This is a very basic explanation of Trisomy 13, heavily focusing on explaining Liam’s type, which is Complete Isodicentric Trisomy 13 Mosaicism.
What is Trisomy 13?
People have 23 pairs of chromosomes. Tri-somy means three-chromosomes. Trisomy 13 means there’s an extra chromosome kickin’ it with chromosome pair 13 (Down Syndrome is Trisomy 21; extra chromosome on pair 21). Get it? Trisomy 13 is also known as Patau Syndrome (named after the dude who first ascertained T13).
It is labeled “incompatible with life,” although that is clearly bullshit.
Variations of Trisomy:
Complete + Partial (chromosome)
Complete and partial characterize the extra chromosome. Complete refers to an entire chromosome being copied. Partial refers to some of the chromosome being copied.
Full + Mosaic (cell)
Full and mosaic characterize the cell but are different in many ways. Full means every cell in the body has trisomy (an extra chromosome) regardless of whether it’s complete or partial and occurs at fertilization.
Mosaic means some cells have a well organized 23 pairs and some cells didn’t get their shit together and there’s a tag along (extra), also regardless of whether it’s complete or partial and occurs after fertilization.
Because cells die and new ones are made, the number of trisomy cells in a mosaic person can change and it’s impossible to do tests to find out an accurate number. (You’d have to test every single ever-changing cell in the body, forever…simply impossible.) However, it’s safe to assume, anywhere there is a defect or anomaly, there is a higher number of trisomy cells.
Being mosaic means a person has two different genetic makeups. Because mosaicism varies so much within one person, another person with the exact same trisomy mosaicism cannot be compared because it varies so greatly and so does its expression. In fact, a person with mosaicism can have all the features of full trisomy, none of the features, or snuggle somewhere in between; meaning mosaicism does not dictate the severity of the expression.
Isodicentric characterizes the arrangement of the extra chromosome. Isodicentric means the extra chromosome is mirror imaged and stuck to its pair. (It’s a copycat with attachment issues).
Complete Isodicentric Trisomy 13 Mosaicism
Liam has an extra, complete, #13 chromosome, mirror imaged on pair 13 in most of his cells. His tests showed over 80% of his blood has Trisomy 13 cells and he has several anomalies. See images above.
This is the absolute most basic and simple way to explain Trisomy 13, which is still a LOT of info. Genetics is crazy complex and there is SO MUCH MORE to it than what is above (but I’m trying to not overwhelm you). If you have any questions about something above, Liam, or about genetics in general, let me know and I will clarify or do my very best to point you in the right direction. Help spread awareness by sharing this page.