Final answer:
Abnormalities in the number of X chromosomes tend to result in milder effects than autosomal abnormalities due to a process called X-inactivation, which leads to dosage compensation and generally mild phenotypic effects.
Step-by-step explanation:
Abnormalities in the number of X chromosomes tend to be milder than the same abnormalities in autosomes because of X-inactivation. This molecular process allows females to compensate for their double genetic dose of the X chromosome. During early development, one X chromosome in each cell is inactivated and becomes a Barr body, ensuring that each cell uses the genes from only one X chromosome, preventing an overdose of X-linked gene expression.
An individual with an abnormal number of X chromosomes will inactivate all but one X chromosome in each cell. This inactivation process is associated with mild mental and physical defects and sometimes sterility. The random nature of which X chromosome is inactivated leads to a phenomenon called dosage compensation, allowing for generally mild phenotypic effects compared to similar abnormalities in autosomes.