This sounds like a scientifically worded question that simply asks: What is the chance that the indentical twin of a person with an affective disorder, could also have the same problem?
Are you asking for an actual statistical reading of this phenomenon? I don't know that it even matters to one suffering, even if it has been studied. Most people with sadness that won't lift, or any affective disorder don't give a rats tail about statistical analysis. The important thing for anyone feeling awful, is to get help! For identical twins? I don't know about "concordance rates"; But genetically, they are at as much disadvantage as anyone else is who has any affective disorders in their family tree. However, just because one twin is clinically ill, it doesn't mean the other will be too..I hope it isn't you, God bless..