A. Multiple alleles and human blood groups : Generally a gene has two alternative forms / versions called alleles . They are present at the same locus in a pair of homologous chromosomes. Two alleles of a.gene can.form three genotypes in a diploid organism: Sometimes a gene may have more than two alleles. When more than two allelic forms occur at the same locus on the homologous. chromosomes of an organism, they are called mutiple alleles when more than two alleles exist in a population of a specific organism, the phenomenon is called mutiple allelism. As mentioned above .multiple alleles. cannot be observed in the genotype of a diploid individual, but can be observed in a population. The number of genotypes that can occur for multiple alleles is given by the expression n (n + 1)/2 where, n = number of alleles. A well known example of multiple allelism in man is the expression of ABO blood types by three alleles of a single gene which can produce six genotypes ABO Blood Types : The ABO blood group system was proposed by Karl Landsteiner. He was awarded the Nobel Prize in Physiology or Medicine in 1930 for his work. The phenotypes (blood types) A, B, AB and O types are characterized by the presence or absence of .antigens. on the plasma membrane of the RBCs. The A and B antigens are actually carbohydrate groups (sugar polymers) that are bound to lipid molecules (fatty acids) protruding from the membrane of the red blood cell. They are also called isoagglutinogens because they cause blood cell agglutination in the case of incompatible blood transfusions. .Blood type A. persons have antigen A on their RBCs and anti-B antibodies in the plasma. .Blood type B. persons have antigen B on their RBCs and anti - A antibodies in the plasma. Blood type. AB person have antigens .A. and .B. on the RBCs and no antibodies in the plasma. .Blood type O persons have no artigens on their RBCs and both anti - A, and anti - B antibodies are present in the plasma Bernstein discovered that these phenotypes were inherited by the interaction of three .autosomal alleles. of the gene named I, located on chromosome 9.`I^(A)`l^(B)`and i (or `l^(O)`)are the three alleles of the gene I. The antibodies anti - A and anti - B are called isoagglutinins (also called isohaemagglutinins) which are usually IgM type. The isoagglutinins of an individual cause agglutination reactions with the antigens of another individual. The alleles `l^(A)` and `l^(B)` are responsible for the production of the respective antigens .A. and B. The allele i does not produce any antigen. The alleles `I^(A)` and `I^(B)` are dominant to the allele i, but co-dominant to each other `(l^(A)=l^(B)gti)`. A child receives one of the three alleles from each parent, giving rise to six possible
genotypes and four possible blood types (phenotypes). The genotypes are `l^(A)l^(A)`,`l^(A)i`,`l^(B)l^(B)`,`l^(B)i`,`l^(A)l^(B)` and `ii//l^(o)l^(o)` The phenotypic expressions of `l^(A)l^(A)` and `l^(A)i` are .A.-type blood, the phenotypic. expressions of `l^(B)l^(B)` and `l^(B)i` are .B. - type blood, and that of `l^(A)l^(B)` is .AB. type blood. The phenotype of ii `(l^(o)l^(o))` is .O. blood
