Hi Folks

Please clarify one doubt.

This is a normal grammatical issue not a SC question .So no Source

People who inherit the sickle cell anemia gene from only one parent seem to be resistant to Typhoid, a hereditary advantage that may explain why a genetic condition such as this has survived in the human population.

Now is it the correct usage

Is the modifier "a hereditary advantage...." modifying Typhoid , as modifier here is touching the noun typhoid

Please help me
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nope, it's modifying the entire description of the advantage (i.e., the resistance to typhoid).

this is an example of what is called an appositive noun modifier (if you care about linguistic terms; you don't have to know what these things are called, as long as you know what they look like and how they're used). these modifiers have more flexibility than relative pronouns, and that's why the gmat likes them so much.

if you have a copy of the o.g. verbal supplement, check out 'a shortcoming' in problem #59 (note: please don't post any details about the problem here). that modifier functions in the same way as the one you've posted here.

another example:
the coach tried to put five wide receivers on the line for third down, a strategy that failed when his quarterback was sacked.
note that the strategy is that of putting 5 receivers on the line (i.e., third down isn't the strategy).

the reason this construction may seem awkward or difficult for you is that it never appears in spoken language. a SPEAKER, SAYING the above sentence, would say something like '...for third down, which failed...' or '...for third down, but it/that failed...', and would of course never utter an appositive noun modifier. unfortunately, as in so many other cases, the rules for spoken and written language are vastly different.