An individual's genetic information is packaged into strings of genes called chromosomes. Normal human cells contain 46 chromosomes, or 23 pairs. These include chromosomes 1 to 22 (the autosomes) and chromosomes X and Y (the sex chromosomes). Females carry two X chromosomes, while males carry an X and a Y chromosome.

What is an X-linked Disease?
An individual's genetic information is packaged into strings of DNA called chromosomes.  Normal human cells contain 46 chromosomes, or 23 pairs.  These include chromosomes 1 to 22 (the autosomes) and chromosomes X and Y (the sex chromosomes).  Females carry two X chromosomes, while males carry an X and a Y chromosome. 
X-linked diseases, such as Haemophilia or Muscular Dystrophy, are caused by different gene changes on the X chromosome.  Because these gene changes are carried on the X chromosome, the probability of a particular embryo being affected is dependent upon its sex.  An example of this is shown in Figure 1.  The potential risks to offspring will vary according to the parental origin of the gene change and whether it is a dominant or recessive disorder.

Figure 1: An example of the possible embryo outcomes following sex selection for a recessive X-linked disease carried by the mother.  The X chromosome carrying the gene change is indicated by Xa.

What is Preimplatation Genetic Diagnosis?
Preimplantation genetic diagnosis (PGD), in conjunction with In Vitro Fertilisation (IVF), is an alternative to traditional prenatal diagnosis (chorionic villus sampling or amniocentesis) and allows genetic testing to be performed on an embryo before a pregnancy is established.
PGD is available for couples at risk of passing on an X-linked genetic disorder to their children.  When genetic testing is not possible for the particular disorder, PGD with testing for embryo sex may be possible.  PGD enables identification of the sex of the embryo and therefore the possible selection for transfer of only embryos of the sex that will not be affected by the particular disorder.
The National Health and Medical Research Council (NHMRC) "Ethical guidelines on the use of assisted reproductive technology in clinical practice and research" September 2004, state that sex selection must not be undertaken except to reduce the risk of transmission of a serious genetic condition.  The guidelines also state that sex selection should not be used to select in favour of a genetic disorder or disability in the person to be born.
 
How does PGD for Sex Selection Work?
Preimplantation genetic diagnosis (PGD) is an option for couples at genetic risk.  PGD utilises assisted reproduction technology to stimulate the woman's ovaries to produce a number of eggs.  These eggs are collected and fertilised using the male partner's sperm.  Resulting embryos are cultured in the laboratory and embryo biopsy is performed on Day 3 after egg collection.  Embryos that have developed to at least 5 cells are suitable for biopsy.  A hole is made in the zona (the outer shell) of the embryo and 1 or 2 cells are removed from the embryo. 
The cells are tested for the sex chromosomes (X and Y) using Fluorescent In Situ Hybridisation (FISH).  Fluorescent dyes are used to tag the sex chromosomes and these show as fluorescent "dots", indicating the chromosome status of the cell.  Additionally, a small number of chromosomes commonly involved in abnormalities in humans (ie: chromosomes 13, 15, 16, 17, 18, 21 and 22) are also included in the test.  Embryos identified with the correct sex (and thus unaffected) and normal for the additional chromosomes may be transferred back to the woman’s uterus.  A maximum of 2 embryos can be transferred.  A blood test is taken 2 weeks after transfer to confirm if pregnancy has occurred.

Figure 2: Embryo biopsy and FISH Analysis

What are the costs?
Information relating to the cost of embryo biopsy and chromosome testing is available from Monash IVF.  Please note that additional costs will be incurred for IVF services.  Please contact Monash IVF for a list of current prices.

What else do I need to know?

• Embryo biopsy has been used extensively at Monash IVF and other major IVF clinics throughout the world and follow up studies have shown that the procedure is safe and does not appear to have an adverse affect on the embryo's potential to implant and develop normally.
• It may not be possible to obtain a FISH result from some or all of the biopsied embryos.  Some embryos may not be of sufficient quality to biopsy, or in some embryos the test results may not be conclusive. 
• PGD will only test for the sex chromosomes and some clinically significant chromosomes (ie: chromosomes 13, 15, 16, 17, 18, 21, 22).  Abnormalities involving other chromosomes cannot be excluded.
• The accuracy of sex selection is high, but not 100%.

Prenatal diagnosis is highly recommended in an ensuing pregnancy.

Quality Systems
Monash IVF employs a very high standard of quality assurance. The laboratories are NATA accredited.  Through the application of quality systems we monitor and provide standards of excellence in quality service, care and advice.