Sex chromosomes are a specific type of chromosome that determine the biological sex of an organism. In humans, there are two sex chromosomes: X and Y. Typically, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). These chromosomes carry genes that influence sexual development and reproductive functions. The presence of the Y chromosome triggers male characteristics, while the absence of it (having two X chromosomes) leads to female characteristics. This genetic system is not unique to humans; many other organisms, including some species of birds and insects, also use sex chromosomes to determine sex.