These two questions are one of the more interesting in the study of intelligence, but also one of the most difficult to answer. The 'Heredity vs. Environment' fight seems to be never-ending, while both sides have solid reasons to claim victory. Essentially, two different types of studies are used, twin studies and adoption studies.
Monozygotic twins have a very important genetic equality, and they also share the same environment.
Dizygotic twins' genes are just as similar as those of ordinary siblings, while assumedly, they share the same environmental similarity monozygotic twins share. This is why the study of twins are a useful tool for 'heredity vs. environment' researchers. Assuming the above premises are true, this leads us to the conclusion that if the differences between monozygotic twins are much lesser than the differences between dizygotic twins, then we can affirm that heredity plays a major role determining intelligence. When these studies took place, the results indicated that heredity seemed to be the main determiner of intelligence. While the first are usually treated as ordinary siblings, monozygotic twins are much more likely to share clothes, classes, games and extracurricular activities.