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The recent American Scientist had an interesting article on modern neuroscience, which included a section on genetics. Here is what it said:

Genetic processes underlie the development and evolution of the brain, and several research teams are studying the genetics of human brain volume and structure. One strategy is to use MRI to look at the brain volumes of identical and fraternal twins. The studies indicate that human cranial capacity is a strongly inherited trait, and most of the variation in total or hemispheric volume can be explained by genetic factors.In one report, by William Baaré and his colleagues at the University Medical Center of Utrecht in the Netherlands, genes accounted for the large majority of brain volume differences: 90 percent for the brain as a whole, 82 percent for gray matter and 88 percent for white-matter.

However, two major neuroanatomical features appear to be free of strong genetic control. In the same paper, Baaré stated that the lateral ventricles—CSF-filled cavities inside the brain—were only mildly influenced by heredity. A separate study by Alycia Bartley and her colleagues at the National Institute of Mental Health explained how patterns of sulci and gyri were more similar in monozygotic (identical) twins than in dizygotic (fraternal) twins. Interestingly, siblings from both groups were still very different from each other, especially in the smaller sulci. Thus, while overall volumes of major brain sectors are under strong genetic control, smaller regions may be more responsive to environmental influence.