A human egg will develop into a female if it is fertilized by a sperm containing:

When a human egg is fertilized by a sperm that carries an X chromosome, the resulting zygote will develop into a female. This is due to the genetic contribution from both the egg and the sperm. The human egg always contains an X chromosome, as females have two X chromosomes (XX). If the sperm also contributes an X chromosome, the combination becomes XX, which corresponds to female development.

In contrast, if a sperm carrying a Y chromosome fertilizes the egg, the zygote will be XY, leading to male development. The presence of XY or XXY combinations corresponds to male and atypical situations, respectively, with XXY typically resulting in a condition called Klinefelter syndrome, which is not a standard male or female designation. Therefore, the fertilization by a sperm containing an X chromosome is what ensures the development of a female embryo.