What is genetic disease?
In this article we will introduce the topic of genetic disease – what it is, how important it can be medically, how common it is and, most importantly, if there is anything you can do to prevent it. Before we discuss further details, it is essential to explain a few commonly used terms that will help us to understand the topic better: the language or jargon of genetics.
The jargon of genetics: is it really that complicated?
Many people are wary of the countless acronyms and confusing technology associated with genetics. I am going to try to focus on a few key ideas using minimal jargon – but there are some terms that it really helps to understand:
- DNA – you can think of DNA as the code that tells your cells how to build your entire body
- Chromosome – a chromosome is a structure that houses large stretches of your DNA. Healthy human beings have 23 pairs of chromosomes – 46 in total
- Sex chromosome – one of the 23 pairs (2 of your 46 chromosomes) is made up of two ‘sex chromosomes’. The two types of sex chromosomes are X and Y – if you have two X chromosomes, you are female. Males have one X and one Y chromosome
- Autosome – the remaining 22 pairs (44 chromosomes) that are not sex chromosomes – these are numbered 1-22 in order of decreasing size
- Gene – a sequence of DNA that contains (codes for) a specific instruction for your cells. You have two copies of each gene – one on each chromosome in a pair. You inherit one gene from each parent.
- Genome – your full, 3-billion-letters-long DNA sequence
- Mutation – a change in the DNA sequence that may have a negative medical consequence in specific circumstances
The diagram to the right shows how a chromosome can be thought of as ‘rolled up’ DNA, which is divided into sections – genes – that code for a specific function:
What is genetic disease?
As we have discussed, each gene codes for a specific function in your body. Genetic diseases are caused by mutations to one or more genes. Whilst genetic diseases caused by mutations to a single are individually rare, they cumulatively affect 1% of individuals1; this figure is much higher when polygenic (multiple gene) disorders are also included.
Cystic fibrosis (CF), for example, is the most common genetic disease in the UK – CF is caused by a mutation to a specific gene called the CFTR gene. The disease itself causes breathing and digestion problems, requires significant and costly medical intervention and can severely reduce the lifespan of affected individuals. Though most CF cases are caused by one specific mutation to the CFTR gene, over 1500 different causative mutations have been identified. This is why a simple test looking at only a single mutation may not be suitable in all cases.
Types of genetic disease
Cystic fibrosis is an example of a type of genetic disease called autosomal recessive (AR) diseases. These AR diseases:
- Are caused by mutations in genes that are present on autosomes
- Only have an effect on you if both of your copies of the gene(s) are mutated
This is a key point for recessive conditions:
If you have two copies of the mutated gene (e.g. a CFTR mutation) then you will have the condition (e.g. Cystic fibrosis)
If you have only one copy of the mutated gene (e.g. a CFTR mutation) then you will not be affected by the condition (e.g. Cystic fibrosis) – instead you are a carrier of the disease.
Additional types of genetic disease exist, including
- Autosomal dominant (AD) – to be affected by the condition you only need a single copy of the mutated gene (e.g. Huntington’s disease or BRCA1 – the so-called ‘breast cancer’ gene)
- X-linked recessive & X-linked dominant – caused by genes on the X (sex) chromosome. The criteria for being affected/a carrier of these conditions is more complex, as males have only one X chromosome
- Other methods of inheritance – including polygenic diseases (e.g. genetic influences in heart disease and diabetes)
What does it mean to be a ‘carrier’?
The idea of being a ‘carrier’ of a genetic disease can be tricky to understand, so I want to reiterate the key points:
- If you have only one copy of the mutated gene for a recessive condition, you are not affected by the condition, you are a silent carrier of the disease
- To be affected by a recessive condition you must have two copies of the mutated gene
Being a carrier of a genetic disease generally has no impact on your health. It does, however, have the potential to impact any offspring that you may have. To understand this, we need to think back to how inheritance works:
When humans reproduce, the offspring inherits two copies of each gene – one from the mother and one from the father.
Therefore, if both parents are carriers of the same specific genetic disease, then there is a chance that the child will inherit two mutated copies of the gene – one from each parent.
This pattern of inheritance of autosomal recessive conditions from carrier parents can be visualised as follows:
For diseases caused by a single gene, this method of inheritance can also be explained using percentage probability. The following table highlights the probability of offspring being: affected by the condition; carriers of the condition; or neither affected nor carriers. I have included data for situations where at least one parent is a carrier, but neither are themselves affected:
If either parent is affected by the condition (and therefore has two copies of the mutated gene) then it is advisable to seek medical advice before trying to conceive as the chance of having an unaffected child may be significantly lower.
How can I find out if I am a carrier?
Carriage of genetic disease is silent – it will generally have no direct effect on you and your health. The only way to find out your so-called ‘carrier status’ is to undertake genetic testing – this is referred to as carrier screening.
Carrier screening is typically carried out on a sample of your blood or saliva (which contains cheek cells). This testing uses advanced techniques to sequence your DNA and identify mutations in your genes, which may cause genetic disease.
Whether or not you have any history of genetic disease in your family, tests are now available that are able to identify important mutations in thousands of genes. By finding out the carrier status for you and your partner, you can quickly and easily identify genetic risk to your future child and this information can help you to make informed choices about your particular reproductive journey.
If you would like more information about carrier screening offered by Igenomix, then please do get in touch with our specialist team.
- 1 – Hoyert DL, Freedman MA, Strobino DM, Guyer B (2001) Annual summary of vital statistics: 2000. Pediatrics 108:1241– 1255
- Image – Chromosome and Gene (Captions altered) – Cancer research UK
- Image – Autosomal Recessive Inheritance (Unaltered) – Kashmiri, based on earlier work by Domaina – This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.