PracticeVideosBlogFlashcardsEspañol

Free exam prep

ASCP Molecular Biology Technologist

Practice QuestionsStudy Guide

10.3 Microarrays & Array Comparative Genomic Hybridization (aCGH)

Key Takeaways

  • Microarray platforms immobilize hundreds of thousands to millions of distinct, sequence-defined oligonucleotide or BAC probe features at known spatial coordinates on a solid glass or silicon surface for parallel nucleic acid interrogation.
  • Array Comparative Genomic Hybridization (aCGH) co-hybridizes differentially labeled patient (Cy5, red) and sex-matched normal reference (Cy3, green) genomic DNA to quantify copy number variations (CNVs), where normalized log2(Cy5/Cy3) ratios distinguish diploid normal (0.0), single-copy duplication (+0.58), and single-copy deletion (-1.0).
  • Single Nucleotide Polymorphism (SNP) microarrays measure both total hybridization intensity (Log R Ratio, LRR) and allelic fraction (B-Allele Frequency, BAF), uniquely identifying copy-neutral loss of heterozygosity (CN-LOH), uniparental disomy (UPD), and runs of homozygosity (ROH).
  • Chromosomal microarray (CMA) is recommended as the primary first-tier clinical diagnostic test for individuals with unexplained developmental delay, intellectual disability, autism spectrum disorder, and multiple congenital anomalies.
  • Microarrays cannot detect balanced chromosomal rearrangements (such as reciprocal translocations, balanced inversions, and Robertsonian translocations), low-level mosaicism (<10%–20%), or single-nucleotide point mutations, requiring complementary karyotyping, FISH, or NGS for comprehensive diagnostic assessment.
Last updated: August 2026

10.3 Microarrays & Array Comparative Genomic Hybridization (aCGH)

Quick Summary: Microarray technology immobilizes hundreds of thousands to millions of microscopic, sequence-defined nucleic acid probe features onto solid glass or silicon substrates, enabling massively parallel genomic interrogation. Array Comparative Genomic Hybridization (aCGH) employs two-color competitive hybridization (patient DNA labeled with Cy5 vs. reference DNA labeled with Cy3) to detect submicroscopic copy number variants (CNVs) across the entire genome based on normalized $\log_2(\text{Cy5/Cy3})$ ratios. Single Nucleotide Polymorphism (SNP) microarrays provide dual orthogonal metrics: Log R Ratio (LRR) for total copy number and B-Allele Frequency (BAF) for allelic distribution. This dual capability allows SNP arrays to uniquely identify copy-neutral loss of heterozygosity (CN-LOH), uniparental disomy (UPD), and consanguinity, which are completely invisible to conventional aCGH. Chromosomal microarray (CMA) serves as the primary first-tier clinical diagnostic standard for developmental delay and congenital anomalies.

1. Microarray Architecture & Physical Substrates

A microarray is a miniaturized, ordered arrangement of immobilized single-stranded DNA oligonucleotides or cloned genomic fragments attached to a planar solid support (typically a silanized microscope glass slide or silica bead chip).

+---------------------------------------------------------------------------------------------------------+
|                                 MICROARRAY PLATFORM ARCHITECTURES                                       |
+---------------------+-----------------------------------+-----------------------------------------------+
| Microarray Platform | Substrate & Probe Synthesis       | Analytical Capabilities & Resolution          |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Oligonucleotide** | High-density 60-mer synthetic     | High resolution (10–25 kb); sequence-specific;|
| **aCGH (Agilent)**  | oligonucleotides synthesized *in* | covers coding exons and intergenic backbones; |
|                     | *situ* via inkjet phosphoramidite | detects microdeletions and microduplications. |
+---------------------+-----------------------------------+-----------------------------------------------+
| **BAC / PAC**       | Bacterial Artificial Chromosomes  | Historical legacy platform; low resolution    |
| **Arrays (Legacy)** | (100–200 kb genomic inserts)      | (1–5 Mb); replaced by high-density synthetic  |
|                     | spotted mechanically onto glass   | oligonucleotide and SNP arrays.               |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Illumina Infinium**| 50-mer probes attached to 3 µm    | Dual CNV + SNP genotyping (500k to >2M SNPs); |
| **BeadArray**       | silica beads residing in etched   | single-base extension incorporates labeled    |
|                     | microwells on a silicon chip      | dideoxynucleotide (ddNTP) hapten.             |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Affymetrix / Cyto**| High-density 25-mer oligos        | Dual CNV + SNP interrogation (>2.6M markers); |
| **Scan (Thermo)**   | photolithographically synthesized | whole-genome coverage of structural gains,    |
|                     | directly on a quartz glass wafer  | losses, LOH, and mosaicism down to 5–10%.     |
+---------------------+-----------------------------------+-----------------------------------------------+

2. Array Comparative Genomic Hybridization (aCGH) Mechanics

In traditional metaphase CGH (developed by Kallioniemi in 1992), labeled patient and reference DNA were hybridized to normal metaphase chromosome spreads on a slide, limiting resolution to $5–10\text{ Mb}$. Array CGH replaced metaphase chromosomes with thousands of sequence-defined genomic probes, improving diagnostic resolution down to $10–50\text{ kb}$.

                           THE aCGH TWO-COLOR HYBRIDIZATION WORKFLOW
                           
   [ Patient Genomic DNA ] (500 ng)               [ Reference Genomic DNA ] (500 ng)
              |                                              |
              v [Random Primed Labeling (Klenow)]            v [Random Primed Labeling (Klenow)]
   [ Labeled with Cy5 (Red, 650 nm) ]             [ Labeled with Cy3 (Green, 550 nm) ]
              \                                              /
               \                                            /
                v                                          v
           [ Combine Equal Masses + Add Human Cot-1 DNA to Block Repetitive Sequences ]
                                        |
                                        v
           [ Co-Hybridize onto Oligonucleotide Glass Array Slide (40–48 hrs at 65°C) ]
                                        |
                                        v
           [ Dual-Laser Microarray Scanner (635 nm Laser for Cy5 / 532 nm Laser for Cy3) ]
                                        |
                                        v
           [ Calculate Normalized Fluorescence Ratio: log2(Cy5 Intensity / Cy3 Intensity) ]

Mathematical Derivation of $\log_2$ Ratios

The relative copy number at any specific genomic locus is calculated from the normalized fluorescence emission of the patient channel (Cy5, Red) divided by the reference channel (Cy3, Green):

Normalized Ratio=Fluorescence Intensity of Cy5 (Patient)Fluorescence Intensity of Cy3 (Reference)\text{Normalized Ratio} = \frac{\text{Fluorescence Intensity of Cy5 (Patient)}}{\text{Fluorescence Intensity of Cy3 (Reference)}}

Theoretical log⁡2 Ratio=log⁡2(Patient Copy NumberReference Copy Number)\text{Theoretical } \log_2\text{ Ratio} = \log_2\left(\frac{\text{Patient Copy Number}}{\text{Reference Copy Number}}\right)

+---------------------------------------------------------------------------------------------------------+
|                                 THEORETICAL aCGH LOG2 RATIO INTERPRETATION                              |
+---------------------+-------------------+-----------------------+---------------------------------------+
| Genomic Copy State  | Ratio (Pt : Ref)  | Theoretical Log2 Value| Mathematical Derivation               |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Diploid Normal**  | 2 : 2 (1.0)       | **0.00**              | $\log_2(2/2) = \log_2(1.0) = 0$       |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Single Deletion** | 1 : 2 (0.5)       | **-1.00**             | $\log_2(1/2) = \log_2(0.5) = -1.00$   |
| (Heterozygous Loss) |                   | (Observed: -0.6 to -1)| (Shifted toward 0 by normal stromal)  |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Homozygous Del**  | 0 : 2 (0.0)       | **$-\infty$**         | $\log_2(0/2) = -\infty$               |
| (Complete Loss)     |                   | (Observed: < -2.0)    | (Residual background fluorescence)    |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Single Duplication| 3 : 2 (1.5)       | **+0.585**            | $\log_2(3/2) = \log_2(1.5) = +0.585$  |
| (Heterozygous Gain) |                   | (Observed: +0.3 to +0.5|                                       |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Double Duplication| 4 : 2 (2.0)       | **+1.00**             | $\log_2(4/2) = \log_2(2.0) = +1.00$   |
| (Tetrasomy / Gain)**|                   | (Observed: +0.8 to +1)|                                       |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **High Amplification| > 5 : 2 (> 2.5)   | **> +1.32**           | $\log_2(>5/2) = \log_2(>2.5) > +1.32$ |
| (e.g., Oncogene amp)|                   |                       | (e.g., MYCN, ERBB2 amplification)     |
+---------------------+-------------------+-----------------------+---------------------------------------+

                              aCGH CHROMOSOME PLOT DISPLAY
                              
   log2 Ratio
     +1.0 |                                    [ DUPLICATION (+0.58) ]
          |                                      * * * * * * * * *
      0.0 | - - * - * - * - * - * - - - - - - - - - - - - - - - - - - * - * - * - -  Diploid Baseline
          |                         * * * * * *
     -1.0 |                     [ DELETION (-1.0) ]
          +--------------------------------------------------------------------> Chromosome Position
           pter                                                             qter

3. SNP Microarrays: Dual LRR & BAF Deconvolution

While aCGH measures total copy number via two-color competitive hybridization, Single Nucleotide Polymorphism (SNP) arrays measure two independent, orthogonal parameters at millions of individual single-nucleotide coordinates.

+---------------------------------------------------------------------------------------------------------+
|                                 ORTHOGONAL SNP ARRAY METRICS                                            |
+---------------------+-----------------------------------+-----------------------------------------------+
| Metric Name         | Mathematical Definition           | Diagnostic Information Provided               |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Log R Ratio**     | $\text{LRR} = \log_2(R_{\text{obs}}| **Copy Number / Total Dosage:**               |
| **(LRR)**           | / R_{\text{exp}})$ where $R$ is   | Reflects total hybridization signal intensity |
|                     | total probe fluorescence $(A + B)$| ($\text{LRR} \approx 0$ normal; $>0$ gain; $<0$ loss) |
+---------------------+-----------------------------------+-----------------------------------------------+
| **B-Allele**        | $\text{BAF} = \frac{I_B}{I_A + I_B}$| **Allelic Imbalance & Genotype:**             |
| **Frequency (BAF)** | Normalized proportion of signal   | Reflects the ratio of B-allele to total A+B;  |
|                     | contributed by the 'B' allele     | reveals discrete genotype tracks ($0, 0.5, 1$) |
+---------------------+-----------------------------------+-----------------------------------------------+

Deconvoluting Genotypes on LRR and BAF Plots

+---------------------------------------------------------------------------------------------------------+
|                             SNP ARRAY GENOTYPIC PROFILES ACROSS LRR & BAF                               |
+---------------------+-------------------+-----------------------+---------------------------------------+
| Biological State    | Log R Ratio (LRR) | BAF Track Pattern     | Observed Genotypes                    |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Diploid Normal**  | $\text{LRR} \approx 0$| **3 Tracks:**         | $AA = 0.0$                            |
| (Disomy, 2 copies)  |                   | $0.0$, $0.5$, $1.0$   | $AB = 0.5$                            |
|                     |                   |                       | $BB = 1.0$                            |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Hemizygous**      | $\text{LRR} < 0$  | **2 Tracks:**         | $A = 0.0$                             |
| **Deletion (Loss)** | ($\approx -0.55$) | $0.0$, $1.0$          | $B = 1.0$                             |
| (1 copy remaining)  |                   | (Loss of $0.5$ track) | (Heterozygous AB state destroyed)     |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Homozygous**      | $\text{LRR} \ll 0$| **No discrete tracks**| No DNA remaining                      |
| **Deletion (Null)** | ($<-2.0$)         | (Random noise)        | (Complete signal loss)                |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Duplication**     | $\text{LRR} > 0$  | **4 Tracks:**         | $AAA = 0.0$                           |
| **(Trisomy, Gain)** | ($\approx +0.35$) | $0.0, 0.33, 0.67, 1.0$| $AAB = 0.33$                          |
| (3 copies)          |                   | (Split middle track)  | $ABB = 0.67$, $BBB = 1.0$             |
+---------------------+-------------------+-----------------------+---------------------------------------+
| **Copy-Neutral LOH**| **LRR = 0.0**     | **2 Tracks:**         | $AA = 0.0$                            |
| **(CN-LOH / aUPD)** | **(Normal Copy!)**| $0.0$, $1.0$          | $BB = 1.0$                            |
| (2 identical copies)|                   | (Absence of $AB=0.5$) | (Megabase loss of heterozygosity!)    |
+---------------------+-------------------+-----------------------+---------------------------------------+

                        SNP ARRAY COPY-NEUTRAL LOH (CN-LOH) SIGNATURE
                        
      Log R Ratio (LRR)
        +1.0 |
         0.0 | - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  Normal Baseline!
        -1.0 |
             +--------------------------------------------------------------------->
             
      B-Allele Frequency (BAF)
         1.0 | * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *  (BB Genotype)
             |
         0.5 | * * * * * * * * * *           [ NO AB TRACK! ]          * * * * * * *  (Heterozygous AB)
             |
         0.0 | * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *  (AA Genotype)
             +--------------------+-----------------------------------+------------->
              Normal Diploid Flank        REGION OF CN-LOH / aUPD      Normal Flank

Why aCGH Cannot Detect Copy-Neutral LOH (CN-LOH)

Copy-Neutral Loss of Heterozygosity (CN-LOH)—also known as acquired uniparental disomy (aUPD)—occurs when a cell loses one parental allele and duplicates the remaining homologous allele via mitotic recombination or nondisjunction rescue.

  • Total copy number remains exactly 2.
  • In aCGH, because total patient DNA hybridizes at a $2:2$ ratio with normal diploid reference DNA, the calculated $\log_2(\text{Ratio})$ is exactly $0.00$. aCGH is completely blind to CN-LOH.
  • In SNP arrays, while the LRR is $0.0$, the BAF track instantly reveals the complete loss of all heterozygous $AB$ SNPs ($0.5$ frequency) over contiguous megabase segments, exposing the abnormal isodisomy.
  • Clinical Significance: Identifies imprinting disorders (e.g., Uniparental Disomy in Prader-Willi / Angelman syndromes on chromosome 15, Beckwith-Wiedemann syndrome on 11p15.5) and oncogenic homozygosity of tumor suppressor mutations (e.g., TP53, FLT3, JAK2 V617F in AML/MDS).

4. Clinical Indications & Professional ACMG Guidelines

The American College of Medical Genetics and Genomics (ACMG) and the American Academy of Pediatrics (AAP) endorse Chromosomal Microarray (CMA) as the first-tier diagnostic test for:

  1. Unexplained Developmental Delay (DD) and Intellectual Disability (ID)
  2. Autism Spectrum Disorders (ASD)
  3. Multiple Congenital Anomalies (MCA) not fitting a well-defined monogenic syndrome
+---------------------------------------------------------------------------------------------------------+
|                                 MICROARRAY VS. G-BAND KARYOTYPING                                       |
+---------------------+-----------------------------------+-----------------------------------------------+
| Parameter           | G-Banded Karyotyping              | Chromosomal Microarray (CMA)                  |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Resolution**      | **5 to 10 Mb** (low resolution)   | **10 to 50 kb** (>100-fold higher resolution!)|
+---------------------+-----------------------------------+-----------------------------------------------+
| **Cell Culture**    | Mandatory; requires living cells  | None; uses extracted DNA from uncultured blood|
|                     | arrested in metaphase             | saliva, skin, or fresh/frozen tissue          |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Diagnostic Yield**| 3% to 5% in DD/ID/ASD cohorts     | **15% to 20%** in DD/ID/ASD cohorts           |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Submicroscopic**  | Invisible (below optical limit)   | Easily detects DiGeorge (22q11.2), Williams   |
| **Microdeletions**  |                                   | (7q11.23), Prader-Willi/Angelman (15q11-q13)  |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Balanced Trans-** | **Detected easily** (visualizes   | **Completely blind**; no copy number change!  |
| **locations**       | chromosome structural movements)  |                                               |
+---------------------+-----------------------------------+-----------------------------------------------+

5. Inherent Limitations of Microarray Platforms

Understanding what microarrays cannot detect is a primary focus of clinical competency and the ASCP MB examination.

+---------------------------------------------------------------------------------------------------------+
|                                 WHAT MICROARRAYS CANNOT DETECT                                          |
+-----------------------------+---------------------------------------------------------------------------+
| Structural Variant Type     | Biochemical / Biophysical Reason for Diagnostic Invisibility              |
+-----------------------------+---------------------------------------------------------------------------+
| **Balanced Reciprocal**     | Involves mutual exchange of chromosomal segments without any gain or loss |
| **Translocations**          | of genomic material ($\log_2\text{ Ratio} = 0.00$; $\text{LRR} = 0.00$).  |
+-----------------------------+---------------------------------------------------------------------------+
| **Balanced Inversions**     | DNA segment is inverted 180° in orientation; total copy number is normal. |
+-----------------------------+---------------------------------------------------------------------------+
| **Robertsonian Trans-**     | Fusion of acrocentric p-arms (containing non-essential rDNA); balanced.   |
| **locations (Balanced)**    |                                                                           |
+-----------------------------+---------------------------------------------------------------------------+
| **Gene Relocations / Tandem**| Arrays identify gain of a segment but cannot determine its spatial        |
| **vs. Insertional Position**| location (e.g., tandem duplication on chr 1 vs inserted into chr 12).     |
+-----------------------------+---------------------------------------------------------------------------+
| **Low-Level Mosaicism**     | Minor clonal sub-populations (<10% to 20% mutant cells) are submerged in  |
|                             | background diploid signal noise.                                          |
+-----------------------------+---------------------------------------------------------------------------+
| **Single-Nucleotide SNVs**  | Point mutations (e.g., *CFTR* F508del, *BRAF* V600E) and tiny indels      |
| **and Small Indels (<10 kb)**| are not detected unless directly interrogated by specific single-base SNPs|
+-----------------------------+---------------------------------------------------------------------------+
| **Trinucleotide Expansions**| Repetitive expansions (*FMR1*, *HTT*) do not alter copy number probes.    |
+-----------------------------+---------------------------------------------------------------------------+

6. Microarray Quality Control Metrics & Clinical Troubleshooting

To guarantee diagnostic validity under CAP/CLIA regulations, laboratories analyze mathematical quality metrics before releasing clinical array data.

+---------------------------------------------------------------------------------------------------------+
|                                 ARRAY QUALITY CONTROL BENCHMARKS                                        |
+---------------------+-----------------------------------+-----------------------------------------------+
| QC Metric           | Clinical Diagnostic Benchmark     | Significance & Failure Root Cause             |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Derivative Log**  | **DLRSpread < 0.20 to 0.30**      | Measures probe-to-probe noise across adjacent |
| **Ratio Spread**    | (Agilent aCGH platforms)          | chromosomal features. Elevated by degraded    |
| **(DLRSpread)**     |                                   | DNA, poor labeling, or uneven hybridization.  |
+---------------------+-----------------------------------+-----------------------------------------------+
| **SNP Call Rate**   | **> 98.5% to 99.0%**              | Percentage of SNP features assigned a high-   |
|                     | (Illumina / Affymetrix arrays)    | confidence discrete genotype. Reduced by poor |
|                     |                                   | DNA quality, low input mass, or inhibitors.   |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Specific**        | **> 8.0 to 10.0 pmol dye / µg**   | Measures fluorophore incorporation efficiency |
| **Activity**        | for Cy5 and Cy3                   | during random priming with Klenow fragment.   |
+---------------------+-----------------------------------+-----------------------------------------------+
| **Signal-to-Noise** | **SNR > 10.0**                    | Ratio of mean feature foreground intensity to |
| **Ratio (SNR)**     | for all scanning channels         | surrounding background noise.                 |
+---------------------+-----------------------------------+-----------------------------------------------+

Systematic Array Troubleshooting Guide

Visual / Analytical AnomalyRoot CauseCorrective Action
Elevated DLRSpread ($>0.35$) / High Wave NoiseDegraded input genomic DNA; salt/solvent contamination; bubbles during hybridizationVerify DNA integrity via Agilent Bioanalyzer / TapeStation (DIN $>7.0$ required); re-purify DNA with silica columns; roll out bubbles during array chamber assembly.
Wavy Baseline Across Chromosomes (GC Wave)Systematic labeling bias or hybridization artifact correlating with local genomic GC contentApply bioinformatic GC-wave correction algorithms (LOESS / spline smoothing); ensure equal DNA input quantities for Cy5 and Cy3.
Severe Dye Bias / Unbalanced ChannelsUnequal labeling efficiency between Cy5 and Cy3; Cy5 ozone degradationMeasure specific activity spectrophotometrically prior to pooling; protect Cy5 from ambient atmospheric ozone ($<5\text{ ppb}$) or use ozone-free enclosures.
Localized Donut / Ring Artifacts on ScanAir bubble trapped under array gasket during 65°C incubation; dust on glassEnsure hybridization rotator oven operates at correct RPM; handle slides in certified cleanroom / HEPA-filtered laminar flow hoods.
Low SNP Call Rate ($<95%$)Inadequate genomic DNA input mass ($<200\text{ ng}$); incomplete single-base extension enzymologyAccurately quantify input DNA using fluorometry (Qubit dsDNA BR); verify reagent expiration dates on enzyme extension master mix.
Loading diagram...
aCGH Two-Color Workflow and SNP Array Dual LRR / BAF Metric Interpretation
Test Your Knowledge

A clinical cytogenetics laboratory evaluates an infant with suspected Beckwith-Wiedemann syndrome using a genome-wide SNP microarray. Analysis of chromosome 11p15.5 demonstrates a normal Log R Ratio (LRR ~ 0.0) across the entire 15-megabase terminal region, but the B-Allele Frequency (BAF) plot reveals a complete absence of heterozygous markers (no 0.5 BAF track), displaying only homozygous 0.0 and 1.0 tracks. What genetic mechanism explains this finding, and why would standard two-color aCGH fail to detect it?

A
B
C
D
Test Your Knowledge

A pediatric patient with severe developmental delay and dysmorphic facial features is evaluated using a high-density oligonucleotide chromosomal microarray. The microarray report is returned as completely normal with no copy number variations detected. However, subsequent classical G-banded karyotyping reveals a de novo balanced reciprocal translocation between chromosomes 4 and 8: t(4;8)(q21;q24). Why was this structural chromosomal abnormality undetectable by chromosomal microarray?

A
B
C
D
Test Your Knowledge

A molecular technologist is analyzing two-color array Comparative Genomic Hybridization (aCGH) data for a patient suspected of having a contiguous gene syndrome. Across a 3-megabase region on chromosome 22q11.2, the software reports a consistent, normalized log2(Cy5/Cy3) ratio of +0.585. What does this quantitative value indicate regarding the patient's copy number status at this locus?

A
B
C
D
Up Next

11.1 Quantitative Viral Load Monitoring (HIV-1, HCV, HBV, CMV)

Molecular Infectious Disease Diagnostics