Final answer:
Sex chromosome aneuploidies are better tolerated than autosomal aneuploidies due to the mechanism of X inactivation in females, which prevents the double gene dosage effect and mitigates phenotypic consequences.
Step-by-step explanation:
Sex chromosome aneuploidies are better tolerated by the cell because of a process known as X inactivation, which occurs early in mammalian embryonic development. In females, one of the two X chromosomes in each cell condenses into a dormant structure called a Barr body and is not expressed. This random process of inactivation ensures that females, despite having two X chromosomes, do not have double the gene dosage compared to males who have only one X chromosome. As a result, variations in the number of sex chromosomes translate into relatively mild phenotypic effects. In contrast, autosomes do not have such an inactivation mechanism, which makes aneuploid conditions involving autosomes often lethal or causing severe developmental issues, such as Down syndrome.