In meiosis, the chromosomes pair up and exchange segments, so that genes from one are replaced by genes from another. Then chance determines which member of each pair will gather with others and end up in the same gamete. Meiosis ensures that no two genes will ever be alike. 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.