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Since the egg already contains an X chromosome (from the mother), the gender of a baby is determined by the X or Y sex chromosome of the sperm cell from the father (thus adding one more thing to the "blame it on the husband" list if things don't go as planned).
A girl baby will result if the final arrangement is XX, and a boy will result if it is XY. As you would expect, about half of the sperm cells contain an X chromosome, and half contain a Y chromosome. It is possible to have an abnormal number of sex chromosomes, however, this is extremely rare. Realizing that gender is dependent on the Y-bearing and X-bearing sperm cells, scientists have attempted for years to separate these two populations from each other. Fortunately -- or unfortunately (depending on your view) -- this has proven to be a very difficult task. The human sperm cell is one of the smallest cells in the body. The head is oval, tapered and flat. The genetic material is tightly packed inside the head. The tail is long and very motile, giving the cell its amazing swimming capability. Sex selection procedures are based on theoretical and actual differences between the two types of sperm cells.
How can you tell X sperm from Y?
Various methods attempting separation have included:
1) spinning the cells to separate by weight
2) having the cells swim through different layers of media
3) placing the cells in an electric field
...along with countless other procedures.
While there have been reports that some of these techniques have separated X and Y cells to various degrees, it has been difficult to reproduce any clinically significant success when using fluorescent in situ hybridization (FISH) to confirm separation. The most successful technique to date is a process called flow cytometry.
With this technique, the nuclear material of the cells is stained with a dye, and the cells are passed through a machine in a high-speed, single-file line. The cells are then separated based on the amount of DNA stain that is read by the machine (the X chromosome is larger, thus emitting more signal). This technique is reported to have the ability to sort X-bearing spermatozoa with approximately 85 percent enrichment, and Y-bearing with 60 percent to 75 percent enrichment.
It is important to know that this technique requires DNA staining with a fluorochrome (fluorescent dye) in addition to ultraviolet laser exposure. The long-term consequences of this procedure are not known, however, several generations of animals appear to be normal.
While attempts have been made over the years to select the gender of a child before conception, there is currently no process that can guarantee the selection of either a boy or a girl. For many couples, the ability to simply have a healthy baby is reward enough.
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