Examples of Multiple Allelism

Gene may has more than 2 alleles  😕....
Is it !!!!!!!!

As we have studied that some genes may exist in three or more allelic forms which we called as multiple alleles. ⇦

Here are some examples of multiple allelism 

1. ABO Blood group in Humans.
2. Skin Colour in Humans.
3. Eye colour in Drosophila.
4. Coat colour in Rabbit.

1. ABO Blood group in Humans.

• A classical example of multiple alleles is found in the ABO blood group system of humans. 
• The RBC's of people with type A blood group contain A antigens, those with blood group B contain B antigens, those with type AB blood group have both A and B antigens and those with O blood group don't have any A or B antigens.
• The IA and IB  alleles are responsible for production of A and B antigens.
• The alleles IA for the A antigen is codominant with the allele IB  for the B antigens.
• Both IA and IB are completely dominant to allele i, which fails to specify A or B antigens. 
• The plasma of type A blood contain anti-B antibodies, type B blood contains anti-A antibodies, no anti-A and anti-B antibodies are present in type AB blood, and both anti-A and anti-B antibodies are present in type O blood. 

Table 1. Human ABO blood group

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2. Eye colour in Drosophila

• Drosophila eye colour is red normally which is coded by a wild type allele. 
• 1st mutation observed in red eye gene to become white eyed Drosophila (mutant type)
• After while different shades between red and white were observed. About a dozens of allelomorphs some are showing below in table 2. 

• red eye is dominant over the other. 
• When any of the two recessive alleles crossed the intermediate eye colour was observed. 

now the question arise how is this white eyed phenotype inherited?

lets have a look ..........

We have a flies with white eyes and that white eyed flies come after the mutation in a gene which is responsible for red eye colour of flies..

First... the parental cross 

• From all the red eyed flies in F1 generation, we revealed that white eyed phenotype is recessive.
• We can say that in white eyed flies there may be a mutation in red colour dominant allele which may result from failure to produce red colour eye. 

now...
take one of F1 progeny & lets do test cross.......

4 different phenotypes

normally a single gene would give us 2 classes of phenotypes 

but here ...

after a test cross we got 4 different phenotypes roughly equal in number 
and from these 4 different phenotypes we observed  that there must at least 2 genes involved here . 

lets understand in more simple way 

• assume 4 allele control eye colour of Drosophila i.e.

               E1 = Wild

            E2 = Apricot

            E3 = Honey

            E4 = White

 ----E1 > E2 > E3 > E4----

  Phenotypes                                 Genotypes

• Wildtype                           E1E1, E1E2, E1E3, E1E4

• Honey                                    E2E2, E2E3, E2E4

• Apricot                                         E3E3, E3E4        

• White                                                 E4E4

Now predict the phenotypic outcome if honey coloured heterozygous fly is crossed with white eyed eyed.

What are the possible phenotypes of the parents in a cross which produces the following offsprings ???

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