Bipolar disorder is a mental disorder that is highly heritable, meaning it is usually passed down from a family member. Bipolar is also found to be heterogeneous and polygenic, meaning it is influenced by two or more genes. This disorder is also found to have a genetic overlap with other psychiatric disorders. Bipolar can be described as alternating manic and depressive episodes. Along with these episodes, there are psychotic symptoms that can occur that lead to hospitalizations. There are different types of bipolar disorder, each showing different cycling in these states of mind. Bipolar 2 can be based on depressive episodes and hypomanic episodes but not true mania. Bipolar can also be associated with schizoaffective which includes more psychotic episodes. There are many different ways this disorder can come out in different people. This image shows the range of symptoms for each type of Bipolar and depressive disorder.
BD (bipolar disorder) is often comorbid with other psychiatric and non-psychiatric disorders as well. Looking into the genetics of BD, there were a few studies done to establish if family history is an important factor in the heritability of this disease. There was a large family study done where over two million individuals were involved, the results showing that the heritability was almost 50%. BD was found to be transmitted homogenous across the family. When looking at the twin studies for heritability it was found to be between 60-90%. Looking at these numbers it was concluded that the heritability of BD is one of the highest of all the psychiatric and behavioral disorders. This article goes into the classic genetic epidemiology of BD as well as molecular genetic epidemiology. The genetic studies done bring light to how this is inherited as well as why.