Q.29. The human major histocompatibility complex (MHC) is
(A)Polygenic and monomorphic
(B)Polygenic and polymorphic
(C)Monogenic and polymorphic
(D)Monogenic and monomorphic
Answer: (A) Polygenic and monomorphic
The human major histocompatibility complex (MHC) is polygenic, meaning multiple genes encode its molecules, and polymorphic, featuring high allelic variation across individuals.
Option Analysis
Option (A): Polygenic and monomorphic
This suggests multiple genes but identical alleles population-wide. Incorrect, as MHC shows extreme polymorphism with thousands of alleles (e.g., >350 for HLA-A).
Option (B): Polygenic and polymorphic
Correct. MHC has several class I (HLA-A, B, C) and class II (HLA-DR, DQ, DP) genes per haplotype, with codominant expression, and is highly polymorphic for broad peptide presentation.
Option (C): Monogenic and polymorphic
Wrong. MHC involves multiple linked genes on chromosome 6, not a single gene.
Option (D): Monogenic and monomorphic
Incorrect on both counts; MHC is neither single-gene nor invariant.
The human major histocompatibility complex (MHC), also called HLA, is polygenic and polymorphic, crucial for antigen presentation in immunity and a frequent CSIR NET topic.
MHC Structure and Genetics
MHC spans chromosome 6 with class I (HLA-A, B, C) and class II (HLA-DR, DQ, DP) genes, making it polygenic—multiple genes per individual enable diverse peptide binding.
Polymorphism arises from thousands of alleles, ensuring population-level diversity against pathogens; no two unrelated people share identical MHC sets.
Immune Function
MHC class I presents intracellular peptides to CD8+ T cells; class II shows extracellular ones to CD4+ T cells, driving adaptive responses.
Polygeny and polymorphism broaden antigen coverage, vital for transplant matching and disease resistance.
CSIR NET Relevance
Questions test MHC as polygenic (multi-gene) and polymorphic (multi-allele), distinguishing from monogenic/monomorphic traits.
Key: Codominant inheritance doubles diversity per person.
