Turner Syndrome (XO) is the only viable monosomy found in humans. Some sex chromosome
trisomies are also viable, including XXX, XXY, and XYY. There are no viable autosomal
monosomies, and a small handful of viable autosomal trisomies exist, such as Trisomy 21 (Down
Syndrome), and Trisomy 13 and 18, both of which are lethal within the first year of life. Clearly
aneuploidy is not tolerated well in humans. Speculate as to why there are so few viable monosomies
(only one) in humans, and then why there are so few viable trisomies. Also, speculate as to why
aneuploidy involving the sex chromosomes is better tolerated than those involving autosomes.
What will be an ideal response?
Monosomic cells are missing an entire chromosome. Apparently, human cells must have two of each
chromosome to be viable, perhaps because some autosomal genes must be present in two copies in
order to be functional for the cell. Trisomic cells have an extra copy of a chromosome. Apparently,
human cells need to have ONLY two copies of each gene, and having three copies of some genes is
lethal as there may be too much gene function. The few viable autosomal trisomies are either lethal
shortly after birth (Trisomy 13 and 18) or have a variety of associated symptoms, including a slightly
shortened lifespan (Trisomy 21). During human development, all X chromosomes over one per cell are
inactivated. Therefore, aneuploidy is better tolerated when involving sex chromosomes because there
is already a natural process in place for partially “haploidizing” those chromosomes. Despite this, there
are symptoms associated with XO, XXY, XYY, and XXX individuals.
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