A 32-yrs-old woman is diagnosed with major depression. What is the chance that her identical twin sister will develop the same disease?
**Core Concept**
The question is testing the concept of heritability of major depression, specifically the genetic contribution to the risk of developing the disease. Major depression is a complex disorder influenced by both genetic and environmental factors.
**Why the Correct Answer is Right**
Identical twins share 100% of their genes, and studies have shown that if one identical twin has major depression, the other twin has a significantly increased risk of developing the same disease. This is because identical twins are more likely to have similar genetic predispositions, environmental exposures, and shared lifestyle factors that contribute to the development of major depression. The concordance rate for major depression in identical twins is estimated to be around 70-80%.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the concordance rate for major depression in identical twins is not 0%. While there is no guarantee that the identical twin sister will develop the disease, the risk is significantly higher than 0%.
**Option B:** This option is incorrect because the concordance rate for major depression in identical twins is not 100%. While there is a strong genetic component, environmental and lifestyle factors also play a role in the development of the disease.
**Option C:** This option is incorrect because the concordance rate for major depression in identical twins is not 50%. While there is a significant genetic component, the risk is not evenly split between the two twins.
**Clinical Pearl / High-Yield Fact**
The heritability of major depression is estimated to be around 40-50%, indicating that while there is a strong genetic component, environmental and lifestyle factors also play a significant role in the development of the disease.
**Correct Answer: A. 70-80%**