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 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?
ANSWER: Monosomic cells are missing an entire chromosome. Because only one monosomy is viable,
this suggests human cells must require two of each chromosome to function normally.
Possibly, some autosomal genes must be present in two copies to achieve adequate
expression levels in order to be functional for the cell. Trisomic cells have an extra copy of a
chromosome. Due to the low tolerance for human trisomies, human cells seem to require 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 beyond 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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