Final answer:
Abnormalities in the X chromosome number have milder effects due to X-inactivation, which compensates for the extra chromosome by inactivating it, leading to milder defects or sterility.
Step-by-step explanation:
Abnormalities in the number of X chromosomes often result in milder phenotypic effects compared to similar abnormalities in autosomes. This phenomenon is largely due to a process called X-inactivation. During early development in females, one of the two X chromosomes in each cell becomes inactivated, condensing into a structure known as a Barr body. The genes on the inactive X chromosome are not expressed. This process allows females to compensate for the potential overdose of genes due to having an extra X chromosome. Consequently, individuals with additional X chromosomes will inactivate all but one in each cell, leading to relatively mild mental and physical defects, and in some cases, sterility. However, if the X chromosome is absent altogether, development will not proceed in utero, demonstrating the necessity of at least one functional X chromosome.