Probability of Donatello being from a different clutch of eggs.

[kelligrafie]

The brothers look different for a few reasons.

Firstly because in the comics where they all had red masks, without speech bubbles where they addressed one another by name, and weapons, I believe it was pretty impossible to tell them apart. Remove the coloured masks (& weapons) in '87 and you have the same problem.

Secondly, and I don't know if it was intentional, but I get the feeling that by making the turtles look different, you're kinda sending out the message to kids that heroes come in all shapes and sizes.

I don't actually find the concept of them not being biologically related that unfathomable. As you correctly state, it doesn't change how they feel about each other. In the same way that they regard Splinter as 'father', even though they originate from opposite sides of the plants.

The reason I don't think the writers would explore it however, is because a similar theme is already getting looked at via Karai/Miwa.

PS My brother and I don't really look alike. He has brown eyes, I blue. He has brown hair that originated from being blonde, I've always been brunette. He tans, I stay like a milk bottle. I've worn glasses since I was five, he's only just started using specs and that's only for reading. Same mother and father.