text: Depends on what actually happened to give us more or less chromosomes.  In humans, the most likely reason someone would have more or less chromosomes would be nondisjunction (an uneven split of chromosomes) in gametogenesis (process of making sperm or eggs).\n\nMost often, nondisjunction does not produce viable offspring.  They die within the first few weeks of gestation.  Some nondisjuncted offspring do come to term, but have some pathological problem.  Some common diseases that arise from nondisjunction are:\n\nDown's syndrome \nEdward's syndrome \nKlinefelter's syndrome\nPallister-Killian syndrome \nPatau syndrome \nTurner syndrome \nXYY syndrome \nXXX syndrome