Table of Contents
What is Multiple Alleles?
Alleles are genetic pairs that share a locus, which is a specific place on a chromosome. A gene in a diploid organism usually contains just two alleles. Multiple allelism is a condition in which a gene exists in more than two allelic variants. Allelism is the term used to describe any of a gene’s numerous variations. Hereditary variations are caused by these genetic differences, which are generally caused by mutation.
Each gene, according to Gregor Mendel, should contain just two alleles. Alleles are gene variants that come in two or more forms. Each gene is passed down in two forms, one from each parent. As a result, having two distinct alleles for a characteristic is also a possibility.
Heterozygous offspring have genotypes made up of various alleles of a gene, whereas homozygous offspring have genotypes made up of the same alleles (i.e., of a gene for a certain characteristic). Multiple allele characteristics may occur at the population level, despite the fact that humans (and other diploid creatures) can only have two alleles for any particular gene in genetics. As a result, many alleles play an essential role in fostering diversity within a species.
A particular chromosomal locus was inhabited by two unique sorts of gene alternatives in Mendelian inheritance-one dominant and one recessive. Both of these options are alleles of the same gene. However, there are times when there are more than two options (alleles) available to a population. When different versions of the same gene exist in the population, this is referred to as “multiple allelism.” Multiple alleles refer to three or more variations of the same gene.
Multiple Alleles Definition
A gene with three or more alleles. Multiple allelism is a biological term that refers to the existence of multiple alleles. The ABO blood group system in humans is one example. IA, IB, and IO are the three allelic variants of human gene I (I stands for “isohaemagglutinin”). On the cell surface of RBCs, IA and IB generate type A and type B antigens, respectively, but IO (or I) is a recessive allele that does not produce antigens. A child with both IA and IB genes, for example, will have blood type AB, while those with IOIO (or ii) will have blood type O.
Multiple Alleles Examples
Let’s look at some examples of many alleles to help us comprehend the notion better.
i. Cat Coat Color
Domestic cats have been bred for thousands of years to produce a wide range of coat colours. The gene that determines the cat’s coat colour appears to have several variations, since coat colours range from black to orange to brown to white. This indicates that the coat colour is determined by many alleles.
In the population, the coat colour gene has several alleles, and the pigment-producing protein will be determined by the inheritance and expression of these alleles. Curliness, shading, patterning, and even texture is all controlled by the same genes. Because of the numerous potential genotype combinations and expressions resulting from these genes, a broad range of breeds exist. Even when two parents share just four alleles for each gene, the variety may be amazing.
Cats’ coat colour phenotype — patches of black and orange (tortoiseshell pattern), black, grey, white, and patches of white hair — strongly shows multiple allelism, since there appear to be more than two alleles for the coat colour phenotype (piebald spotting).
The genotype of a cat may usually be detected by looking at its coat colouring and pattern. It is typically feasible to estimate the colouring possibilities of kittens if the phenotypes of the parents are known, but the computations would be difficult in most situations.
ii. Multiple Alleles in Fruit Flies
The genomic mapping of the common fruit fly, Drosophila melanogaster, was finished in 2000. Because of its rapid reproductive rate and the simplicity of keeping and analysing large numbers of flies, the fruit fly has been and continues to be a valuable laboratory animal. The DNA of a fruit fly is far smaller than that of a human, with just 165 million base pairs.
Fruit flies have just four chromosomes, whereas humans have 23. Despite this, just four chromosomes contain about 17,000 genes. Each gene controls a different element of the fly’s appearance and is susceptible to mutation and the development of new alleles.
Drosophila melanogaster’s wings are normally fairly long. Two mutations at the same gene happened in different flies, one of which resulted in vestigial (reduced) wings and the other in antlered (less developed) wings.
When a fly with vestigial wings crosses with one with antlered wings, the F1 hybrids have intermediate wing lengths, suggesting that none of the mutant genes is dominant. This hybrid, also known as the vestigial antlered compound, is made up of two mutant genes at the same locus. There is evidence of Mendelian segregation and recombination.
Other phenotypes include nicked wings, strapped wings, and no wings at all, in addition to vestigial and antlered wings. Multiple alleles in this fruit fly population are the gene variations responsible for various characteristics.
iii. Multiple Alleles in Humans
Humans and other organisms have traits with three or more different types of alleles (genes). Multiple allele inheritance refers to the inheritance of three or more different alleles in a characteristic. The ABO blood type alleles/trait in humans is an example of a multiple allele trait. There are three types of alleles: allele A (IA), allele B (IB), and allele i. (IO or i).
Protein A is generated when the allele A is present on the chromosome, and protein A is found on the membranes of the individual’s red blood cells. Protein B will be generated if allele B is present on the chromosome, and protein B will be found in the membranes of red blood cells. Finally, neither protein A nor protein B will be produced if allele I is present on the chromosome. The ABO blood group characteristic is made up of these three alleles.
Allele A and allele B have a codominant inheritance pattern (co-dominance). When neither allele is dominant over the other and a heterozygous individual displays both traits, this is known as co-dominance.
For example, if an individual has allele A on one homologous chromosome and allele B on the other, both proteins are expressed, and red blood cells have both proteins A and B on their cell membranes.
In humans, the ABO blood type genetic system is an example of many alleles blood type. Blood group A, blood group B, blood group AB, and blood group O are the four types of phenotypes. In this example, there are three alleles in the population. When the IA allele is expressed, A molecules are present on red blood cells; when the IB allele is expressed, B molecules are present on red blood cells; and when the IO allele is expressed, no such antigens are present on red blood cells. Not only are the IA and IB alleles codominant, but they are also dominant over the IO allele. Because the IO allele is recessive, it will be expressed even if IA or IB are not present.
Despite the fact that a population has three alleles, each person gets only two of them from their parents. The genotypes and phenotypes shown below are the consequence of this. Consider that there are six genotypes when three alleles are present. The dominance relationships between the three alleles dictate the number of phenotypes that can be produced.
iv. Multiple Alleles in Plants
While it is commonly assumed that the form of a potato tuber is constant, visual traits such as round or long tubers can be distinguished at the diploid level. Although this is the first report of experimental evidence for the occurrence of several allele systems in a potato tuber, this study may be compared to one in maize. The tuber shape recessive allele can be considered a qualitatively acknowledged null or near-null allele.
The difference between dominant alleles is quantifiable. The concept of a null or near-null allele for the (most) recessive allele is compatible with how quantitative effects at a multiallelic locus are characterised. When extra metric characteristics are resolved into Mendelian factors in experimental designs utilising heterozygous parents, inferences regarding the relative significance of various allele traits to numerous loci in explaining quantitative genetic allele variance can be reached.
v. Multiple Alleles in Bacteria
Bacteria carry a combination of genes, and some of them have many alleles. These wild-type alleles are typically linked to different forms of pathogenicity and can be used to categorise subspecies (e.g., housekeeping genes for Multi Locus Sequence Typing, MLST). As a result, identifying not just the target gene but also the appropriate allele is important.
Sequencing-based techniques now available are confined to mapping reads to each known allele reference, which is a time-consuming procedure. More than knowing the species responsible for the infection, it is required to understand and forecast the pathogenic impact and epidemic potential of a bacterial infection.
Bacterial virulence is typically controlled at the sub-species level by a set of genes or even alleles, necessitating the adoption of different treatment methods for infections caused by the same bacterial species.
Minor changes in a gene, for example, might result in a varied array of antibiotic resistance profiles within a single taxonomic group.
Varied alleles of the same gene may be responsible for different adhesion and invasion tactics, immunological responses to the infected organism, and toxin synthesis, among other things.
Identifying alleles of certain genes leads to a more exact categorization of bacteria, in addition to its relevance in understanding pathogenicity.
Multiple Allelism, Pleiotropy and Epistasis
Pleiotropy and epistasis are two additional popular concepts in genetics that might be confused with multiple allelism. Differences between multiple allelism (defined as the presence of many alleles) and other genetic disorders.
Pleiotropy is a situation in which more than one gene can affect the phenotype in numerous ways. Pleiotropy entails more than one gene that influences phenotype,
Whereas multiple allelism simply comprises a single gene with numerous variations (referred to as many alleles).
Albino people, for example, are more likely than pigmented ones to have crossed eyes. Albinos may also have a cross-eyed characteristic, in addition to having insufficient colour production in their skin and hair. However, not all albinos have this feature, showing that the two qualities are not connected in these situations. (BioL110F2012 – Confluence, 2012).
The Complex Expression Patterns of Multiple Alleles Another example is the colour of one’s eyes. More than one gene influences the characteristic. OCA2 and HERC2 are two of these genes.
When one gene influences the expression of another, this is known as epistasis. When genes cooperate to generate a certain characteristic, this happens.
One example is the determination of coat colour in some species (such as horses), where the effect of one gene is influenced by the influence of another gene that controls hair pigment deposition. (BioL110F2012 – Confluence, 2012). Multiple Alleles’ Complex Expression Patterns.
Multiple Alleles Citations
- The Involvement of HLA Class II Alleles in Multiple Sclerosis: A Systematic Review with Meta-analysis. Dis Markers . 2019 Nov 6;2019:1409069.
- Schizophrenia: a common disease caused by multiple rare alleles. Br J Psychiatry . 2007 Mar;190:194-9.
- HLA-DRB1 alleles and cervical cancer: A meta-analysis of 36 case-control studies. Cancer Epidemiol . 2020 Aug;67:101748.