Your symptoms and family history will give some indication of whether it takes only one faulty copy of a gene to cause your sight loss, or whether both copies have to be faulty. This in turn dictates the chances of your children inheriting the condition.
Understanding your family’s inheritance pattern is not the same as having a genetic diagnosis, although it is not always possible to confirm the inheritance pattern in a family without a genetic test.
The risk to children is often one of the most important issues for families affected by inherited sight loss conditions. The way in which a genetic condition is passed down through the generations is often called the inheritance pattern. For example, in some families, the condition might appear at least once in every generation. In others, it might appear to crop up out of the blue, but it has still been caused by an inherited genetic fault.
Why genes matter explained that we all have two copies of each gene. The inheritance pattern mostly depends on whether it takes only one faulty copy of a gene to cause sight loss, or whether both copies have to be faulty. Inherited retinal conditions can result from either one faulty gene copy or two, depending on which genetic mutation is involved and what its knock-on effects are, so different affected families can experience different inheritance patterns.
In the clinic, your ophthalmologist, genetic counsellor or another healthcare professional might spend some time asking about any sight loss affecting other people in your family to try to work out the inheritance pattern. Your symptoms and family history may give an indication of whether it takes one or two faulty copies of the gene to cause sight loss, but it isn’t always clear without genetic testing.
It is useful to build a picture of your family tree even if no one else is affected as far as you are aware, as other family information may be relevant, such as general medical problems. It also gives a picture of who might be affected by implications of a genetic test result. Whoever asks you for this information should handle the discussion with sensitivity and will understand that our families come in all sizes & shapes, or that we might not know much about them e.g. if we are adopted. Family information will always be kept confidential.
Types of inheritance patterns
Inheritance patterns are described in terms such as autosomal dominant, autosomal recessive or X-linked. Knowing the inheritance pattern in your family is not the same as having a genetic diagnosis, which identifies the individual named gene involved and can only be discovered through genetic testing. However, a genetic diagnosis can confirm the suspected inheritance pattern so that you can be more certain of the risk to other family members (see Genetic testing).
Autosomal dominant
This means that the faulty gene is on an autosome (any chromosome other than the sex chromosomes; see Why genes matter) and that only one faulty copy is needed to cause sight loss. If you have an autosomal dominant condition, there would be a 50% chance of you passing the condition on to each of your children. Just like tossing a coin and getting heads twice in a row, the chances do not always balance out in families; if you have two children, it’s possible that they could both be affected, one could be affected, or neither of them could inherit the faulty gene.
Some families affected by autosomal dominant conditions will already be aware of the gene running in their family as they may have an affected parent, affected siblings or affected aunts, uncles and cousins. However, this is not always the case. In some individuals, an autosomal dominant gene change may start for the first time without any family history.
More information is available from: Autosomal dominant inheritance.
Autosomal recessive
This means that the faulty gene is on an autosome (any chromosome other than the sex chromosomes; see Why Genes Matter) and that both copies need to be faulty to result in sight loss. Autosomal recessive conditions usually occur when the affected person inherits one faulty copy of the gene from each of their parents. Individuals with one faulty copy of an autosomal recessive condition are called carriers. Having one faulty copy doesn’t cause any symptoms and the parents are usually not aware that they are carriers. The condition may therefore have seemed to crop up out of the blue in the family, having not appeared at all in previous generations.
If you are living with an autosomal recessive condition, you will pass one faulty copy of the gene on to each one of your children. However, your children will usually inherit a working copy of the gene from your partner, unless your partner is also affected or known to be a carrier of this condition. This means that your children are likely to be unaffected carriers (like your parents). More information is available from Autosomal recessive inheritance.
X-linked
This means that the faulty gene is on the X chromosome (sex chromosome). X-linked conditions affect men far more frequently than women. Women have two X chromosomes, so even if one of them contains a faulty gene, its healthy counterpart on the other X chromosome can often compensate. Women sometimes experience no symptoms, or milder symptoms than men, but have a 50% chance of passing one copy of the faulty gene on to both sons and daughters. Sons who inherit the faulty gene will go on to develop sight loss.
Men only have one X chromosome; if this contains a faulty gene there is no second copy to compensate, and they will definitely develop sight loss. Affected men cannot pass the faulty gene on to their sons, but they will pass it to their daughters, who will be carriers and will probably experience mild or no symptoms. However, some women with an X-linked condition may experience severe symptoms.
Where X-linked inheritance occurs in retinitis pigmentosa, the condition is sometimes referred to as X-linked RP, or XLRP. Choroideremia is also an X-linked condition.
More information is available from X-linked inheritance