Trisomy is an umbrella term for several genetic conditions in which three copies of one or more chromosomes is present in the cells of an individual rather than two copies of each chromosome. Usually human cells have 23 pairs of chromosomes for a total of 46. During the process of producing eggs and sperm (Meiosis - scroll down to "Making New Cells and Cell Types" in this link for detailed description and diagrams) the parent's chromosomes (2n) are divided in half among "daughter" cells. Usually each daughter cell gets half (n) of each chromosome pair. Trisomy occurs when one daughter cell gets an extra copy of a chromosome (n+1) due to nondisjunction (the chromosomes do not separate). The other daughter cell loses a copy (n-1), which results in monosomy. Upon mating an n+1 gamete combining with an n cell from the other parent produces an offspring with 2n+1 chromosomes (trisomy). An n-1 gamete combining with an n cell from the other parent produces an offspring 2n-1 chromosomes (monosomy) -- these zygotes are generally not viable.
In the process of researching this post I drew myself a diagram to consolidate some of the information I was gathering, to illustrate the formation of a 2n+1 zygote. I'm sharing it here with a couple of disclaimers. Please note that this is only one sequence that could lead to trisomy. Also, I am not a molecular biologist, so my diagram may be overly simplified, but hopefully is clear enough to explain the basic steps. Click on the picture to see a larger version...
Trisomy may result in conditions that are life-threatening to the offspring. In some cases the child will have severe physical or cognitive challenges. In other cases the issues can be so mild as to go undiagnosed. The latter is particularly true of some sex chromosome trisomies.The degree of severity appears to be related to the amount of genetic information that is affected. Because the chromosomes are numbered according to size, with 1 being the largest chromosome, the lower numbered chromosomes are responsible for more DNA or genetic information than the higher numbered chromosomes. Trisomy 21 (Down Syndrome) affects one of the smaller chromosomes, fewer genes are affected, and so fewer challenges affect the developing child relative to trisomy of lower numbered chromosomes.
There are several kinds of trisomy that affect the autosomes (the non-sex determining chromosomes):
Trisomy 8 - Warkany Syndrome
Trisomy 13 - Patau Syndrome
Trisomy 18 - Edwards Syndrome
Trisomy 21 - Down Syndrome
Trisomy 22 - Cat eye Syndrome
There are also several kinds of trisomy affecting the sex chromosomes:
XXX - Triple X Syndrome
XXY - Klinefelter's Syndrome
XYY - XYY Syndrome
I hope to explore each trisomy in more detail in future posts. For now I hope understanding more about how these conditions arise has been helpful. I also wanted to share a link for SOFT (Support Organization For Trisomy 18, 13, and Related Disorders), an organization that supports families affected particularly by Trisomy 18 and 13. Besides Down Syndrome, these are the most common forms of trisomy, and finding support is a critical step.