Hi Divya,
Starting with Passage 3- Q36:
In order to understand this question, you will have to understand Figure 1 and what it represents. The colored-in shapes indicate the affected individuals i.e. the preterm newborns. In these individuals you can see that the DNA variations are only found on either the paternal or maternal chromosome. The question is asking why this might be.
A) We do not have any processes that check to see whether any mutations have been inherited once an embryo is formed. If we had this, it would solve several of our human diseases to this date (i.e like cancer)
B) While this is a possibility, preterm births seem to be prevalent (passage says it accounts for an estimated 2 million deaths worldwide)...therefore the possibility that 2 people carrying the mutation have NEVER conceived seems improbable. [Watch out for extreme language like 'NEVER' used in MCAT Qs, definitely double check these answer choices]
c) This answer choice says that perhaps the reason why we only see preterm babies with 1 copy of the mutation is because embryos with 2 copies of the mutation don't make it to birth i.e. they die. This would most definitely answer why we only see either a maternal/paternal copy. In these preterm babies with one copy, the 2nd wildtype/normal copy of the gene could make up for the mutation and therefore they survive.
d) We know that IGF1R is located on chromosome 15
Remember with MCAT Qs: rule out incorrect answers first. This helps you narrow down your choices :)
I hope this helped!
- Shin