Turner syndrome is caused by monosomy of the sex chromosomes XO. Why is there no corresponding condition for monosomy of the sex chromosome YO Turner syndrome is caused by monosomy of the sex chromosomes XO. Why is there no corresponding condition for monosomy of the sex chromosome YO Solution Nondisjunction leading to YO cells are non viable.At least one X chromosome is needed for viability. YO condition is lethal hence there are no corresponding conditions for monosomy of the sex chromosome YO..