Generally, a single gene has two alternative forms or alleles, one is found in each of the homologous chromosomes. And when a gene has more than two alleles, it is known to exhibit multiple alleles. Multiple alleles do arise by mutation of the wild type gene and occupy the same locus on the homologous chromosomes. Which means multiple alleles are a different expression of normal or wild type genes? Multiple alleles shouldn’t be confused with multiple factors or polygenic inheritance. In which 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 the occurrence of four types of blood groups as first discovered by Carl Landsteiner (1912).
Human being has 4 blood groups—A, B, AB, and O. Which are identified by a specific glycoprotein substance, called antigen on the surface of RBCs. This kind of blood group is known by a gene (l) that contains 3 alleles viz., (I, I, I). I and l genes are dominant over gene I but are co-dominant to each other and both are expressed when they present together.
So when a man with homozygous blood group A ‘l I’ marries a woman with blood group O ‘I I’or if the reverse is the case, all the children will have blood group A as shown below. And in the same vein, If a man homozygous for blood group A ‘I I’ marries a woman homozygous for blood group B ‘I I’, then all their children will have blood group AB. The same thing will be applicable to a man with homozygous for blood group A (I I ) if marries a woman homozygous for blood group B (I I ), all their children will have blood group AB.
So also, If a man with blood group AB marries a woman with the same AB blood group, their children will possess three types of blood groups. The above explanations and image show how blood group can be inherited from parents and this makes it possible for father and his children to be having the same blood group or related. The above explanation also explains different blood group and how they can be transferred from parents to children which bring about related in blood groups.