Multiple Alleles- Definition, Principle, Examples, vs. Polygenic Traits

Multiple alleles are the presence of two or more alternative forms of the same gene present in the population because of mutation in the haploid and diploid organism, which usually have two copies of the gene i.e, homozygous alleles (copy of the same kind) or heterozygous alleles (copy of a different kind), in case of diploid and in the haploid organism only one copy of the gene is present.

Multiple Alleles
Multiple Alleles

Multiple alleles principle

  • Gene carries a unique piece of information for every individual, but due to the presence of mutation, it creates different alleles. 
  • With a population, the number of alleles has to be constant i.e 2 but sometimes due to mutation, 3 or more alleles are present in the same gene aka multiple alleles. 
  • It can only be studied in a population and its presence in a population changes the mode of inheritance. 
  • Different alleles occupy at most 2 homologous loci that are present in an individual diploid organism. But many other forms of the same gene can exist among members of a species.

Examples of Multiple Alleles

1. ABO blood grouping

  • Designed by Karl Landsteiner in 1900s
  • the  ABO blood group just like the MN blood grouping  is determined using the presence of antigen on the surface of RBC
  • this is different from the MN system, in such a way that A and B antigens on the surface of RBC are controlled by gene locus present on chromosome no. 9
  • In one gene, there 3 different forms of alleles.
  • The determination is made by mixing the blood with antiserum solutions containing a and B antibodies. The mixing and further clumping of blood cells are indicative of the presence of antigen on the surface of RBC and one of the four phenotypes present in an individual.
  • An individual can have only A antigen, only B antigen, both of them or neither of them.
ABO blood grouping Multiple Alleles
Figure: ABO blood grouping Multiple Alleles. Reference: Klug, W.S., Cummings, M.R., Spencer, C.A., et al. (2017) Essentials of Genetics. 7th Edition, Higher Education Press, Beijing, 446-464
  • I in this stands for isoagglutinogen which is generally known as an antigen. IA, IB, and i are 3 alternative forms of the same gene. 
  • We can comprehend from the diagram above that IA and IB form A and B respectively and i don’t carry any antigen on its surface, where IA and IB are dominant alleles and i is recessive.
  • Biochemically, there is an H substance that is synthesized by combining—galactose (Gal), N-acetylglucosamine (AcGluNH), and fucose, present on all 3 types of alleles.
  • Later for the IA allele, N-acetylgalactosamine (AcGalNH) is added to its H substance and for the IB allele, simple galactose residue attaches to it. Blood group type AB will have both sugar residue. While blood group O does not have any extra sugar residue attached to the H substance.
  • These are the sugars that specifically interact with the antiserum and determine the blood type. 

Bombay phenotype

  • Discovered in 1952, in a female belonging to Bombay thus named Bombay phenotype.
  • The woman was found to have homozygous for a  rare mutation in the FSUT1 gene, that is responsible for the addition of fucose sugar residue to H substance. Thus these women, do not have a proper H substance but the rest is the same.
  • Thus, normal ABO genotype can not be determined from her blood sample, as she was type B genetically but functionally she is having type O blood group.

2. Drosophila melanogaster

  • Multiple allelic inheritance influences many phenotypes in plants and animals.
  • Drosophila has multiple alleles at every locus.
  • A discovery by Thomas H. Morgan and Calvin Bridges in 1912, demonstrated the recessive mutation in the drosophila that causes it to have white eyes. 
  • There is a great variety of allelic combinations that cause the change in color of drosophila eyes from white to deep ruby in the white-satsuma allele, orange in the white-apricot allele, and buff in the white-buff allele. 
  • w, wsat, wa, and wbf are the abbreviations used to donate the allelic series.
  • The total amount of pigment decreases 20 percent in eas case from that of brick red wild type eye. 
  • Thus, it can be comprehended that multiple allelism is contributing to the pigment formation through a biochemical process that makes up the brick red eye wild-type fly. 
  • In the mutate white phenotype, flies can produce the pigment known as ommatidia but are unable to transport it to the structural unit of the eye, thus giving it a white color.
  • Also, drosophila has multiple allelism in other genes as well including its body color. 
  • Homozygous recessive flies have dark color body due to the presence of two alleles in the gene controlling body color.
Figure: Drosophila melanogaster Multiple Alleles. Reference: Klug, W.S., Cummings, M.R., Spencer, C.A., et al. (2017) Essentials of Genetics. 7th Edition, Higher Education Press, Beijing, 446-464

3. Coat color in cats

  • The genes responsible for cat hair means whether the cat will have long hair, short hair, or no hair at all, also the coat color, curliness, texture, pattern, shading, etc all have multiple copies of alleles in one gene. 
  • Due to multiple allelism, cats can have a completely different pattern of hair for all offsprings of the same pattern.
  • Using multiple allelism in cats and dogs, breeders can create many different types of cats and dogs.
  • In cats, long hairs are coded by two recessive alleles, while short hairs are coded by two dominant alleles.

4. Human leukocyte antigen (HLA) genes

  • HLA is the major histocompatibility complex present in humans responsible for the regulation of immune responses and it is the major locus on the genome that is responsible for transplant rejection.
  • There are two classes of HLA known as class 1 and class 2 which has a role in the presentation of the processed antigen.
  • It is found to be the most polymorphic region present in the human system, which means that have many alleles.
  • Multiple allelism in HLA genes produce numerous varieties of HLA molecules, each specific for the antigen and thus providing greater immunity.

Multiple Alleles vs Polygenic Traits

Multiple allelesPolygenic traits
Two or more alternate forms of the same geneControlled by a group of non-allelic genes
Present on same DNA strandFound on multiple DNA strands
Unaffected by environmental factorsAffected by environmental factors
Trait determination by co-dominance or complete dominanceTrait determination by co-dominance or incomplete dominance
There is no homologous crossing overBetween two alleles of each polygene, homologous crossing over can take place
Responsible for qualitative traitsResponsible for quantitative traits.
Multiple alleles found in the populationPolygenic traits found in individual
E g: ABO blood grouping, white gene locus in drosophila, cat coat color, HLA geneE g: kernel color in wheat

References

  1. Klug, W.S., Cummings, M.R., Spencer, C.A., et al. (2017) Essentials of Genetics. 7th Edition, Higher Education Press, Beijing, 446-464
  2. Yonsei Med J. 2007 Feb 28; 48(1): 11–23. DOI: 10.3349/ymj.2007.48.1.11
  3. https://www.embibe.com/exams/multiple-allelism/
  4. https://www.slideshare.net/nehamahor1/multiple-allelism
  5. https://medical-dictionary.thefreedictionary.com/multiple+allelism
  6. https://biologydictionary.net/multiple-alleles/
  7. https://byjus.com/neet/multiple-alleles-notes/
  8. https://researchtweet.com/multiple-alleles-definition-types-examples/
  9. https://www.biologydiscussion.com/genetics/multiple-alleles/multiple-alleles-meaning-characteristics-and-examples-genes/35452
  10. https://pediaa.com/difference-between-multiple-alleles-and-polygenic-traits/
  11. https://byjus.com/neet-questions/what-is-the-difference-between-multiple-alleles-and-polygenic/

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