because a regular sharingan came about after the brothers eyes thinned
when a uchiha awakens a mangekyou sharingan it gives them more yin chakra
when sasuke awakened the ms his chakra resembled genjutsu genjutsu is yin element
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sasuke felt itachi's power flow through him and his susuano'o both look the same now
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so ms(yin chakra)+ms(yin chakra)=equals the older son's yin chakra and his eyes
you have to have the older son's yin chakra also then add yang chakra to make rinnegan
or the eyes in danzo's arm would have been rinnegan