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In vitro fertilisation has helped many overcome difficulties with having children. However, it raises a number of ethical issues, one of which is the ability to select for particular traits or diseases using a technique called preimplantation genetic diagnosis (PGD).

The first successful PGD test was performed in Britain in 1989, where female embryos were selected for couples at risk of having baby boys with X-linked genetic conditions. PGD is now often used to test for genetic diseases such as cystic fibrosis, Duchenne muscular dystrophy and Fragile-X syndrome or for conditions resulting from aneuploidy (having one more or one less chromosome), such as Down’s Syndrome. After testing, unaffected embryos can be transferred into the mother’s uterus.

Prior to PGD, the only testing option for couples at risk of passing an inherited condition to their children was by sampling foetal cells from the placenta or amniotic fluid. If a foetus was found to be affected, parents would then have to decide whether to proceed with the pregnancy, or to disrupt the pregnancy for medical reasons.

How is PGD done?

 

During in vitro fertilisation (IVF), eggs are removed from a woman’s ovaries and fertilised in vitro with sperm. The fertilised eggs undergo normal mitotic cell division in the lab until they have eight cells. At this stage, one or two of these cells are removed. DNA from the cells is extracted and tested for the presence of genetic disorders.

Typically, the genetic contents of the cells are tested using polymerase chain reaction (PCR) or fluorescence in situ hybridisation (FISH). PCR is used to look for single gene abnormalities, whereas FISH is used to look for abnormal chromosomes.

Embryos that genetically normal can be then transferred into the woman’s uterus where they may implant and develop. The removal of cells for testing does not affect the development of the embryo, as these cells are replaced as the embryo’s cells continue to divide and grow.

When is PGD used?

PGD is an expensive procedure and is normally used to test embryos of couples who have a high risk of passing an inherited condition to their children. For example, if both parents have a family history of cystic fibrosis, there is a chance that they could be carriers of the disease and have children with cystic fibrosis, even if neither of them actually has it.

PGD can also be used to screen for chromosomal aneuploidy to increase the chances of a successful pregnancy for couples who are undergoing IVF.

PGD has been used by couples to select an embryo that will make a compatible donor of stem cells to treat a sick sibling.

PGD can also be used to test for traits, such as gender or deafness, but these tests raise a number of ethical issues and are not permitted in most countries.

Ethical issues of PGD

PGD gives parents the ability to select against particular combinations of their genes (that could occur naturally) by choosing which embryos are implanted and which are destroyed. However, others believe PGD is preferable to finding out during pregnancy that the baby has a particular genetic disease or condition - at which point the parents decide whether to continue with the pregnancy, or interrupt it for medical reasons (a medical termination).

The IVF procedure creates a surplus of embryos, which are usually destroyed. There are many people who believe this is unethical.

Some people fear that PGD will lead to selecting for particular traits, such as gender, hair colour or behaviour, and will allow people to make ‘designer babies’. However, most human traits are the result of the action of more than one gene, and it is unlikely that people will be able to select for complex traits like these.

PGD in New Zealand

Guidelines for using PGD in New Zealand are available from the Advisory Committee on Assisted Reproductive Technology (ACART). These guidelines were developed in March 2005 after consultation with the New Zealand people.

In New Zealand, PGD may be used to test for inherited diseases and chromosomal abnormalities. PGD may also be used to match embryo tissue for stem cell therapy of a sibling with approval from the Ethics Committee on Assisted Reproductive Technology (ECART). In New Zealand, PGD cannot be used:

  • for social reasons, including sex selection
  • to alter the genetic constitution of an embryo
  • to select embryos with a genetic impairment seen in a parent.

Useful link

Read this article, Preimplantation genetic diagnosis after 20 years, by Professor Alan Handyside, who pioneered PGD in the 1980s and continues to advocate for its use.

 

    Published 15 November 2007, Updated 12 June 2018 Referencing Hub articles