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UPSC MainsZOOLOGY-PAPER-II201120 Marks
Q27.

How many alleles are concerned with the determination of A, B, AB and O blood groups? Tabulate the genotype and phenotype of parents to work out the genotype of offsprings resulting from mating of A, B, AB and O blood groups.

How to Approach

This question requires a detailed understanding of Mendelian genetics, specifically the ABO blood group system. The approach should involve first stating the number of alleles involved, then constructing Punnett squares for each possible mating combination (A x B, A x AB, A x O, B x AB, B x O, AB x AB, AB x O, O x O). The genotype and phenotype ratios for each cross should be clearly presented. Focus on accurate genetic notation and clear explanation of inheritance patterns.

Model Answer

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Introduction

The ABO blood group system, discovered by Karl Landsteiner in 1901, is a classic example of multiple alleles and codominance in human genetics. It’s determined by three alleles of a single gene – *I<sup>A</sup>*, *I<sup>B</sup>*, and *i*. The *I<sup>A</sup>* and *I<sup>B</sup>* alleles are codominant, meaning both are expressed when present together, resulting in the AB blood type. The *i* allele is recessive, meaning it is only expressed when two copies are present, resulting in the O blood type. Understanding the inheritance patterns of these alleles is crucial for predicting blood types in offspring and has significant implications in blood transfusions and genetic counseling. This answer will detail the allelic basis of the ABO system and demonstrate the possible genotypes and phenotypes resulting from various parental combinations.

Alleles Determining ABO Blood Groups

Three alleles are concerned with the determination of A, B, AB, and O blood groups: IA, IB, and i. IA leads to the production of A antigens, IB leads to the production of B antigens, and i leads to no antigen production.

Genotype and Phenotype Relationship

The relationship between genotype and phenotype in the ABO blood group system is as follows:

Blood Group (Phenotype) Possible Genotypes
A IAIA or IAi
B IBIB or IBi
AB IAIB
O ii

Mating Combinations and Offspring Genotypes/Phenotypes

1. A x B (IAIA x IBIB)

All offspring will be heterozygous IAIB, resulting in the AB blood type.

2. A x AB (IAIA x IAIB)

Punnett Square:

IA IB
IA IAIA IAIB
IA IAIA IAIB

Genotype Ratio: 50% IAIA, 50% IAIB. Phenotype Ratio: 50% A, 50% AB.

3. A x O (IAi x ii)

Punnett Square:

i i
IA IAi IAi
i ii ii

Genotype Ratio: 50% IAi, 50% ii. Phenotype Ratio: 50% A, 50% O.

4. B x AB (IBIB x IAIB)

Punnett Square:

IA IB
IB IAIB IBIB
IB IAIB IBIB

Genotype Ratio: 50% IAIB, 50% IBIB. Phenotype Ratio: 50% AB, 50% B.

5. B x O (IBi x ii)

Similar to A x O, resulting in 50% B and 50% O.

6. AB x AB (IAIB x IAIB)

Punnett Square:

IA IB
IA IAIA IAIB
IB IAIB IBIB

Genotype Ratio: 25% IAIA, 50% IAIB, 25% IBIB. Phenotype Ratio: 25% A, 50% AB, 25% B.

7. AB x O (IAIB x ii)

Punnett Square:

i i
IA IAi IAi
IB IBi IBi

Genotype Ratio: 50% IAi, 50% IBi. Phenotype Ratio: 50% A, 50% B.

8. O x O (ii x ii)

All offspring will be ii, resulting in the O blood type.

Conclusion

In conclusion, the ABO blood group system is governed by three alleles – *I<sup>A</sup>*, *I<sup>B</sup>*, and *i* – demonstrating the principles of multiple alleles and codominance. The Punnett square analysis for various parental combinations clearly illustrates the probabilities of different blood types in offspring. Understanding these inheritance patterns is vital in medical contexts, particularly for blood transfusions and genetic counseling, ensuring compatibility and minimizing risks. Further research into blood group genetics continues to reveal complexities and associations with various diseases.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Codominance
A genetic phenomenon where both alleles in a heterozygote are fully expressed, resulting in a phenotype that displays both traits simultaneously. In the ABO system, the *I<sup>A</sup>* and *I<sup>B</sup>* alleles are codominant, leading to the AB blood type.
Antigen
A substance that triggers an immune response in the body, specifically the production of antibodies. In the context of blood groups, A and B antigens are present on the surface of red blood cells.

Key Statistics

Approximately 45% of the Caucasian population has blood type O, making it the most common blood type in this group.

Source: American Red Cross (as of knowledge cutoff 2023)

Approximately 85% of people are Rh-positive, while 15% are Rh-negative (data varies slightly by ethnicity).

Source: National Heart, Lung, and Blood Institute (NHLBI) (as of knowledge cutoff 2023)

Examples

Hemolytic Disease of the Newborn (HDN)

Incompatibility in ABO blood groups between a mother and fetus can lead to HDN, although it's typically milder than Rh incompatibility. If a mother has type O blood and the fetus has type A or B, maternal antibodies against A or B antigens can cross the placenta and attack fetal red blood cells.

Frequently Asked Questions

▶Can a person with type O blood receive blood from someone with type A blood?

No, a person with type O blood has antibodies against both A and B antigens. Receiving type A blood would cause a potentially fatal transfusion reaction.

Topics Covered

BiologyGeneticsInheritanceBlood GroupsGenetics
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