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

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation5 Marks
Question
Explain the following terms with examples: Co-dominance
✓

Answer

Co-dominance: In Mendel's experiment, trait of $F_1$ resembles either of the two parents (dominance) and was in between two parents in incomplete dominance. But in the case of co-dominance, $F_1$ individual is similar to both the parents and expresses both the alleles (dominant and recessive) independently.
→ One of the good example of co-dominance is different types of Red blood cells that determine the ABO blood grouping in human being. → There are found four types of blood groups named-A, B, AB and O in human population which were discovered by Landsteiner (1902).
→ ABO blood groups are controlled by gene 'I'. The Plasma membrane of the RBC has sugar polymer (glycoprotein) protruding from its surface and the gene 'I' governs the synthesis and type of sugar polymer.
→ Gene 'I' exists in three allelic forms $I^A$, $I^B$ and i. Alleles $I^A$ and $I^B$ produce different types of sugar whereas allele does not produce any sugar.
→ $I^A$ and $I^B$ are dominant over I. It means that when I and i are present together in an individual, only I expresses itself and when Gamma ^ 0 and i are present only expresses.
→ When $I^A$ and $I^B$ co-exist, they express their type of sugar independently, this is called co-dominance
→ Since gene 'I' has three alleles, there is the possibility of six types of combinations of genotypes for ABO blood grouping in human beings as shown in Table given below:
Allele from
Parent 1		Allele from
Parent 2		Genotype of offspringBlood types of offspring
$I ^{ A }$$I ^{ A }$$I ^{ A } I ^{ A }$A
$I ^{ A }$$I ^{ B }$$I ^{ A } I ^{ B }$AB
$I ^{ A }$i$I ^{ A } i$A
$I ^{ B }$$I ^{ A }$$I ^{ A } I ^{ B }$AB
$I ^{ B }$$I ^{ B }$$I ^{ B } I ^{ B }$B
$I ^{ B }$i$I ^{ B } i$B
iiiiO

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