a. 2 > The 23rd pair, the sex chromosomes, differ between males and females (females have two copies of the X chromosome, while males have one X and one Y chromosome). In humans, the sex chromosomes comprise one pair of the total of 23 pairs of chromosomes. The other 22 pairs of chromosomes are called autosomes. Chromosomes are thread-like molecules that carry hereditary information (they are made of protein and one molecule of DNA, which contains an organism's genetic instructions).