Restrictive vs Non-Restrictive

[rohan]

Unlike transplants between identical twins, whose genetic endowment is the same, all patients receiving hearts of other organs must take antirejection drugs for the rest of their lives.
1. Unlike transplants between identical twins, whose genetic endowment is the same
2. Besides transplants involving identical twins with the same genetic endowment
3. Unless the transplant involves identical twins who have the same genetic endowment
4. Aside from a transplant between identical twins with the same genetic endowment
5. Other than transplants between identical twins, whose genetic endowment is the same

The GMAC explains that in [C] "who have the same genetic endowment" is non-restrictive and is required because the other choice(presumably restrictive) suggest that only some twins have the same genetic endowment.

But I thought non-restrictive clauses have to be separated by a comma.

[RonPurewal]

Unlike transplants between identical twins, whose genetic endowment is the same, all patients receiving hearts of other organs must take antirejection drugs for the rest of their lives.
1. Unlike transplants between identical twins, whose genetic endowment is the same
2. Besides transplants involving identical twins with the same genetic endowment
3. Unless the transplant involves identical twins who have the same genetic endowment
4. Aside from a transplant between identical twins with the same genetic endowment
5. Other than transplants between identical twins, whose genetic endowment is the same

The GMAC explains that in [C] "who have the same genetic endowment" is non-restrictive and is required because the other choice(presumably restrictive) suggest that only some twins have the same genetic endowment.

But I thought non-restrictive clauses have to be separated by a comma.

Choice (C) needs a comma. If this problem is actually from GMAC, it should have one.

Did you see this problem, with your own eyes, in a GMAC source?
If so, please say the name of the source. If it's an OG book, please provide the problem number. If it's GMAT Prep, please post a screen shot.

If you did not see this problem with your own eyes, then you're just copying a second- (or third- or fourth- or 782364th-)hand transcription, would could very easily contain errors in punctuation.

[rohan]

Hi Ron,
The problem was posted on the mba.com. Here is the screenshot of the problem with the explanation

http://postimg.org/image/5tfuqf335/

This is really confusing!

[RonPurewal]

Hi Ron,
The problem was posted on the mba.com. Here is the screenshot of the problem with the explanation

http://postimg.org/image/5tfuqf335/

This is really confusing!

Yep, it's a typo. They meant to put a comma there.

* First, that's what they mean by "non-restrictive" (= NOT narrowing anything down).
I don't generally know these terms, but I figured that out mostly by looking at the word "non-restrictive"... and verifying with google searches. (:

* Second, choice C currently has the same problem that they mention in eliminating choices B and D!

[RonPurewal]

It's reassuring that this problem isn't from the test software itself, which is much more carefully and extensively edited than some random website problem. So, that's good, at least.

More importantly, it's reassuring that they haven't started to test the presence/absence of commas! (B and D can also be eliminated because they compare trasnplants with people.)
GMAC has always avoided testing punctuation"”especially this difference, since many eastern European languages punctuate modifiers in ways that are almost exactly the opposite.

[rohan]

Thanks Ron. I was really confused because there was no distinction in B/D in terms of restrictive non restrictive. This seems like a typo..Although i have seen the same question discussed on the forum before.. Hope its not one of the old OG questions.

[RonPurewal]

Thanks Ron. I was really confused because there was no distinction in B/D in terms of restrictive non restrictive. This seems like a typo..Although i have seen the same question discussed on the forum before.. Hope its not one of the old OG questions.

I'm sure it's not.

I searched for it on the internet, and, interestingly, the comma is correctly placed in some places.
Most of those pages are older"”suggesting that the problem was posted correctly somewhere else, earlier, but improperly transcribed into the page you quoted.