In humans, sex is determined by the presence or absence of the Y chromosome. The sex chromosomes in humans are labeled as X and Y. Females have two X chromosomes (XX) while males have one X and one Y chromosome (XY).
During fertilization, an egg cell from the mother always contains one X chromosome, while a sperm cell from the father contains either an X or a Y chromosome. If a sperm cell carrying an X chromosome fertilizes the egg, the resulting zygote will have two X chromosomes and develop into a female. If a sperm cell carrying a Y chromosome fertilizes the egg, the resulting zygote will have one X and one Y chromosome and develop into a male.
Therefore, the sex of the offspring is determined by the sex chromosome contributed by the father.