genetic testing

Genetic Testing

Genetic screening options for healthy couples contemplating pregnancy are increasingly available. Many genetic disorders are rare and affected children are often born to families where there is no family history of the condition and the parents did not know that they were at increased risk. The main inheritance patterns that can lead to a healthy couple having a child affected by a serious genetic disorder are autosomal recessive conditions and x-linked conditions.

In autosomal recessive conditions, 2 faulty copies of the gene are required, one from each parent in order to be affected by the condition. Both parents possess one normal copy and one faulty copy of the gene and so are healthy carriers for the condition. They have a one in four chance of a child who inherits both faulty copies of the gene and has the condition. They also have a two in four chance of a child who is a healthy carrier of the condition as they are and a one in four chance of a child who is neither affected nor a carrier.

X linked conditions occur when the fault gene is contained within the x chromosome. Males have one X and one Y chromosome and females have two X chromosomes. Because they only have one X chromosome, if a male has a faulty gene on that chromosome, they will be affected by the condition. A female will be a healthy carrier for the condition as they have a normal X chromosome to balance that faulty gene. In women who are healthy carriers for X linked conditions, one in two of their male children will be affected by the condition and one in two of their daughters will be carriers.

It is now possible to screen for many inherited conditions and ideally this testing is done prior to pregnancy. Testing prior to pregnancy allows couples to be aware of the risk they may have of an affected pregnancy and allows them time to consider and discuss their reproductive options. There are many genetic screening panels now available for prospective parents. Some limited panels test for the three commonest inherited genetic conditions: cystic fibrosis, fragile X syndrome and spinal muscular atrophy.

Chance of being a carrier

Chance of having the condition

Cystic fibrosis (CF)

1 in 25

1 in 2500

Fragile X

1 in 250

1 in 4000

Spinal muscular atrophy (SMA)

1 in 40

1 in 6000-10000

It is now also possible to have screening for the small number of common inherited conditions as well as a large number of rare inherited conditions. With these expanded carrier screening panels it is very likely that you will be found to be a carrier for one or more conditions due to the sheer number of conditions being screened for. The chance of you and your partner both being carriers for the same rare condition however remains low. Sometimes it can cause worry or anxiety waiting for test results to understand the risk of a rare genetic condition prior to pregnancy therefore we often discuss doing expanded carrier screening on both partners simultaneously.

Carrier screening is not yet public funded and so there are out of pocket costs for testing. I am happy to discuss the pros and cons of testing to assist you in making this individual decision as prospective parents.

For couples with a known or suspected risk of a genetic condition the testing and reproductive options are different and often discussed in collaboration with a genetic counsellor.