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Principles of Inheritance and Variations — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variations4 Marks
Question
Write about multiple allelism with suitable example.###Describe multiple albinism showing the inheritance pattern of blood groups in human beings.
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Answer

Multiple Allele : There are certain genes which have more than two forms. For example, in man there are four blood groups A, B, AB and O. Letters A and B symbolise two different types of glycoprotein which are found in blood groups A and B respectively. In AB type, both the types of glycoprotein are found. In blood group O, neither of the two types of glycoproteins are found.
Three different alleles $I ^{ A }, I ^{ B }$ and i of a gene determine the phenotypes of four blood groups. In the presence of $I ^{ A }$ glycoprotein A is formed while in the presence of allele $I ^{ B }$ glycoprotein B is formed. Any person may possess only two of these alleles one from each parent. But since there are three types of alleles hence there can be 6 types of genotypes as shown below :
Image
Thus, six types of genotypes are :
(1) $I ^{ A } I ^{ A }$ (2) $I ^{ A } I ^{ B }$, (3) $I ^{ A } i$, (4) $I ^{ B } I ^{ B }$, (5) $I ^{ B } i$, (6) i i .
The phenotypes of these genotypes are of the following types :
Image
ABO system is, therefore, an example of multiple allelism.
Besides, $I ^{ A }$ and $I ^{ B }$ both are dominant over i. Please note that in this example due to the presence of $I ^{ A } I ^{ B }$both the glycoproteins are formed. When both the alleles are dominant then it is called codominance. They are expressed in the phenotype as AB. In this way we find that human blood groups system ABO is example of multiple alleles as well as codominance. But it is not essential that all the multiple alleles will exhibit codominance also.
Inheritance Pattern of Blood Groups in Human Beings
From the blood group of a child, the parents blood groups can be rightly predicted or vice versa. This is depicted in the following tables :
Image
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