally indicates failure to complement.
• Complementation is a functional test and should not be confused with recombination.
• Intragenic complementation is a special case involving mutations within the same gene.

11. Common Mistakes in Competitive Exams

1. Mistake: Thinking that multiple alleles mean one individual has three or more alleles.
   Correction: Multiple allelism generally refers to a population.
2. Mistake: Assuming dominant means more common.
   Correction: Dominance describes phenotypic expression in a heterozygote, not population frequency.
3. Mistake: Confusing codominance with multiple allelism.
   Correction: Codominance describes an allelic relationship; multiple allelism describes the presence of several alleles in a population.
4. Mistake: Treating pseudo-alleles as true alleles.
   Correction: Pseudo-alleles are at different but closely linked loci.
5. Mistake: Thinking that a conditional mutation disappears under permissive conditions.
   Correction: The mutation remains present; its phenotypic effect is condition-dependent.
6. Mistake: Assuming failure of complementation means that mutations are always molecularly identical.
   Correction: Failure to complement generally indicates that the mutations affect the same functional unit under the conditions tested.

12. Quick Revision Sheet

⚡ One-Minute Revision

• Gene: functional unit of hereditary information.
• Locus: position of a gene on a chromosome.
• Allele: alternative form of a gene.
• Multiple alleles: >2 allelic forms in a population.
• ABO: Iᴬ, Iᴮ and i.
• Iᴬ + Iᴮ: codominant.
• i: recessive to Iᴬ and Iᴮ.
• Conditional mutant: phenotype depends on environment.
• Permissive: mutant function sufficiently maintained.
• Restrictive: mutant phenotype becomes evident.
• Pseudo-alleles: different but closely linked loci.
• Complementation: functional test for mutations.
• Wild type after crossing recessive mutants: usually complementation.
• Mutant after crossing: usually failure to complement.

13. Practice MCQs – Concept of Gene & Multiple Alleles

Test yourself with the following 10 questions. Select one option for each question and click Submit Quiz. The correct answers and explanations remain hidden until submission.

1. Which statement best describes an allele?

A. A physical position on a chromosome B. An alternative form of a gene C. A complete chromosome D. An environmental factor

2. Multiple allelism refers to:

A. Three or more alternative alleles occurring in a population B. Three or more genes controlling one trait C. Three or more chromosomes in a cell D. Three or more mutations in different chromosomes

3. Which is a classical example of multiple allelism in humans?

A. ABO blood group B. Haemophilia only C. Turner syndrome D. Down syndrome

4. In the ABO blood group system, which relationship exists between Iᴬ and Iᴮ?

A. Complete dominance B. Complete recessiveness C. Codominance D. Epistasis

5. A conditional mutant is best described as a mutant:

A. That cannot reproduce B. Whose phenotype depends on environmental conditions C. That contains no DNA D. That always produces a lethal phenotype

6. A temperature-sensitive mutant is usually studied by changing:

A. DNA sequence after every generation B. Environmental temperature C. Chromosome number D. Genetic code

7. Pseudo-alleles are generally located:

A. At exactly the same locus B. On unrelated chromosomes only C. At different but closely linked loci D. Only in mitochondrial DNA

8. If two recessive mutations complement each other, the resulting phenotype is generally:

A. Wild type B. Always lethal C. More severely mutant D. Haploid

9. Failure of complementation between two recessive mutations usually suggests that:

A. The mutations affect different chromosomes B. The mutations affect the same functional gene C. The mutations are always dominant D. No mutation is present

10. Which statement correctly distinguishes multiple allelism from polygenic inheritance?

A. Multiple allelism involves several alleles of one gene in a population, whereas polygenic inheritance involves several genes affecting a trait B. Both terms mean exactly the same thing C. Multiple allelism always involves three chromosomes D. Polygenic inheritance involves only one allele

🎯 Quiz Result

14. Final Exam-Oriented Summary

The concept of gene and multiple alleles forms an important foundation for understanding classical genetics, molecular genetics and genetic analysis. A gene represents a functional unit of hereditary information, while an allele represents an alternative form of a gene. Multiple allelism occurs when more than two alternative alleles are present within a population. The ABO blood-group system is a particularly important example because it demonstrates both multiple allelism and codominance.

Conditional mutants are especially useful experimental tools because they allow researchers to study genes whose complete loss may otherwise be lethal. Temperature-sensitive mutants are a common example. Under permissive conditions, the altered protein may retain sufficient activity, while restrictive conditions reveal the mutant phenotype.

Pseudo-alleles are important historically because they helped demonstrate that genetic units that appear allelic may actually represent closely linked loci. The complementation test provides another powerful method for determining whether two recessive mutations affect the same functional gene or different genes.

🎯 Most Important 5 Points to Remember:

1. Alleles are alternative forms of the same gene.
2. Multiple alleles means more than two allelic forms in a population.
3. ABO blood group demonstrates multiple allelism and codominance.
4. Conditional mutants show phenotype depending on environmental conditions.
5. Complementation helps determine whether recessive mutations affect the same or different functional genes.