Every 28 days an ovum is released from a woman's ovary and remains viable for approximately 24 hours. Once released into the vaginal canal, sperm can live for up to six days. Conception takes place in the fallopian tubes where sperm and ovum unite to form the zygote. The 22 pairs of matching chromosomes within a human cell are called autosomes. The 23rd pair of chromosomes is called the sex chromosomes.
Females have an XX pair of sex sex chromosomes, while males have an XY pair.
While in theory all men should produce an equal number of X and Y sperm, studies of men who have produced a family of all girls or all boys have shown that these people are producing exclusively X or Y sperm. Since this is theoretically impossible, there would have to be some mechanism in the man's body that is not permitting all of the sperm to survive. It is now possible to choose the sex of your child in advance , if you have the money and have a compelling reason for wanting one sex over the other. Fraternal, or dizygotic, twins result when two separate ova are fertilized by two separate sperm. Identical, or monozygotic, twins result when a single zygote that has started to duplicate separates into two clusters of cells that develop into two individuals.