Why is there a 50 50 chance of having a girl or boy?

The 50/50 chance of having a girl or boy is primarily due to the way sex is determined in humans, which involves the combination of sex chromosomes from each parent. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). When a sperm (carrying either an X or Y chromosome) fertilizes an egg (which always carries an X chromosome), the combination will either be XX (girl) or XY (boy), resulting in approximately equal probabilities for each sex. This genetic mechanism leads to a roughly equal likelihood of having a girl or boy.