Last Reviewed 21/07/2016
Genetic conditions are caused by one or more faulty genes. You can learn more about what genes are and how they are arranged in our cells here. Depending on how the faulty gene works and on which chromosome they are located, they can be inherited in different ways. Here we explain how autosomal dominant, autosomal recessive, xlinked and mitochondrial inheritance can affect a family.
Autosomal dominant conditions
These conditions are passed down from generation to generation. Only one copy of the faulty gene is needed to cause the condition. If one member of a couple is affected by one of these conditions then they will have a 50% (one in two) chance of having a child affected by this condition. Each of their children will have a 50% chance of inheriting the faulty gene. Unfortunately, just like tossing a coin and getting heads twice in a row, the chances do not always balance out in families.
Rarely, both members of a couple might be affected by the same dominant condition. This gives the chance that a pregnancy might inherit two copies of the faulty gene. For most dominant conditions, the condition caused is so serious that the pregnancy will not survive. Others will be very severe versions of the autosomal dominant condition that affects their parents. For these conditions, you either have the condition, and you can pass it on, or you don’t have the condition. The majority of families affected by these conditions will know them well, as any person who is affected will also have an affected parent, and may have affected siblings or aunts and uncles. When family members come to decide about having children, they are usually in an informed position as to their risk, or at least will be aware that they may have a risk.
In some cases though, a gene can become faulty in the process of making sperm or eggs. These are called de novo (latin for ‘anew’) cases of the condition. In this case, having an affected child will ‘come out of the blue’ to families, just like autosomal recessive conditions below.
What does autosomal mean? This means that the faulty gene causing the condition is on one of our chromosomes that do not decide whether we are male or female. We call these autosomal chromosomes.
What does dominant mean? This term is used here to show that only one copy of the faulty gene is needed to cause the genetic condition.
Autosomal recessive conditions
These conditions occur only when both parents of a child are carriers of the condition. Two copies of the faulty gene are needed to cause the condition. A carrier of a condition has just one copy of a faulty gene. They will usually be unaffected and will therefore not know that they are at risk of having an affected child. Conditions like this ‘come out of the blue’ to families. It is only once a couple have had one affected child that they discover they are at risk of having more affected children. They will have a 25% (one in four) chance of having a child affected by this condition. Additionally, there is a 50% (two in four) chance that their child will be an unaffected carrier.
There are some special situations that apply to autosomal recessive conditions. These are when one parent is affected a condition that their partner is an unaffected carrier for, or when both parents are affected by the same recessive condition. In these cases the above explanations don’t apply. There are more copies of the faulty gene, so the chances of having an affected child are raised. The chances are 50% (two in four) when one parent is affected, and when both parents are affected, there are no copies of the gene that are not faulty, so every child will have the condition.
As families will usually not have any experience of autosomal recessive conditions, they are unlikely to be aware of the risk, at least in their first child. This means that it can take some time to diagnose these conditions in children, as healthcare professionals need to understand the symptoms of the condition and select the appropriate test to make a positive diagnosis. Parents can struggle to find adequate information about the condition quickly enough. Our member groups can help provide information about these conditions.
What does autosomal mean? This just means that the faulty gene causing the condition is on one of our chromosomes that do not decide whether we are male or female. We call these autosomal chromosomes.
What does recessive mean? This term is used here to show that just one copy of the faulty gene is not sufficient to cause the genetic condition, both copies of the gene need to be faulty to cause the condition.
X-linked recessive conditions
These conditions occur in boys and men. Their mothers are usually unaffected carriers and will therefore not know that they are at risk of having an affected son. Each of a couple’s sons has a 50% (1 in 2) chance of being affected by the condition, and so each child has a 25% (1 in 4) chance of being affected, once we take account of the girls. Depending on family histories, some couples can be aware that there has been a history of the condition in the family, perhaps there is an affected brother or uncle; for other couples this condition can come with no warning.
What does X-linked mean? This means that the faulty gene is on the X chromosome. Men and boys have one X and one Y chromosome.
What does recessive mean? This term is used here to show that women, who have two X chromosomes will not be affected, as one copy of the faulty gene is not sufficient to cause the genetic condition. There are rare instances though of girls being affected by these conditions. We do not fully understand the causes for this yet.
X-linked dominant conditions
These conditions behave very similarly to autosomal dominant conditions [LINK]. The main difference is that the faulty gene is located on the X-chromosome. The outcomes for families are the same though. There is a 50% (one in two) chance that a child is born with the condition.
In this case, it is important which member of a couple is affected. If it is the father, then any female child will be affected by the condition. This is because it is the father’s X and Y chromosomes that decide whether the child will be a boy or a girl. If the mother is affected then there is a 50% (1 in 2) chance of any child being affected, whatever their sex.
There is a very small amount of our DNA that is stored in a different part of our cells from the rest of the DNA. Instead of the nucleus, this DNA is stored in mitochondria, and is called mitochondrial DNA. We all get our mitochondria from our mother, and all of the mitochondria we have reproduced from the mitochondria that were in the egg cell that was fertilised to create us. If some of those mitochondria had a faulty gene in their DNA, then we will have that gene in all of our cells.
Conditions that are caused by faulty genes in mitochondrial DNA can affect all children from a couple where the mother has a faulty gene in her mitochondrial DNA. Whether the child gets the condition or not depends on the proportion of mitochondria with a faulty gene that they inherit. For some couples this means some of their children will be unaffected, but for other couples all of their children will be affected by the condition caused by the faulty gene.
What does mitochondrial mean? This means that the inheritance depends on mitochondria. These are small things in our cells that produce energy and have a tiny bit of DNA in them. A single one is called a mitochondrion.