פירוש על קידושין 7:6

"From where do we know [that the verse includes] an unfit seed (i.e. child)?" Rabbenu Yitzhak said: [The midrash] was talking about an unfit grandchild (lit. seed of a seed), for since she had sex to make her unfit, she was made unfit [to marry a man] from the priesthood and she can no longer eat terumah. Therefore it is not possible to explain literally "an unfit child" but "an unfit grandchild".

The midrash here teaches that if a daughter of a priest who is married to a non-priest and then the non-priest died had any offspring, even grandchildren or illegitimate offspring (such as mamzerim) she no longer may eat priestly food such as terumah. The midrash uses the letter “yod” in “eyn” as if the word was written “ayen” which means to “search.” Her lineage must be examined to see if she has any offspring. The reason this midrash is cited is to prove that derashot (rabbinic exegesis) may be based on the letter yod.