What defines a sex chromosome?

A sex chromosome is specifically defined by its role in determining the sex of an organism and is characterized by differences in morphology or number between males and females. In many species, including humans, males and females possess different sex chromosomes (for example, XY for males and XX for females). These differences can manifest in variations in size, shape, or the presence of specific genes that are crucial for sex determination and the development of sexual characteristics.

This definition is foundational in genetics because it underpins the understanding of how traits related to sex are inherited and expressed. In contrast, sex chromosomes are distinct from autosomes, which carry genes unrelated to sex and are the same in both sexes; therefore, the other options do not accurately capture the essential characteristic of sex chromosomes.