African Americans are generally underrepresented, both in the universities, and in the sciences. Berkeley is no exception in this case.
During my time in grad school I have interviewed well over 100 undergraduates who were applying to work with me, and taken on (as volunteers or paid workers) about 30 of them. Currently, I have 18 undergraduate collaborators. I've not given a great deal of thought to the demographics of this group, other than to notice that the great majority of my applicants (and therefore of my assistants) are female. A recent conversation (about Pres. Elect Obama) made me stop and think about the race and religion of this group. It is a very diverse group. I have had assistants who are Christian, Jewish, Hindui, Muslim and non-religious. Maybe other religions, I don't know. I have had assistants whose ancestors (or they themselves) came from East Asia, South Asia, the Middle East, Eastern Europe, Western Europe, Pacific Islands, Latin America and possibly other places I am not aware of. They have been male and female, heterosexual and homosexual. There are few places in the world where I could have ended up with a more diverse group, but I have no one of obvious African decent.
African Americans are not represented in my lab for a simple but sad reason. I have had not one African American applicant (that I am aware of), out of maybe 120. It is striking that African American representation in this group is lower than among our nation's elected officials. I am not sure why exactly this is, what combination of bias, cultural factors and public policies to blame, but I know this is one area where African Americans don't yet seem to have made sufficient inroads.