Final answer:
The biological sex of a human is determined by the sex chromosomes contributed by the parents. If a baby inherits an X chromosome from the mother and a Y chromosome from the father, the baby will be male (XY). If both inherited chromosomes are X, the baby will be female (XX).
Step-by-step explanation:
The biological sex of a human is determined by the presence or absence of specific sex chromosomes. If a baby inherits an X chromosome from both the mother and the father, the baby will be female (XX). On the other hand, if the baby inherits an X chromosome from the mother and a Y chromosome from the father, the baby will be male (XY). Therefore, it is actually the presence of the Y chromosome that determines whether a human is born a boy or a girl.
Each parent contributes one sex chromosome to their offspring. Females have two X chromosomes and can only pass on an X chromosome, while males have both an X and a Y chromosome, giving them the ability to pass on either, which ultimately determines the sex of the child. The process is random, with a 50:50 chance of passing on either an X or a Y chromosome from the father, making it equally likely for a child to be born male or female.
It is a common misconception that the mother's ovum determines the sex of the baby as, in reality, ova only contain an X chromosome. Instead, it is the sperm's chromosomal content (either X or Y) that is the deciding factor for the sex of the baby. The sex-determining region Y gene (SRY) on the Y chromosome initiates the development of male characteristics, while the absence of a Y chromosome results in female characteristics. Interestingly, except for the SRY gene, most genes on the X chromosome are unrelated to sex determination.