GMAT Sentence Correction (SC) | Re: Medical researchers, who have identified a genetic

Medical researchers, who have identified a genetic abnormality in parents of children with Down's syndrome, which they believe, instead of the age of the mother, may be the cause of this congenital birth defect.

Error:
1) comma+which refers to the noun just before it and thus in this sentence comma+ which incorrectly refers to Down's syndrome. Comma+ which must refer to genetic abnormality.
2) Also this sentence is a fragment as there is no verb for medical researchers.
3) Correct idiom is: believe
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