No specific reason for this, just popped into my head this morning.

I’ve read somewhere (probably Terry Pratchett somewhere) that there are 12 viable human karyotypes. I assume this refers solely to the sex chromosomes as well. Looking at Wikipedia I can see some of them. Now, by viable I mean just capable of living, not necessarily of breeding.

OK, we all know XX and XY, I assume most have heard of XXY and XYY, XO is new to me and apparently YO never happens. I can imagine that XXX happens and that YY and YYY do not for the same reason YO doesn’t.

So that’s six possibilities. What are the other 6? Any ideas?