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The below mentioned article provides an overview on the inheritance of blood groups in man.
Generally a gene has two alternative forms or alleles, one in each of the homologous chromosomes. If a gene has more than two alleles, it is known to exhibit multiple alleles.
Multiple alleles arise by mutation of the wild type gene and occupy the same locus on the homologous chromosomes. It means multiple alleles are different expression of normal or wild type genes.
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Multiple alleles should not be confused with ‘multiple factors’ or polygenic inheritance. While the former refers to alleles of a gene at a given locus, multiple factors refer to genes at different loci on the homologous chromosomes pair. The well known example of multiple alleles in humans is occurrence of four types of blood groups discovered first by Carl Landsteiner (1912).
Human population has 4 blood groups—A, B, AB and O. These are identified by a specific glycoprotein substance, called antigen on the surface of RBCs. The blood group is determined by a gene “I” which has three forms or alleles viz., IA, IB, IO. The genes IA and lB are dominant to gene IO but are co-dominant to each other and both are expressed when present together.
(i) If a person homozygous for blood group A (IA IA) marries a woman with blood group O (IOIO) or vice versa, all the children will have blood group A as shown below:
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(ii) If a person homozygous for blood group B (IB IB) marries a woman with blood group (IOIO) or vice versa, all the children will have blood group B as shown below:
(iii) If a man homozygous for blood group A (IA IA) marries a woman homozygous for blood group B (IBIB), then all their children will have blood group AB.
(iv) If a man with blood group AB marries a woman with same blood group AB, their children will have 3 types of blood groups as follows:
