How is the sex of the baby determined?

The sex of a baby is determined by the type of sperm that fertilises the egg, carrying either an X or Y chromosome.

In more detail, human cells typically contain 23 pairs of chromosomes, making a total of 46. One pair of these chromosomes, known as the sex chromosomes, determines the sex of an individual. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

During the process of meiosis, which produces the sex cells (sperm in males and eggs in females), these pairs of chromosomes are separated so that each sex cell contains only one set of 23 chromosomes. This means that all egg cells produced by a female will contain an X chromosome, while half of the sperm cells produced by a male will contain an X chromosome and the other half will contain a Y chromosome.

Fertilisation is a random process, and the sex of the baby is determined by whether an X or a Y sperm cell reaches the egg first. If an X sperm fertilises the egg, the resulting zygote will have two X chromosomes and develop into a female. If a Y sperm fertilises the egg, the zygote will have one X and one Y chromosome and develop into a male.

This process is a perfect example of Mendelian inheritance, where traits are determined by discrete units of inheritance, or genes. In this case, the gene for sex determination is located on the Y chromosome. If this chromosome is present, the baby will develop as a male. If it is not, the baby will develop as a female. This is a simplified explanation, as other factors can influence sexual development, but it provides a basic understanding of how the sex of a baby is determined.