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Why does the recurrence risk for a multifactorial disorder depend on the number of affected relatives?

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BRCA2 tumor suppressor gene on chromosome 13 — human genetic disease Photomicrograph of the *BRCA2* tumor suppressor gene on chromosome 13 of the human genome. Inactivation of this growth-regulating gene is associated with a higher risk of developing breast cancer.

In the context of human genetic disease, multifactorial disorders arise from the interaction of multiple genes and environmental factors. Because the specific genetic and environmental contributors are often unknown, recurrence risks are determined empirically rather than through simple Mendelian ratios.12

The recurrence risk increases with the number of affected relatives because each additional affected family member provides evidence of a higher concentration of predisposing genetic factors within that specific family lineage. For example, while the empirical risk for a future child after one affected sibling is typically around 3 percent, this risk rises to approximately 10 percent if two children are affected.12

Essentially, having multiple affected relatives suggests that the family carries a greater "genetic load" or threshold of susceptibility, making the manifestation of the disorder more likely in subsequent offspring. This principle also applies to the severity of the condition; close relatives of individuals with more severe forms of a disorder are generally at a higher risk than those related to individuals with less severe presentations.12