ABMGG Cytogenetics and Chromosomal Analysis 1 — Questions and Answers
Question 1: Which banding technique is most commonly used to produce a standard karyotype in clinical cytogenetics laboratories?
- Q-banding
- R-banding
- G-banding (Correct answer)
- C-banding
Correct answer: G-banding
G-banding (Giemsa banding) is the standard technique used in clinical cytogenetics to produce a karyotype with characteristic dark and light band patterns.
Question 2: According to the ISCN (International System for Human Cytogenomic Nomenclature), how is a male with trisomy 21 correctly written?
- 47,XY,+21 (Correct answer)
- 46,XY,+21
- 47,XX,+21
- 46,XY,21q+
Correct answer: 47,XY,+21
47,XY,+21 indicates a male (XY) with 47 total chromosomes due to an extra chromosome 21, which is the correct ISCN notation for Down syndrome in a male.
Question 3: Fluorescence in situ hybridization (FISH) is best suited for detecting which type of chromosomal abnormality?
- Whole genome aneuploidy
- Submicroscopic deletions at targeted loci (Correct answer)
- Balanced reciprocal translocations with no copy number change
- Polyploidy
Correct answer: Submicroscopic deletions at targeted loci
FISH uses locus-specific probes to detect submicroscopic deletions or duplications at targeted regions that are too small to be seen by conventional karyotyping.
Question 4: A chromosomal microarray (CMA) using SNP probes has an advantage over array CGH in that it can detect:
- Larger deletions
- Balanced translocations
- Uniparental disomy (UPD) (Correct answer)
- Telomeric fusions
Correct answer: Uniparental disomy (UPD)
SNP-based arrays can identify stretches of homozygosity that indicate uniparental disomy, which array CGH cannot detect because UPD does not involve copy number change.
Question 5: The most common cause of trisomy 21 (Down syndrome) is:
- Robertsonian translocation
- Mosaicism
- Meiotic nondisjunction (Correct answer)
- Uniparental disomy
Correct answer: Meiotic nondisjunction
Approximately 95% of Down syndrome cases result from meiotic nondisjunction, most often in maternal meiosis I, leading to a gamete with two copies of chromosome 21.
Question 6: Turner syndrome (45,X) most commonly results from loss of the sex chromosome derived from which parent?
- Maternal X is retained; paternal sex chromosome is lost (Correct answer)
- Paternal X is retained; maternal sex chromosome is lost
- Equal frequency from either parent
- Always due to maternal nondisjunction
Correct answer: Maternal X is retained; paternal sex chromosome is lost
Studies show that in ~75% of 45,X cases the retained X is of maternal origin, meaning the paternal sex chromosome (X or Y) was lost.
Question 7: Fragile X syndrome is caused by a CGG trinucleotide repeat expansion in the promoter region of which gene?
- DMPK
- FMR1 (Correct answer)
- HTT
- ATM
Correct answer: FMR1
Fragile X syndrome results from a >200 CGG repeat expansion (full mutation) in the 5' UTR of FMR1 on the X chromosome, leading to gene silencing via methylation.
ABMGG Cytogenetics and Chromosomal Analysis 1
Which banding technique is most commonly used to produce a standard karyotype in clinical cytogenetics laboratories?