When in mammals is the sex of the offspring determined?

The sex of mammals is determined at conception, which occurs when the sperm fertilizes the egg. At this moment, the genetic material from both parents combines to form a zygote. The zygote contains a set of chromosomes that determines the offspring's sex. In mammals, this is typically established by the combination of the X and Y chromosomes contributed by each parent. If the sperm carries an X chromosome and fertilizes the egg, the resulting offspring will be female (XX). If the sperm carries a Y chromosome, the offspring will be male (XY).

Understanding this concept is fundamental in genetics and reproductive biology, as it highlights the pivotal role of chromosomal combinations in determining biological sex right from the earliest stages of development, rather than during gestation or childbirth.