CHT HLA Genetics & Molecular Biology 3

Question 1

What phenomenon occurs when a heterozygous individual at HLA-DQA1 and HLA-DQB1 can form up to four different DQ heterodimers?

Accepted Answer

Trans (in-trans) DQ pairing

Answer

In heterozygous individuals, DQA1 and DQB1 chains from opposite chromosomes can pair in trans, generating up to four different DQ heterodimers.

Question 2

The HLA system is the most polymorphic genetic system in the human genome. Which locus currently has the greatest number of recognized alleles?

Accepted Answer

HLA-B

Answer

HLA-B has the greatest number of recognized alleles of any HLA locus, with thousands catalogued in the IMGT/HLA database.

Question 3

What is a 'null' allele in HLA nomenclature?

Accepted Answer

An allele with a stop codon or splice mutation resulting in no cell-surface expression

Answer

Null alleles (designated 'N' in the allele name) harbor mutations such as premature stop codons that prevent surface expression of the HLA protein.

Question 4

Which feature distinguishes HLA class II molecules from class I in terms of their antigen presentation?

Accepted Answer

Class II presents exogenous peptides to CD4+ T cells; class I presents endogenous peptides to CD8+ T cells

Answer

HLA class II molecules present peptides derived from extracellular (exogenous) antigens via the endosomal pathway to CD4+ helper T cells.

Question 5

In NGS-based HLA typing, what is a key advantage over Sanger sequencing?

Accepted Answer

Resolves phase ambiguities and provides allele-level phased genotypes

Answer

NGS typing can resolve cis/trans (phase) ambiguities that Sanger sequencing cannot, producing fully phased allele-level results.

CHT - Certified Histocompatibility Technologist Practice Test