Genetic testing and cystic fibrosis

Gene technology and the human genome

Family tree diagram of cystic fibrosis inheritance

Cystic fibrosis is an inherited disease. It is caused by a recessive genetic variant of the CFTR gene. This causes cells to make a very thick mucus. The thick mucus causes problems for breathing as it blocks the lungs. It also interferes with digestion and reproduction. About 1 baby in every 2500 is born with cystic fibrosis. At present there is no cure.

In order for a baby to be born with cystic fibrosis, both of its parents need to be carriers. The healthy variant in this Punnett square is F.

Ff = mother

Ff = father

f = cystic fibrosis allele

Ff
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fFfff

Testing the parents

About 1 in 25 people in the UK carries the harmful variant. One type of gene technology is genetic testing. This can be done using a sample of saliva, which allows doctors to predict the probability of affected parents having an affected child. Then there needs to be a discussion of the risk, which also takes into account the consequences of having an affected child.

There are two methods to test a fetus to find out if it is affected by a genetic condition:

  • amniocentesis - a sample of the amniotic fluid is removed and a genetic test done on cells in the fluid
  • chorionic villus sampling (CVS) - a suction tube is used to remove cells from the placenta and a genetic test done on the cells

Both procedures carry a small risk of miscarriage (estimated lower than 1%), which is an important consideration when deciding whether to be tested.

Pre-implantation Genetic Diagnosis (PGD) can also be used. Eggs are removed from the mother's ovaries and fertilised with the father's sperm in a laboratory. Embryos which are 3 to 6 days old each have a cell removed, and these are tested for harmful genetic variants. An embryo without harmful variants is selected and implanted into the mother's womb.

False positive and negative test results

A false positive test occurs when a genetic test has wrongly detected a certain allele or faulty chromosome when it is not present. The individual or family could believe something is wrong when it is not. This may lead them to decide not to start a family, or to choose an abortion to avoid having a baby with a genetic disorder.

A false negative test fails to detect a harmful variant that is present.