[Huck Finn]

Humans do not tolerate variation in chromosome number well. We have 23 pairs of chromosomes, 1 pair is our sex chromosomes, XY for men, XX for women. We have 22 pairs of autosomes. If a embryo is missing an autosome, it will terminate, humans cannot be monosomic(45 chromosomes resulting in one autosome missing), variation in sex chromosome number is often less severe and may lead to a "viable" fetus. Trisomy 21 is the most common form of down syndrome and it occurs from when instead of having 2xchromosome 21(autosome), the individual has 3xchromosome 21. Gene dosage is the reason problems are caused by the extra 21 chromosome, we are finely balanced biological machines, when things are thrown out of joint, there are many flow on effects, essentially chromosome variation is always very bad.