Explain the concept of 'sex determination' in humans. What role do the X and Y chromosomes play in this process?

In humans, sex determination is primarily chromosomal. Females typically have two X chromosomes (XX), while males typically have one X and one Y chromosome (XY). During reproduction, the mother always contributes an X chromosome to the offspring. 

The father, however, can contribute either an X or a Y chromosome. If the father contributes an X chromosome, the offspring will be XX (female). If the father contributes a Y chromosome, the offspring will be XY (male). The presence or absence of the Y chromosome, specifically the SRY gene on it, is crucial in triggering the development of male characteristics.