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. There are some special situations that apply to autosomal recessive conditions. Additionally, there is a 50% (two in four) chance that their child will be an unaffected carrier. They will have a 25% (one in four) chance of having a child affected by this condition.

It is only once a couple have had one affected child that they discover they are at risk of having more affected children. Conditions like this ‘come out of the blue’ to families. They will usually be unaffected and will therefore not know that they are at risk of having an affected child. A carrier of a condition has just one copy of a faulty gene. Two copies of the faulty gene are needed to cause the condition. These conditions occur only when both parents of a child are carriers of the condition. 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. 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. In this case, having an affected child will ‘come out of the blue’ to families, just like autosomal recessive conditions below. These are called de novo (latin for ‘anew’) cases of the condition. In some cases though, a gene can become faulty in the process of making sperm or eggs. 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. 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. For these conditions, you either have the condition, and you can pass it on, or you don’t have the condition. Others will be very severe versions of the autosomal dominant condition that affects their parents. For most dominant conditions, the condition caused is so serious that the pregnancy will not survive. This gives the chance that a pregnancy might inherit two copies of the faulty gene. Rarely, both members of a couple might be affected by the same dominant condition. Unfortunately, just like tossing a coin and getting heads twice in a row, the chances do not always balance out in families.

Each of their children will have a 50% chance of inheriting the faulty gene. 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.

Only one copy of the faulty gene is needed to cause the condition. These conditions are passed down from generation to generation. Here we explain how autosomal dominant, autosomal recessive, xlinked and mitochondrial inheritance can affect a family. Depending on how the faulty gene works and on which chromosome they are located, they can be inherited in different ways. You can learn more about what genes are and how they are arranged in our cells here. Genetic conditions are caused by one or more faulty genes.

Mode of inheritance