In the general population in the UK, about 1 in 600 people has MS.
MS is not considered hereditary. A hereditary condition is directly transmitted from parent to child or further generations, which is not the case in MS.
One of the elements that combine to cause an individual to develop MS is a genetic susceptibility to the condition. More than 230 genes have been found that each increase the risk of developing MS to a small degree. None of them directly causes the condition itself and someone with MS will have a combination of many of these genes.
As family members will share some of the same genes, this does mean that there is a higher risk of developing MS if there is already MS in the family. However, there is no certainty that the particular combination of genes that make a person in one generation susceptible will occur in subsequent generations. Many people diagnosed with MS do not have a family history of the condition.
Whilst the rate of MS within families suggests that there is a genetic factor involved in developing the condition, studies of identical twins show that genes are not the whole story. Identical twins have exactly the same genetic make up as each other. If MS were solely dependent on genes, it would be expected that if one identical twin developed the condition, so would the other, which isn't the case.
Studies have shown that, whilst still small, the risk of developing multiple sclerosis is higher for people who already have someone with MS in their family.
An Australian study published in 2013 combined the results of 18 previous studies to get a better estimate of the risk of MS in family members. In families where one member was already diagnosed with MS, the risk was highest for more closely related members.