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How do X-bearing and Y-bearing sperm influence a child's chromosomal sex?

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Front view anatomical diagrams of the male and female reproductive systems, with labeled organs including penis, testis, prostate gland, uterus, ovary, and vagina. — human reproductive system Organs and structures of the male and female reproductive systems.

In humans, chromosomal sex is determined at the moment of fertilization based on the genetic contribution of the sperm.12

Because all human eggs carry a single X chromosome, the sex of the zygote depends on whether the fertilizing sperm carries an X or a Y chromosome.13

X-bearing sperm: When an X-bearing sperm fertilizes the egg, the resulting combination is XX, which leads to the development of female chromosomal sex.1
Y-bearing sperm: When a Y-bearing sperm fertilizes the egg, the resulting combination is XY. The presence of the Y chromosome stimulates the undifferentiated gonad to develop into a testicle, directing the development toward male chromosomal and anatomic sex.12

Since males produce X-bearing and Y-bearing sperm in roughly equal numbers, there is an approximately equal statistical chance of conceiving a boy or a girl at any single fertilization event.12