Final answer:
The sex of a son is determined by the sex chromosome carried by the father's sperm. If the sperm carries a Y chromosome, the child will be male. This is due to the presence of either XX (female) or XY (male) sex chromosomes during fertilization.
Step-by-step explanation:
The sex of a child is not determined by the phenotype of the parents but by the sex chromosomes involved in the fertilization process. In humans, these chromosomes are called sex chromosomes, typically consisting of XX in females and XY in males. When a sperm cell fuses with an egg, the sperm contributes either an X or a Y chromosome, while the egg always contributes an X chromosome. If the sperm contributes an X chromosome, the child will be female (XX). If the sperm contributes a Y chromosome, the child will be male (XY). This means that the sex of the child is determined by the sex chromosome carried by the father's sperm.
Therefore, the sex of a son is determined by whether the father's sperm carries an X or a Y chromosome. If the father's sperm carries the Y chromosome, then the fertilized egg will develop into a male child. This is a 50:50 chance for each pregnancy, regardless of the number or sex of any previous children.