as作不同连词时的用法

as在引导一个倒装句描述相同情形时，如John is tall, as is his mother.，此时的as是作为一个并列连词，连接两个主句吗？

另外，假如as此时要做关系代词，那样此句是不是应该是John is tall, as his mother does.?

as永远都不可能是并列连词。

as在本句引导比较分句，对主句的表语作补充说明。

as不是关系代词，而是表示比较意义。

John is tall, as his mother is. 用does为错误。

约翰个子高，和他妈一样。这是比较分句。as为比较代词。

John is tall,which his mother is too.

约翰个子高，他妈也个子高。这才是关系分句。which为关系代词。