All are fair games for APOE4 carriers who are desperate to ward off the frightening tumble into dementia that they’ve seen afflicts far too many of their relatives.

 that they’ve seen afflicts far too many of their relatives.为嵌入式定语从句。即定语从句本身是个主从复合句，关系代词在不是主句的成分，而是从句的成分。本句中，关系代词that是定语从句中的宾语从句的主语。

有的语法书将这种嵌入式定语从句的主句分析为插入语。这种分析似乎比较简单，但对有些从句则无法解释。所以我建议网友按照嵌入式定语从句理解。