The Question is from neet-zoology body-fluids-and-circulation , and it was created by the author A SUBRAMANYAM

Question

Which rare blood group phenotype arises from the absence of H substance, preventing the expression of A and B antigens even with the presence of $I^A$ or $I^B$ alleles?

A SUBRAMANYAM · Accepted Answer

## CORE INFORMATION
✓ **Answer**: B
## CONCEPT FRAMEWORK
 **Primary Concept**: Blood group genetics, specifically the ABO blood group system and the H antigen.
 **Related Topics**:  ABO blood group antigens, glycosyltransferases, inheritance of blood groups.
 **Prerequisites**: Understanding of Mendelian inheritance, basic genetics of blood groups.
## SOLUTION PATHWAY
**Given Information**:
  The question describes a rare blood group phenotype where A and B antigens are absent despite the presence of $I^A$ or $I^B$ alleles.
  This absence is caused by a lack of H substance.
**Method & Steps**:
 1. Understand the role of H substance: The H antigen is a precursor substance necessary for the expression of A and B antigens.  The A and B antigens are formed by modifications of the H antigen.  Individuals with the $h$ allele lack the enzyme necessary to produce the H substance.
  Reasoning:  The H antigen is the foundation upon which A and B antigens are built. Without it, the A and B alleles are essentially inactive.
  Formula/Rule:  None directly applicable, this is a qualitative understanding.
 2. Analyze the options:  The Bombay phenotype (Oh) results from the homozygous recessive genotype $hh$, leading to the absence of H substance. This prevents the expression of A and B antigens, regardless of the presence of $I^A$ or $I^B$ alleles.
  Working: The other options are incorrect because they don't directly relate to the absence of the H substance.
  Key Point: The Bombay phenotype is characterized by the lack of A, B, and H antigens on the red blood cells.
 3. Final step:
  Result: The correct answer is the Bombay phenotype.
  Verification: The description in the question matches the characteristics of the Bombay phenotype.
 ## OPTION ANALYSIS
A :x: Para-Bombay phenotype is a weaker expression of the H antigen, not a complete absence.
B ✓: Bombay phenotype correctly describes the absence of H substance, leading to the non-expression of A and B antigens, even when the $I^A$ or $I^B$ alleles are present.
C :x: Rh null phenotype is related to the absence of Rh antigens, not the ABO system.
D :x: Secretor phenotype refers to the secretion of ABO antigens in body fluids, unrelated to the absence of the H substance.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing Bombay phenotype with other rare blood groups:  Understanding the specific genetic basis of the Bombay phenotype is crucial to avoid this.
  2. Focusing solely on A and B antigens:  The key is recognizing the role of the H substance as a prerequisite for A and B antigen expression.
 **Quick Tips**:
  1.Remember the hierarchical relationship: H substance → A/B antigens.
  2.Associate Bombay phenotype with the lack of H antigen.

A SUBRAMANYAM · Answer

Para-Bombay phenotype

A SUBRAMANYAM · Answer

The correct Option is "Bombay phenotype"

A SUBRAMANYAM · Answer

Rh null phenotype

A SUBRAMANYAM · Answer

Secretor phenotype

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