# Multiple Alleles

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If more than two alternate forms of a gene are present on the same locus, those alleles are called multiple alleles.

There is absence of crossing over in multiple alleles and the mode of inheritance is called multiple allelism.

Well known example is human ABO blood group system, which also show co-dominance.

Landsteiner discovered the three blood groups in man (A, B and O). Alfred von deCastello & Adriano Sturli discovered the fourth blood group - AB

There can be more than two alleles of the same gene. Drosophila has 15 alleles for eye color.

Multiple alleles express different alternatives of the same character. Different alleles may show complete dominance, incomplete dominance or co-dominance among themselves.

They, however, follow Mendelian pattern of inheritance.

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## Multiple Alleles Example in ABO Blood Group System

ABO Blood group system is an excellent examples of all these:
 Multiple Alleles IA, IB and i Complete Dominance Between IA and i Complete Dominance Between IB and i Co-Dominance Between IA and IB
There are three possibles alleles of the blood group gene: IA, IB and i. However, in an individual, there can be only two alleles; one contributed by father and another by mother. Together there can be six genotypes and four distinct genotypes.
 Genotype Phenotype (Resultant Blood Group) IA IA A IA i A IA IB AB IB  IB B IB i B i i O

## Multiple Choice Questions on Multiple Alleles and ABO Blood Group System

ABO Blood group system is an example of
(1) Complete dominance and multiple allele
(2) Complete dominance and Lethal genes
(3) Co-dominance and multiple allele
(4) Incomplete dominance and multiple allele



Father is of blood group O and mother of blood group O. The percentage of children with blood group O would be
(1) 12.5%
(2) 25.0%
(3) 50.0%
(4) 100%

Explanation: The alleles from father would be {i} only. And also allele from mother side would be {i} only. So "all" children would be of genotype {i i}, i.e blood group 'O'. So, answer is (4) 100%


Father is of blood group A and mother of blood group AB. The percentage of children with blood group A would be
(1) 12.5%
(2) 25.0%
(3) 50.0%
(4) 100%

Explanation: The alleles from father would be either (IA) or {i} only. And allele from mother side would be {IA} or (IB) only. So children would be of genotypes
{IA IA}, or
{IA IB}, or
{i IA}, or
{i IB}.
So, 50% children would be of blood group 'A'.


An example of Intragenic gene interaction is:
(1) Supplementary genes
(2) Epistasis
(3) Multiple alleles
(4) Polymeric Genes