strain

A tune, melody or song.

Example: Alone she cuts and binds the grain, And sings a melancholy strain. – William Wordsworth

In biology, a particular species or type of animal or microbe.

Extreme effort to do some work.

Family or lineage.

You strain when it is difficult to do something and you must work hard to do it.

Example: He strained to see his friend from so far away.