Which chromosome determines the gender of a baby at fertilization?

The determination of an individual's gender is fundamentally linked to the combination of sex chromosomes inherited from each parent. In humans, females typically possess two X chromosomes (XX), while males have one X and one Y chromosome (XY).

At fertilization, the genetic contribution from the mother is an egg cell which always carries an X chromosome. The genetic contribution from the father comes from the sperm, which can carry either an X chromosome or a Y chromosome. If the sperm that fertilizes the egg carries a Y chromosome, the resulting combination will be XY, leading to the development of a male. Conversely, if an X-bearing sperm fertilizes the egg, the combination will be XX, resulting in a female.

Thus, the presence of the Y chromosome from the sperm is crucial for determining male gender at fertilization. Without the Y chromosome contributing to the genetic make-up, the offspring will be female. This highlights the pivotal role of the Y chromosome from the sperm in establishing the male gender during the earliest stages of development.