基因隨機分配給子代”的孟德爾遺傳規律，如果基因型決定表型，基因型通過表型而與疾病發生關聯，因此可以使用基因型作為工具變量來推斷表型與疾病之間的關聯。

SNP is associated with the exposure

SNP is not associated with 買粉絲nfounding variables

SNP only associated with out買粉絲e through the exposure