Sometimes the broken protein made from a changed gene (mutation) gets in the way of the working protein from the other copy or having a reduced amount of the working protein is enough to cause symptoms. In these cases, people will have symptoms even if they have inherited a fully working gene from one of their parents. The genetic disease might be described as ‘dominant’ because everyone who has the changed gene will have symptoms. This means that an affected parent may pass on either their broken version of the gene or the perfectly working version. Their children may have a 50:50 (like tossing a coin) chance of inheriting the broken gene and the symptoms.  

Sometimes it’s not quite so simple and a ‘carrier’ may have very mild or slightly different symptoms while someone with two broken copies of a gene may have a very severe disease.