...I got the X. I got the Y. What does the O stand for?
Turners Syndrome
https://www.mayoclinic.org/diseases-conditions/turner-syndrome/symptoms-causes/syc-20360782
https://www.reference.com/science/can-boy-girl-identical-twins-f20c37b6ec0408c1
I read somewhere that the boy of the twin pair can be affected also so that he has only a Y chromosome, but that is not mentioned in this article.