The quotations taken from the work of Ramón y Cajal and Lashley that introduced this chapter emphasize that the most prominent differences between the brains of mice, chimpanzees, and humans are quantitative; and yet, despite the pride we take in our burgeoning understanding of neuronal development, we have not yet identified a single gene or allele responsible for any part of this astonishing quantitative variation. This is humbling. In our drive to understand the fundamentals of development we can initially be forgiven the blind eye that we have collectively turned to normal variation, but to get beyond descriptions of neuronal development—to get to a second stage of purposeful design, modification, and repair–we will need to understand the subtle, intertwined, and messy molecular genetics that makes a human brain different from that of a mouse and the brain of one human different from that of another (Bartley et al. 1997).
I thank Dr. Richelle C. Strom for sharing data on the Bsc5 locus. My thanks to Drs. Guomin Zhou, Dan Goldowitz, David C. Airey, and Glenn D. Rosen for their contributions to this work. This research was supported by NINDS RO1 35485.
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