Mishnà su Levitico 13:31

Yellow thin hair causes uncleanness whether it is clustered together or dispersed, whether it is encompassed or unencompassed, or whether it came after the scall or before it, the words of Rabbi Judah. Rabbi Shimon says: it causes uncleanness only when it came after the scall. Rabbi Shimon said: Is it not logical: if white hair, against which other hair affords no protection, causes uncleanness only when it comes after the nega, how much more should yellow thin hair, against which other hair does afford protection, cause uncleanness only when it comes after the scall? Rabbi Judah says: Whenever it was necessary to say, "it turns" Scripture says, "it turns." But the scall, since about it Scripture says, "there is no yellow hair in it," it causes uncleanness whether it came before or after it.