During PGT-A, the cells of an embryo biopsy have their DNA extracted and sequenced. The DNA sequencing platform evaluates this sequence and reports the number of copies of each chromosome. In humans, we normally have two copies of the 23 chromosomes (so the copy number is 2 for these chromosomes).
We usually talk about how this copy number increases from this baseline of 2.
If we saw 3 copies of chromosome 1, for example, you could say that it’s chromosome copy number has increased by 100%. Usually 100% isn’t used because of noise in the system, so a copy number change of more than 80% is called an aneuploid. A copy number of 20%-80% is a mosaic, and less than 20% is euploid.
The image below shows a typical next-generation sequencing profile for PGT-A. Most of the chromosomes have a copy number of 2.0, except for chromosome 7 and 12. There are two copies of the X chromosome, so this embryo is a female.
