Final answer:
Chromosomes come in pairs because humans have 23 pairs of chromosomes. The 23rd pair, known as the sex chromosomes, determines our gender. Females have two X chromosomes (XX) while males have one X and one Y chromosome (XY).
Step-by-step explanation:
Chromosomes come in pairs because humans have 23 pairs of chromosomes. One of these pairs is the sex chromosomes, which determine our gender. The pair of sex chromosomes that determines our gender is the 23rd pair, with females having two X chromosomes (XX) and males having one X and one Y chromosome (XY).