tamal99 - let me try to help you.
Split 1: where vs if. where can be used for places, usage of 'where' is not correct here. Eliminate A and D.
E. if parents have certain gene variants, their children, more likely to go to college, ---- Notice the ';' before underline sentence, you need an independent clause after it. (means subject and verb is must.) 'if clause' is dependent clause. it again need an independent clause, and there is not even a clause, just 'their children' . wrong.
C. if parents, having certain gene variants, have children more likely to go to college, ---- 'having clause' is modifier, modifying parents. if clause is dependent one, need another dependent clause. there are none.
B. if parents have certain gene variants, their children are more likely to go to college, - Correct
First let see why it is correct.
1. we need an independent clause ---- "their children are more likely to go to college" --- found it.
2. sv agreements are correct. ---- if X then Y is correctly preserved.
your queries:long thought to be irrelevant to genetics research --- this is not answering to parents, its for Genes. Look at it this way . What long thought to be irrelevant to genetics research - geans or parents.
not inherited from your parents --- this one again modifying Genes. Take the What test.
not inherited from your parents [but] long thought to be irrelevant to genetics research --- you need to see a hidden but in the meaning. [not X but Y]. I hope you can connect dots now. Let me know if you need more exploration around it.