Karyotype versus chromosomal microarray
A karyotype (a stained picture of all of a person's chromosomes) sees large changes, while a chromosomal microarray (a count of DNA gains and losses) sees much finer ones.
Remember
Match the test to the size and type of change you expect: a karyotype for large structural changes, a microarray for gains and losses too small for a karyotype to see.
Memory aid
karyotype sees chapters of the genome, microarray sees paragraphs, and sequencing reads individual letters
Choose the test by the size of the change you are looking for.
Every genetic test has blind spots, so the method must match the size and type of change you expect. A wider test is not automatically better, because it adds interpretation work and the chance of uncertain or unrelated findings.
A karyotype is a picture of all of a person's chromosomes, laid out and stained so that the banding pattern can be inspected. It is a low-resolution test, able to detect changes larger than about 5 megabases (5 million base pairs). Because it shows whole chromosomes, it can reveal aneuploidy (a missing or extra whole chromosome, such as trisomy 21) and large balanced rearrangements. A reciprocal translocation is one example: two chromosomes swap pieces without any net gain or loss of material.
A chromosomal microarray is a genome-wide test for copy number variants, which are gains (duplications) and losses (deletions) of DNA. It measures the amount of DNA across the genome: where there is a deletion the signal falls, and where there is a duplication it rises. It can detect imbalances of about 50 to 200 kilobases, which are stretches of DNA far smaller than the 5 megabases (5 million base pairs) that a karyotype can see. It is not a sequencing test (sequencing reads the order of the DNA letters), and it does not ordinarily detect balanced rearrangements, because no DNA amount has changed.
That is why a karyotype is the test when a balanced translocation is suspected in a parent of a couple with recurrent pregnancy loss. The carrier is usually healthy, but eggs or sperm can end up with an unbalanced chromosome set. When a deletion too small for a karyotype to see is suspected but its exact location is not known, a microarray scans the whole genome for gains and losses.
Key facts
- Karyotype
- a picture of all of a person's chromosomes, stained so that the banding pattern can be inspected.
- Karyotype resolution
- low, detecting changes larger than about 5 megabases.
- Karyotype and rearrangements
- large balanced rearrangements such as a reciprocal translocation are visible because the whole chromosome is seen.
- Chromosomal microarray
- a genome-wide test for copy number variants, which are gains and losses of DNA.
- Microarray resolution
- about 50 to 200 kilobases, far finer than the 5 megabases that a karyotype can see.
- Microarray limits
- not a sequencing test, and it does not ordinarily detect balanced rearrangements.
- Best test for a suspected balanced translocation
- a karyotype shows the arrangement, whereas microarray only measures the amount and can miss it.
Exam traps
- A balanced exchange involves no net gain or loss of DNA, so a dosage-based microarray may look normal. Karyotyping shows whole chromosomes and their arrangement.
- Chromosome banding has lower resolution than microarray, so conventional karyotyping can miss a small deletion that a microarray would pick up.
- Karyotyping alone does not cover single-gene changes or many smaller copy number changes, because it shows large chromosome changes.
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