Examples of multiple allele systems in genetics

Alleles are a fundamental concept in genetics that play a significant role in determining an organism’s traits and characteristics. They are alternate forms of a gene that occupy the same position, or locus, on a chromosome. Each individual inherits two alleles for each gene, one from each parent.

Alleles can exist in different forms, known as variants. For example, in a gene that determines eye color, one allele may code for blue eyes, while another allele may code for brown eyes. These different alleles result in variations in the expression of a particular trait.

There are two main types of alleles: dominant and recessive. Dominant alleles are expressed in the phenotype, or physical appearance, of an organism even if only one copy is present. In contrast, recessive alleles are only expressed if an individual has two copies of the allele. If an individual has one dominant allele and one recessive allele, the dominant allele will be expressed.

The inheritance of alleles follows specific patterns. In Mendelian genetics, offspring inherit one allele from each parent, resulting in a combination of alleles that determine their traits. This process is governed by laws such as the law of segregation and the law of independent assortment.

In addition to dominant and recessive alleles, there are also co-dominant and incomplete dominant alleles. Co-dominant alleles occur when both alleles are expressed in the phenotype. For example, in the ABO blood group system, the A and B alleles are co-dominant, resulting in individuals with both A and B antigens on their red blood cells if they inherit both alleles. Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in an intermediate phenotype. For instance, in snapdragons, a red-flowered plant crossed with a white-flowered plant produces offspring with pink flowers.

Alleles can also undergo mutations, which are changes in their DNA sequence. These mutations can lead to the creation of new alleles or alter the function of existing alleles. Mutations can occur spontaneously or be induced by various factors such as radiation or chemicals.

The study of alleles and their inheritance patterns provides insights into the genetic basis of traits and the diversity observed within populations. It helps scientists understand how genetic variations contribute to the variation in physical characteristics among individuals. Furthermore, the identification of specific alleles associated with certain diseases or conditions is vital in medical research and genetic counseling.

In conclusion, alleles are alternate forms of a gene that determine an organism’s traits. They can be dominant, recessive, co-dominant, or exhibit incomplete dominance. The inheritance of alleles follows specific patterns and can be influenced by mutations. Understanding alleles and their interactions is crucial for comprehending the genetic basis of inheritance and the diversity observed in living organisms.


In genetics, alleles are different forms of a gene that occupy the same position on a specific chromosome. While most genes have two alleles (one inherited from each parent), there are instances where multiple alleles exist for a particular gene. These multiple allele systems can lead to a wide range of genetic variations within a population. In this article, we will explore some examples of multiple allele systems in genetics.

1. ABO blood group system

1.1 Background

The ABO blood group system is one of the most well-known examples of a multiple allele system. It determines the types of antigens present on the surface of red blood cells. There are three alleles involved in this system: A, B, and O. The A allele codes for the A antigen, the B allele codes for the B antigen, and the O allele does not produce any antigens.

1.2 Allelic combinations

Individuals can have six possible genotypes in the ABO blood group system: AA, AO, BB, BO, AB, and OO. These genotypes result in four blood types: A, B, AB, and O. The A and B antigens can be expressed in various combinations, leading to different blood types. The presence of the O allele results in the absence of both A and B antigens, resulting in blood type O.

1.3 Importance

The ABO blood group system is significant in blood transfusions and organ transplantation. Individuals with type O blood are considered universal donors because their blood lacks A or B antigens, reducing the risk of an immune reaction in the recipient. In contrast, individuals with type AB blood are universal recipients since they can receive blood from any ABO blood type.

2. Human leukocyte antigen (HLA) system

2.1 Background

The human leukocyte antigen (HLA) system is a complex set of genes located on chromosome 6. These genes code for proteins that play a crucial role in the immune system, specifically in recognizing and distinguishing self from non-self cells. The HLA system has a high degree of polymorphism, with multiple alleles associated with each gene.

2.2 Allelic diversity

The HLA system consists of several genes, including HLA-A, HLA-B, and HLA-DR. Each gene has numerous alleles, resulting in a vast array of potential HLA types. For example, the HLA-B gene has over 1,000 known alleles. The diversity in HLA alleles allows for a diverse immune response and helps protect against a wide range of pathogens.

2.3 Transplant compatibility

The HLA system plays a crucial role in organ and tissue transplantation. Matching the HLA types between the donor and recipient is essential to minimize the risk of rejection. The more closely the HLA types match, the lower the chance of rejection. The high allelic diversity within the HLA system provides a greater likelihood of finding compatible donors for transplantation.

3. Coat color in rabbits (C locus)

3.1 Background

The C locus in rabbits is responsible for determining coat color. This multiple allele system involves three alleles: C, c(ch), and c. The C allele codes for full pigmentation (wild-type), c(ch) allele produces chinchilla (light gray) color, and c allele results in albino rabbits with no pigmentation.

3.2 Phenotypic outcomes

The different combinations of alleles at the C locus result in various coat colors. Rabbits with the genotype CC or Cc(ch) exhibit full pigmentation or chinchilla color, respectively. On the other hand, rabbits with the genotype cc lack pigmentation and appear white or albino.

3.3 Inheritance patterns

The C allele is dominant over both the c(ch) and c alleles. Therefore, rabbits with at least one C allele will exhibit full pigmentation. The c(ch) allele is incompletely dominant, meaning rabbits with two copies of the allele have a lighter coat color than those with just one copy. The c allele is recessive, and rabbits must inherit two copies of this allele to display the albino phenotype.


Multiple allele systems in genetics provide a fascinating insight into the complexity and diversity of genetic variations within a population. Examples such as the ABO blood group system, the human leukocyte antigen (HLA) system, and the coat color in rabbits (C locus) demonstrate how multiple alleles can lead to diverse phenotypic outcomes. Understanding these multiple allele systems is crucial for various fields, including blood transfusions, organ transplantation, and animal breeding. By studying and unraveling the intricacies of these multiple allele systems, researchers can gain a deeper understanding of genetic diversity and its impact on various biological processes.

FAQs: Allele

1. What is an allele?

An allele is one of two or more possible versions of a gene. Genes are the basic units of heredity, and they contain the instructions for specific traits. Each gene can have different variations, and these variations are called alleles.

2. How do alleles work?

Alleles are responsible for the variations we see in different traits, such as hair color, eye color, and blood type. Every person has two alleles for each gene, one inherited from each parent. These two alleles can be the same (homozygous) or different (heterozygous) versions of the gene.

3. What are the different types of alleles?

There are several types of alleles:

  • Dominant alleles: These alleles are expressed in the phenotype (the observed trait) even if the individual has only one copy of the allele.
  • Recessive alleles: These alleles are only expressed in the phenotype if the individual has two copies of the allele.
  • Codominant alleles: These alleles are both expressed in the phenotype, resulting in a blended or intermediate trait.

4. How are alleles inherited?

Alleles are inherited from an individual’s parents. During sexual reproduction, each parent contributes one allele for each gene, and the offspring inherits a combination of the two alleles. The way these alleles are inherited follows the principles of Mendelian genetics.

5. What is the role of alleles in genetic diversity?

Alleles play a crucial role in genetic diversity within a population. The presence of multiple alleles for a single gene allows for a wider range of possible phenotypes, which can provide advantages in adapting to different environmental conditions. Genetic diversity also helps to reduce the risk of genetic diseases by providing alternative alleles that may not be affected by harmful mutations.

6. How can alleles be used in genetic testing and analysis?

Alleles can be used in genetic testing and analysis for various purposes, such as:

  • Identifying genetic disorders: Genetic tests can detect the presence of specific alleles associated with genetic diseases.
  • Determining paternity and ancestry: Allele patterns can be used to establish genetic relationships and trace ancestral lineages.
  • Forensic identification: Unique allele patterns can be used to identify individuals in criminal investigations.