Some of the genes also show the presence of multiple alleles. These alleles constitute a series of alleles. Hence, they are known as multiple allelic series. Multiple allelism is an existence of several known alleles of a gene. The concept gets clearer when we look into the examples.
ABO blood groups:
A scientist known as Karl Landsteiner worked on the human blood groups. He set an example of the existence of multiple alleles of a gene in human ABO blood groups. The four blood group types in an ABO system include O, A, B, and AB blood groups. The ABO blood group alleles get denoted as IA, IB, and i giving rise to their respective phenotypes. O blood group individuals are homozygotes for the recessive allele i. Since IA and IB are dominant to the recessive I, those individuals homozygous for i allele get O blood group. Hence, we write their genotype as i/i. The individuals with A blood group either have IA/IA genotype or IA/i genotype. Similarly, the individuals with B blood group either have IB/IB genotype or IB/i genotype. The heterozygous IA/IB individuals have AB blood group.
The cellular antigens get attached to the cell surface of the red blood cells. Here we talk about the antigen-antibody interactions. The antigens often get recognized as foreign molecules and trigger the immune response. The antibodies are the immune molecules that recognize and bind to the antigens for processing them further. An individual consists of a large number of cell surface antigens. Hence, the blood group matching plays a very important role, especially during the blood transfusion. Mismatched blood groups trigger serious immune response and also lead to the death of the individual. Hence, a careful blood group matching procedure gets conducted before the blood transfusions. The people with A blood group have A antigen on their cell surface. The people with B blood group have B antigen on their cell surface. The O blood group individuals have none of the above antigens on their cell surface. Only the AB blood group individuals have A and B antigens on their cell surface. Agglutination involves clumping of red blood cells due to the interactions between the antigen and the antibody. The procedure of finding a blood group of an individual is known as blood typing. The antibodies against the antigen A agglutinate only the red blood cells having A antigen on their cell surface. Similarly, the antibodies against the antigen B agglutinate or clump the cells having B antigen on their cell surface. The blood serum prepared from the individuals with A blood group consists of anti-B antibodies and no anti-A antibodies. The blood serum of the B blood group individuals consists of anti-A antibodies. The individuals with AB blood group neither have anti-A nor have anti-B antibodies in their blood serum. The blood serum of the individuals having O blood group consists of both anti-A and anti-B antibodies.
The blood of the A blood group individuals can be transfused only to the individuals having A or AB blood group. The blood of the B blood group individuals can be transfused only to the individuals having B or AB blood group. People with AB blood group produce both A and B antigens. Hence their blood gets transfused to the individuals having AB blood group. The blood of the people with O blood group gets transfused to all the individuals such as A, B, AB, and O groups. Hence, individuals with AB blood group are known as universal recipients. The individuals with O blood groups are known as universal donors.
An enzyme known as glycosyltransferase adds sugar groups to the polysaccharide which combines with the lipid molecules and forms glycolipids. This enzyme gets encoded by the ABO locus. The association of the glycolipids with the RBC membrane lead to the formation of blood group antigens. H antigen is the most common blood group antigen. In the A and B blood group individuals, the H antigen remains unconverted. In the IA/IB heterozygotes, the H antigen gets converted to A and B antigens. The homozygotes for O allele produce no enzymes for converting the H antigen glycolipid.
Image: Drosophila eye color
Drosophila eye color:
Consider a cross between a white-eyed female and a vermilion eyed-male in the Drosophila species. The F1 generation involved all the females having a red eye. Morgan proposed a concept that the white and the vermilion colors got specified by two different genes. The wild-type allele for the eye color involved brick red color. The presence of the wild-type allele in these females led to the expression of that allele leading to the brick-red eyes. Another cross involved eosin-eyed female and a white-eyed male. The F1 females had eosin eyes. The above crosses easily depict that the dominant allele is present on the X chromosome. Hence, the phenotypic trait is visible in the females. Sturtevant observed that the red color was dominant over the white and the eosin colors. Eosin was dominant over the white. Hence, a single gene consisted of both eosin and white mutant alleles. Hence, the white-eyed gene had multiple alleles. Then, both the scientists considered a cross between eosin eyed females and red-eyed males. All the F1 females were red-eyed. However, they were heterozygotes. Half of the male progeny had eosin eyes. The remaining half had white eyes. The eye color in the Drosophila depends on the amount of the pigment deposited in the eye cells of the fly. For example, the relative amount of the pigment in a wild-type red-eyed fly is 1.0000.
The gene products ranging from null mutants to wild-type alleles exist due to the level of the gene product. The overproduction of the gene product leads to the manifestation of the mutant phenotype. Hence, the number of alleles existing in a given gene decides the number of genotypes. The number of genotypes indicates n(n+1)/2, where n is the number of alleles in the gene.
A red clover plant consists of a gene known as the S gene. It shows the presence of multiple alleles involved in the prevention of the self-fertilization. The self-incompatibility alleles in the parent plant prevent pollen tube in the style. Hence, there are many such examples of multiple alleles. References:
[1] Advanced Biology, Michael Kent
[2] Schaum's Outline Of Genetics, Susan Elrod
[3] Principles Of Genetics, Tamarin
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