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Q. Multiple allelism is observed in

KEAMKEAM 2014Principles of Inheritance and Variation

Solution:

Multiple allelism is the condition when more than two alternate forms of a gene are present on the same locus. They are produced due to repeated mutation of the same gene but in different directions. ABO blood groups are caused due to multiple allelism where three alleles of the gene for blood group i.e. $I^A, I^B$ and $1°$ are present.
$I^A I^A$ and $I^AI^O$ form blood group $A, I^BI^B$ and $I^BI^O$ form blood group $B$ and $I^AI^B$ form blood group $AB$ (effect of codominance). When alleles $I^OI^O$ are present the resultant blood group is $O$.