Certain genetic mutations which cause genetic diseases are found on the X chromosome. These diseases are called X-linked diseases. In most cases, X-linked diseases are passed from carrier or affected mothers to their sons (see images below). Thus, sex selection can be used in these cases to have a girl child, who may be a carrier of the x-linked disease, but who will not be affected by the disease itself.

Other diseases may be inherited by a certain gender, but not the other. For example, the BRCA-1 and BRCA-2 gene mutations cause affected women to have an extremely high risk of breast and ovarian cancers. Thus, women with a strong family history of breast and ovarian cancer and who are BRCA-1 and/or BRCA-2 positive can choose to have a boy, who might inherit the gene, but will have a much lower risk of getting breast cancer, and no risk of ovarian cancer. Similarly, men with a strong family history of cancer of the prostate can choose to have a girl, as only males suffer from this disease.

Using a technique called preimplantation genetic diagnosis, we can examine embryos created through in vitro fertilization for the chromosome indicating if the embryos are male or female. Only embryos of the selected gender will then be transferred back into the mother's uterus. This technique has a 99% chance of having a baby with the gender of your choice, if conception occurs. Embryos of the opposite gender can be frozen and "adopted" by infertile couples who can't produce embryos of their own.

For more information on Preimplantation Genetic Diagnosis, click here.

Sperm sorting

One method of determining the sex of the child is to sort the sperm used for insemination into "boy sperm" (sperm carrying a Y chromosome) and "girl sperm" (sperm carrying the X chromosome). Since a woman's oocytes only carry the X chromosome, sperm sorting will determine the sex of the baby conceived.

Sperm which carry the X chromosome contain more DNA than sperm carrying the Y chromosome. Several methods have been developed to use this difference in size and weight to "enrich" sperm with the X chromosome and Y chromosome. Using a technique called "Flow Cytometry," sperm can be sorted by size, and the "girl sperm" separated from the "boy sperm" for use in insemination.

Advantages and disadvantages

The advantage of sperm sorting is that, because it is used with insemination, it is much less expensive than in vitro fertilization and PGD.

The disadvantage of sperm sorting is that it is less reliable than IVF/PGD, with the likelihood of conceiving a chosen gender being anywhere from 50%-80%.