What are multiple alleles? Describe multiple alleles with the help of ABO blood groups in man.

Multiple Alleles:

• If a gene has more than two alleles in a population, then it is referred to as multiple alleles.
• Possible genotypes are calculated by using the expression $\frac{\mathrm{n}(\mathrm{n}+1)}{2}$.
• The ABO blood group system was proposed by Karl Landsteiner. It is an example of multiple allelism.
• These phenotypes are inherited by three 'alleles' of the autosomal gene named 'I' located on chromosome 9.
• Three alleles for the gene I is I^A, I^B and i (or I⁰). I^A and I^B are dominant over I⁰, but codominant to each other.

Blood typing: 

• Blood type is based on antigen on RBC. Blood group of a person is identified by mixing of blood sample with an antiserum.
• If clump is formed with 'Anti-A' serum-'A' blood group.
• If clump is formed with 'Anti-B' serum-'B' blood group.
• If clump is formed with both 'Anti-A and Anti-'B' antisera-AB - blood group.
• If no clump is formed with either of the antisera -'O' blood group.
• Blood Group A:  Antigen 'A' is present on surface of RBC, and anti 'B' antibodies are present in plasma.
  Blood Group B: Antigen 'B' is present on surface of RBC, and anti 'A' antibodies are present in plasma.
  Blood Group AB: Both antigen 'A' and 'B' are present on surface of RBC and antibodies are absent in plasma.
• They are universal recipients as they can receive blood from other blood types.
• Blood Group O: Antigens are absent on surface of RBC but both anti 'A 'anti 'B' anti bodies are present in plasma. They are universal donors as they can donate blood to all other types.