Final answer:
In humans, the sex of a child is determined by the sex chromosome in the sperm. If the sperm carries an 'X' chromosome, the baby will be female (XX). If the sperm carries a 'Y' chromosome, the baby will be male (XY).
Step-by-step explanation:
The sex of a human child is determined by the sex chromosome in the sperm. This refers to the 'Y' or 'X' chromosome that the male sperm carries. All human females carry an 'X' chromosome in their eggs. Thus, if the sperm also carries an 'X' chromosome, the baby will be female (XX). If the sperm carries a 'Y' chromosome, the baby will be male (XY). Therefore, the sex assignment at birth depends on whether the sperm that fertilizes the egg carries an 'X' or a 'Y' chromosome. Because the male has a 50 percent chance of passing on either the 'X' or 'Y' chromosome, there is an about 50 percent chance each baby will be male or female, regardless of the siblings' sexes.
Learn more about Sex Determination