What are single gene conditions?

What causes single gene conditions?

In your DNA, there are around 20,000 genes. Your genes give you traits such as your hair and eye colour. They also control how your body works. If there’s a change in your genes, it might mean the gene doesn’t work properly and this can affect your health.

An example of a single gene condition is cystic fibrosis. It’s caused by a change in the CFTR gene. This gene codes for the CFTR protein, which is a channel found on the surface of cells that line your airway and gut. The CFTR channel is important for water moving in and out of cells, which helps keep mucus flowing. In cystic fibrosis, a problem with the CFTR gene affects how this channel works. One of the effects of this is a build-up of thick mucus in your airways and other parts of your body.

Another single gene condition is sickle cell anaemia. It’s caused by a change in the HBB gene, which leads to abnormal haemoglobin. Haemoglobin is found in red blood cells and is needed to transport oxygen from your lungs to the rest of your body. In sickle cell anaemia, abnormal haemoglobin causes red blood cells to be misshapen and stiff, so they become clustered and block blood flow.

Can single gene conditions be inherited?

Single gene conditions can run in families. You inherit one copy of a gene from each of your biological parents. In certain single gene conditions, one or both genes from your parents may cause you to have a genetic condition.

Recessive and dominant conditions

Cystic fibrosis is known as an autosomal recessive condition. If you have two altered CFTR genes, you will have the condition. But if you have one altered CFTR gene, and one normal CFTR gene, you won’t be affected. However, you will still be a carrier for the altered gene. Sickle cell anaemia is also recessive.

Other single gene conditions can be autosomal dominant. This is when having just one copy of the altered gene means you’ll be affected, even if the other copy of the gene is normal. The neurological condition Huntington’s disease, for example, is an autosomal dominant condition.

X-linked conditions

Single gene conditions can also be X-linked. This means the altered gene is on the X chromosome. Chromosomes are tightly packaged long strands of DNA. You usually have 46 chromosomes in your cells, 23 from each parent. Your sex chromosomes are called X and Y chromosomes. Males have one X chromosome and one Y chromosome, and females have two X chromosomes.

An example of an X-linked recessive condition is haemophilia, where your blood doesn’t clot properly. X-linked recessive conditions are passed on via the X chromosome. But if you have another X chromosome without the altered gene, you normally don’t get the condition. This is why males mainly get affected by X-linked recessive conditions such as haemophilia, as they only have one X chromosome.

How to test for a single gene condition?

You can have a genetic test if you have symptoms and your doctor thinks there might be a genetic cause. If you have a family history of a condition, you may be offered a genetic test depending on the condition and if testing is appropriate. Genetic testing (also called genomic testing) involves taking a DNA sample, for example your blood or saliva. This is sent to a lab where it is analysed.

In the UK, there is a newborn screening programme for 10 genetic conditions. This involves a heel prick test. It screens newborn babies for single gene conditions including cystic fibrosis, sickle cell, and some metabolic disorders.

If you’re planning on having children, you and your partner may want to think about carrier testing. This can show whether you carry an altered gene for an inherited condition that could be passed on to your children. You can then decide what you want to do next. One possible option is preimplantation genetic testing (PGT). This involves making embryos outside the body through IVF. The embryos are then tested for the condition. Embryos that are not affected by the condition are selected and transferred to the womb (uterus).