Multiple Alleles

The genetic basis of the ABO blood group system is an example of multiple alleles. There are three alleles, A, B, and O, at the ABO locus on chromosome 9. The expression of the O allele is recessive to that of A and B, which are said to be co-dominant. Thus, the genotypes AO and AA express blood type A, BO and BB express blood type B, AB expresses blood type AB, and OO expresses blood type O. In the past, ABO blood group typing was used extensively both in forensic cases as well as for paternity testing. More recently, DNA testing, which is much more informative, has superseded these tests.

The ABO blood group substances are glycoproteins, the basic molecule of which is known as the H substance. This H substance is present in unmodified form in individuals with blood type O. Adding extra sugar molecules to the H substance produces the A and B substances. The frequency of the ABO blood types varies widely across the globe. For example, blood group B has a frequency of 25 percent in Asians, 17 percent in Africans, but only 8 percent in Caucasians. The frequency of blood group O in Europe increases as one travels from southern to northern countries.

Alleles at a locus independent of the ABO blood group locus, known as the secretor locus, determine an individual's ability to secrete the ABO blood group substances in saliva and other body fluids. There are two genes, Se and se, where Se is dominant to se. In other words, an individual with at antibody immune-system protein that binds to foreign molecules alleles particular forms of genes locus site on a chromosome (plural, loci)

dominant controlling the phenotype when one allele is present

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