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

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation3 Marks
Question
Explain the following terms with examples: Incomplete 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, 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 IA, IB and i. Alleles IA and IBproduce different types of sugar whereas allele does not produce any sugar.
→ IA and IB 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 and 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 1Allele from Parent 2Genotype of offspringBlood types of offspring
IAIAIAIAA
IAIBIAAB
IAi$I ^{ A } i$A
IBIAIAIBAB
IBIBIBIBB
IBi$I ^{ B } i$B
iiiiO
Incomplete dominance: Mendel proposed the law of dominance by explaining the concept of dominant and recessive traits based on his experiments but when experiments on pea were repeated using other traits in other plants, it was observed that the trait expressed in F1 generation did not resemble with any of the two parent but had expression in between the two and therefore it shows deviation from law of dominance. It is exception to law of dominance and it is called Incomplete dominance. 
When a cross was made between true breeding red flowered (RR) and true breeding white flowered (rr) plants of snapdragon (Antirrhinum sp.) the F1 produced Pink flowered (Rr) plants and when the F, was self-pollinated, F2 resulted in 1 Red (RR): 2 Pink (Rr): 1 white (rr); a genotypic ratio similar to typical monohybrid cross with the difference that phenotypic ratio is also 1:2:1 instead of 3:1. 
In this experiment, unit factor R was not completely dominant over r and due to this reason, plant with Rr genotype produced the phenotype pink.

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