Sex selection (gender selection)
Human beings have been trying to influence the sex of their children for as long as anyone can remember. The techniques in the past were crude, based on superstition and 50% of the time — unsuccessful. Recently, PGD, preimplantation genetic diagnosis has become the method of choice to influence the gender of a baby.
Basic Biology of Gender Determination
The sex of a baby is determined at the moment of conception by the chromosomes present in the egg and sperm. Human beings have two "sex" chromosomes: The "X" and "Y" chromosomes. Female eggs contain only X chromosomes but male sperm contains either an X chromosome or a Y chromosome.
An embryo is female if an X bearing sperm fertilizes the egg resulting in an "XX" embryo. A male will result if a Y bearing sperm fertilizes the egg resulting in an XY embryo.
PGD for gender selection
PGD (Preimplantation Genetic Diagnosis) can be used to detect which embryos are XX or XY. The desired embryos can be placed into the female's uterus to achieve pregnancy.
PGD is the only method for gender selection that is close to 100% accurate. Other methods for gender selection attempt to separate the X and Y bearing sperm. To date, none of these techniques has achieved a success rate greater than 75-80%.
The technology used during PGD to determine the sex of an embryo is called FISH or flourescent in-situ hybridization. With FISH for gender selection, two specific DNA "probes" are used. One probe will identify the X chromosome. The other probe will identify the Y chromosome. Attached to the probes are moecules which give off light of different colors.
In this example the X chromosome is represented by the yellow colored spots. Since there are two yellow spots, we know that this embryo has two X chromsomes and is therefore female.
What are the problems with PGD for gender selection?
PGD is performed as part of an in vitro fertilization – IVF cycle. In order to successfully achieve pregnancy with IVF, there are several important factors :
- A woman must take and respond to fertility medications in order to retrieve multiple eggs and therefore create multiple embryos
- The embryos that are created must grow and develop normally
- The embryos that are created must not have any chromosome abnormalities
- Older women are less likely to get pregnant with IVF even if their fertility is normal
Some examples of what could happen during an IVF/PGD cycle for gender selection
By random chance, there may not be any embryos of the desired gender produced.
There may be embryos of the desired gender, but they may contain chromosome abnormalities or have failed to develop properly. These embryos cannot be used for attempting pregnancy.
There may be embryos of the desired gender that have no detectable chromosome abnormalities and their growth and development has occurred but they don't look as good under the microscope as some other embryos of the non-desired gender. In other words, if gender selection was not being performed, these embryos would not be the first choice for transfer into the uterus because they were thought to have a lower potential for implantation.
This presents a dilemma for the couple. Do they transfer the best looking embryos to maximize their chances for achieving pregnancy? Or do they accept a lower chance for pregnancy but be assured that if pregnancy does occur that the baby will be of the desired gender?
The final example if the most desired option. The embryos produced are of the desired gender, normally developed, high quality under the microscope and with no chromosome abnormalities.
IVF with PGD is the most successful and accurate way to perform gender selection. Its only limitation is the fertility potential of the couple attempting pregnancy.