Final answer:
Sex chromosomes differ between males (XY) and females (XX), with the X and Y chromosomes containing different gene sets. Nettie Stevens discovered this difference, which is essential for sex determination, and females have a Barr body because they possess two X chromosomes, one of which is inactivated.
Step-by-step explanation:
Chromosomes that differ in size and composition between males and females are called sex chromosomes. In humans, females have two X chromosomes, one of which is inactivated to a Barr body in each cell, whereas males possess one X chromosome and one Y chromosome. This XY sex chromosome pair is unique because the genes on the X and Y are different, unlike the genes on autosomes that have the same organization within each pair. Notable scientist Nettie Stevens first observed these differences and linked them to sex determination.
The existence of Barr bodies in females and not males is because females have two copies of the X chromosome, yet only one is needed for normal function, while males have a single X chromosome that must remain active. The sex chromosomes present the only notable exception to the uniformity of homologous chromosome pairs, with the Y chromosome inducing male characteristics, and its absence resulting in female characteristics. The X and Y chromosomes play a crucial role in the genetic determination of an individual's sex.