Genetic Testing

1 Genetic Testing

Using various assisted conception procedures, embryos can be produced artificially by fertilisation methods in the laboratory, ready to be transferred into the uterus for implantation. However, because couples who are being treated at assisted conception units by definition struggle to conceive naturally, they stand a higher than average chance of carrying faulty genes and genetic disorders, even though they themselves may not actually have any such disease. Their embryos are therefore at higher risk of inheriting these faulty genes and suffering from genetic disorders.

In the past, such embryos could only be tested for genetic disorders at around 12 weeks gestation. By this stage, the pregnancy is almost a third of the way complete. Any abnormality detected would entail a difficult decision for parents, i.e. to continue the pregnancy and have a potentially disabled baby or have a termination (abortion) and start again.

However, the latest genetic testing technology enables embryos to be tested before implantation, while they are only around 3 days old. Thus, only healthy embryos are selected for implantation. These tests are expensive and are not yet available in all centres or clinics.

Pre-implantation Genetic Diagnosis (PGD)

Pre-implantation Genetic Diagnosis (PGD) is a useful test under the following circumstances;

you have had previous terminations due to late testing of your embryo revealing genetic disease,
you already have a child with a genetic disease,
you are deemed to be at high risk of passing on an inherited condition for other reasons.

Examples of inherited diseases include haemophilia, cystic fibrosis and others. Some conditions only affect males such as Duchenne Muscular Dystrophy. In these cases, the test involves examination of extracted cells for male chromosomes which are then excluded. Only female embryos are then used. Gender selection is a controversial area and is banned in the UK as well as several other countries. It may be used only for specific medical (rather then social) reasons.

PGD is carried out by extracting 1 or 2 cells from a 3 day old embryo. These are examined to detect the presence of various abnormal genes responsible for various genetic disorders.

Pre-implantation Genetic Screening (PGS) also called Aneuploidy Screening

Pre-implantation Genetic Screening (PGS) is a useful tool in women over 35 years of age. For these older women, there is a greater risk of producing embryos with chromosomal disorders. An example of this is Down's syndrome (Trisomy 21), where there is an extra chromosome. In these instances, PGS may be considered. This may also be useful in women who have suffered recurrent miscarriages or unsuccessful IVF cycles.

PGS involves the extraction of cells for analysis of chromosomes. Down's syndrome embryos for example have 3 chromosome 21's instead of 2. This can be detected at a much earlier stage than using more traditional techniques.

What is a Chromosome?

Each cell in the body contains a nucleus in the centre. This as a "store" for all the genetic information that governs that cells structure and function. In turn, this governs how the whole body is made and functions.

All genetic information is stored in long, thin structures within the nucleus called chromosomes. Chromosomes in turn are composed of thousands of genes. Most human cells contain 23 pairs (46) chromosomes. However, gametes (eggs and sperm) contain 23 single chromosomes. When an egg and sperm cell fuse together in the process called fertilisation, each brings its 23 single chromosomes to form embryo cells containing 46 chromosomes (23 pairs) in total.

What is a gene?

Chromosomes store the vast amounts of genetic information required to produce and maintain a human body. This information is arranged so that specific areas within each chromosome are responsible for specific functions. These areas are called genes. Genes carry the genetic code for specific tasks within each cell. They are part of a tightly packed double helical molecule called deoxyribonucleic acid (DNA). DNA molecules are very long and form a whole chromosome.