Question

Aneuploidy of an autosome is always more harmful than an aneuploidy of a sex chromosome. One of the explanations as to why sex-chromosome aneuploids are less harmful is due to X inactivation. X-inactivation assures that all human cells only express a single X chromosome regardless of how many X chromosomes are present. Interestingly, sex chromosome aneuploids (i.e. XXY, XO) still have some different phenotypes compared to either an XX or XY individual.  For instance, XXY individuals (Klinefelter Syndrome) are taller, have underdeveloped testes, and have larger breast tissues and hips and XO individuals (Turner Syndrome) are shorter in stature, heavier, and are usually sterile. If X-inactivation is supposed to make all cells equal in regards to the X-chromosome, then why do XXY and XO individuals have any phenotypes that are different than an XX or XY individual?

Answer 1

X chromosomes are known to possess Pseudoautosomonal regions known as PARs.The PARs are not X-inactivated and take part in cellular activity hence causing problem in XXY individuals.

Similarly in XO individuals, due to unavailability of one X chromosome the protein which was to be produced by that is missing and hence low concentration of protein is present hence leading to developmental problems.