Bipolar Disorder, mental illnesses in general, any health conditions, and just about any physical or mental trait imaginable are all weighed against three primary factors: the mother's genetics, the father's genetics, and the experiences of the person in question.
This being said, one can never say that any trait or condition can absolutely be passed down from parent to child, save for diseases such as AIDS, where the active ailment is able to affect the baby en-utero. But most conditions have a delayed development as it is passed down to the next generation.
With the genetics of both parents there is a spectrum effect as the two sets of DNA mix and form the new child's DNA. Say, for instance a woman has a severe case of Bipolar disorder, which her mother had as did her grand mother and possibly even her great-grandmother. Her partner does not have Bipolar Disorder, and has a fairly resilient personality against mental trauma.
What we can expect to find is that when the couple conceives a child there is a greater chance that if that child is a girl she will be Bipolar. But genetics only holds so far as giving a given person a disposition toward any trait or condition albeit sometimes the disposition is overwhelming, and the child then ends up with that condition. But the father's genetics, regardless of if the child is a girl, still has a say in the child's health. Because the father is generally healthy, the Bipolar passed down from the mother is "diluted".
Symptoms are prone to being less severe and appear less often thanks to the father's DNA. And if the father's genetics are stronger than the mother's, the child might even end up with only a moody personality. As with any health condition, birth is the key event that tells the rest of the world whether the child will start with a condition or just a predisposition towards it.
With Bipolar Disorder, if the genetics are strong enough the child begins their life with a variation of the disorder known as Early Onset Bipolar. In this condition the child starts showing less predictable symptoms as early as infancy. Usually the baby is more active and lacks a set sleeping schedule. As the baby grows into a toddler, the active infant grows into a raging child. Raging, a symptom in mania in teens and adults, becomes the predominant behavior in young children. Depression usually only starts up around the time the child goes through puberty.
If the child is not born with Early Onset Bipolar, they go into life with a greater risk of becoming Bipolar later on. In the case of our example, the young girl has several life factors influencing her mentality. First, since her mother is Bipolar, there is a greater chance of traumatic events occurring throughout her childhood.
It becomes the father's place to provide a certain amount of stability that the mother's often changing moods may not allow for. The father's personality may also provide help via the instilling patience and resilience against stress. With the right type and/or amount of stress/trauma in a child's life they may cross the line between mental stability and becoming Bipolar.
Can Bipolar Disorder be passed down genetically? In more severe cases, where either one parent is severely Bipolar and/or has a longstanding family history of Bipolar Disorder, or both parents have the disorder. Genetics alone, however, cannot guarantee a child being Bipolar.. With only a disposition, a child may gain the disorder via experiencing the right types and/or amounts of stress and trauma in their life.