What factors determine whether a baby is a male or a female?
All the traits an individual inherits come from the parent's gametes--sperm and egg cells which unite at fertilization. The chromosomal sex of the baby is determined by which combination of sex chromosomes are inherited. If a baby inherits the chromosome pair XY, then, it is a male. If a baby inherits the chromosome pair XX, it is a female. It is the presence of the Y chromosome that causes the gonads which are undifferentiated at first, to develop into male testes. Gonad development is a result of the presence or absence of the sex determination gene SRY on the Y chromosome. In the absence of this gene, the baby will develop a female reproductive system and if it is present, the baby will develop male gonads. These gonads in turn, produce hormones and sexual differentiation occurs as a result of these powerful chemicals in the body.
The gender of a baby is decided by the presence of the X or Y chromosome in the sperm. The egg contains only the X chromosome. If it is fertilized by a sperm that has the X chromosome the gender of the child born is female and if it is fertilized by a sperm with a Y chromosome a girl is born.
Studies have shown that the sperm cells with the X chromosome have a longer life but they are not able to move as well as the sperm cells with Y chromosome. This reduces their chances of being able to reach the egg and fertilizing it to result in a girl being born.