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Genetics and Inheritance
Genetics and Genetic Testing
Retinal diseases caused by one or more genetic mutations; changes to the amount or structure of DNA, which are passed down through generations are called inherited retinal diseases (IRDs). To date, more than 260 genes associated with retinal diseases have been identified. IRDs are complex retinal conditions because mutations in different genes or different mutations in the same gene can cause the same retinal disease to develop. As a result, this can make discovering specific and effective treatments and cures for IRDs more challenging. This also highlights the importance of genetic testing for guiding effective disease management and treatment selection.
Genetic testing involves analysing DNA, the hereditary material which provides information and direction to our bodies to grow, mature and function. Genetic tests look for changes in an individual’s genes and chromosomes called mutations, to identify the mutated or “faulty” gene(s) responsible for an IRD developing. If your eye doctor (ophthalmologist) suspects you may have an IRD, they will encourage you to receive a genetic test to confirm the diagnosis. Genetic screening is an essential part of our healthcare service, which can inform us about how we can best manage our condition and mitigate its onset. However, there are also many other reasons a person may seek a genetic diagnosis:
- There is a history of vision loss in your family, but the genetic contributors are unknown and you wish to understand the nature and pattern of inheritance for this condition.
- You have concerns that you or your partner has an IRD in your family that might be passed on to your children.
- You want to participate in clinical trials relevant for your genetic diagnosis and have access to suitable treatments.
Note: It is very important to know that an accurate genetic diagnosis is a prerequisite for participation in a clinical trial and in accessing appropriate therapies, should one become available for your condition.
Undertaking a genetic test can be an emotional experience for many, and may cause you to feel anxious and overwhelmed in anticipation of your results. It is vital that you consider the potential consequences that a genetic test will have on you and your family, and that although you receive your genetic diagnosis, there may be no treatment or cure currently available for your condition. Members of Retina International can guide you in the direction of a professional genetic counsellor and qualified doctor to discuss your options, so that you are fully informed about the procedure, benefits and limitations and possible consequences of a genetic test for you and your family.
Read more about genetic testing, how it works and the different types of genetic tests available on our Genetic Testing Services Resource.
Inheritance Patterns
We inherit our DNA from our parents in equal amounts, 23 chromosomes from our mother and 23 chromosomes from our father. Therefore, each individual has 46 chromosomes. Of particular importance are the X and Y chromosomes, which determine the biological sex of each new born child. Males have one X chromosome and one Y chromosome, while females possess two X chromosomes. Each mother contributes one X chromosome, while the father provides either an X or Y chromosome, a process which occurs by chance and determines whether the child is male or female. Understanding the mode of inheritance for a particular disease is crucial so we can accurately predict the likelihood or chance of other family members having the same disease.
In some cases, only one malfunctioning gene copy (dominant inheritance) is required, while in other circumstances, two copies (recessive inheritance) of the mutated gene are necessary for the disease to develop. The location of the mutated gene on a particular chromosome (X-linked inheritance) can also determine who will be primarily affected by the disease. In more complex cases, our genes and environment can interact and work in combination to cause a genetic condition to develop.
Autosomal dominant inheritance describes a form of disease transmission through generations whereby one copy of the gene mutation is sufficient to cause an individual to be affected by the disease. The condition arises and expresses itself even though the second copy of the gene present is normal.
In autosomal inheritance, the gene mutation is not located on the X or Y sex chromosomes and thus males and females are equally affected. Each individual affected by autosomal dominant inheritance has a 50% chance of passing the condition to their child.
This short audio-visual by Genomics Education Programme helps to explain the pattern of Autosomal Dominant Inheritance:
In autosomal recessive inheritance, two mutated or malfunctioning copies of the gene must be present for the disease to develop in an individual. This means that the individual receives one mutated gene from their mother and one from their father. People who contain one copy of the mutated gene are not affected by the disease but are referred to as “carriers” and have the potential to pass this mutated copy to their children.
Typically, only one generation is affected and both males and females are at equal risk of the disease. If both parents are carriers for the mutated gene:
- There is a 25% chance their child will not receive any mutated copy of the gene and will not be affected or a carrier.
- There is a 50% chance that their child will be a carrier of this mutated gene copy but will not be affected.
- There is a 25% risk that their child will receive two mutated copies, one from each parent and will have the condition.
This short audio-visual by Genomics Education Programme helps to explain the pattern of Autosomal Recessive Inheritance:
X-linked recessive inheritance means that the mutation is located on the X-chromosome and primarily affects males as a result. This is because males receive one X chromosome from their mother and one Y chromosome from their father. Thus, if the X chromosome inherited from their mother has the mutated gene copy, they will develop the condition. For females to be affected, both of their X chromosomes must have the mutated gene; one from their affected father and one from their carrier mother. However, due to the phenomenon of non-random X chromosome inactivation, female carriers may display some symptoms of the disease.
In families where X-linked recessive diseases inheritance exists, multiple generations of affected males are observed, connected through females who are carriers and largely unaffected. All daughters of affected males will be carriers for the disease and none of his sons will be affected. When a female is a carrier, each daughter has a 50% chance of being a carrier and each son has a 50% chance of being affected.
These short audio-visuals by Genomics Education Programme help to explain X-linked recessive inheritance from the perspective of a carrier mother and affected father.
X-linked recessive inheritance – where the mother is a carrier for a condition.
X-linked recessive inheritance – where the father has the condition.
Mitochondrial inheritance, also referred to as maternal inheritance applies specifically to genes contained within mitochondrial DNA. Mitochondria are structures found within cells and responsible for producing energy for the cell. Egg cells and not sperm cells contribute mitochondria to a developing embryo and explains why only females can pass mitochondrial mutations to their children. These conditions can affect both males and females and can appear in every generation of a family, but fathers do not pass down these conditions to their children.
The severity of symptoms may vary from one affected individual to another; even within the same family due to a ‘dosage’ effect. This is due to the fact that we have many mitochondria in each cell. In one individual, if only a small proportion of mitochondria in each cell have the mutation, symptoms will be mild. In another individual, if a higher proportion of mitochondria in each cell carry the mutation, symptoms will be more severe.
This short audio-visual by Children’s national Hospital Rare Disease Institute helps to explain the phenomenon of Mitochondrial inheritance: Mitochondrial Inheritance
Very often, a person diagnosed with an inherited condition has no other family members with the disease. There are several possible reasons for only finding one person affected in the family. The mutation may be a new event in that person, or other family members may have the same mutation, but may have not yet been diagnosed with the condition. Alternatively, other family members may have a later age of onset, or have milder signs of the disease.
The mutation might also have been in the family for a long time, but by chance, no other family members have been affected. For autosomal recessive disease, carriers may have been present in the mother’s and father’s side of the family for several generations, but a child won’t develop a condition unless both parents are carriers and both pass on a mutation to their child.