What variant of sex chromosome nondisjunction is not observed because maternal chromosomes are necessary for viability.?

The variant of sex chromosome nondisjunction that is not observed due to the necessity of maternal chromosomes for viability is the presence of a Y chromosome without an X chromosome, specifically the 45,Y (Turner syndrome) condition. In humans, a viable organism requires at least one X chromosome; thus, a monosomy for the X chromosome (45,Y) is nonviable. In contrast, conditions like Turner syndrome (45,X) involve the loss of one X chromosome but still have one functioning X, allowing for some degree of viability.