Although we don't really see that many observers in Fringe I can't help but notice that they are all white and they are all males. Has the series ever given an explanation for this?
In season 5 episode 11 it is revealed that since a neurological breakthrough in 2167 all children are grown (with considerable modification) ex vitro with a relatively small genetic range considered "acceptable".
He was born hundreds of years from now. Like all of our kind, he was created in a lab, using the genetic material of a designated donor. It was a process that began generations earlier, with a discovery on February 20th, 2167, by a scientist in Oslo, Norway, who was trying to find ways to increase human intelligence. He realized that if he could rewire the portion of the human brain that induces jealousy, he could increase cognitive function, sacrificing emotion for intelligence. This discovery was the catalyst of the creation of the beings that you called Observers, and his work carried on, and more and more emotions were seen as roadblocks to higher intelligence. Things like anger, greed, aggression, they were abandoned in the pursuit of intellect. Eventually, Humanity became so intelligent and efficient, they lost perspective of the value of these emotions - - Not only the negative ones, but the positive as well, and soon empathy, compassion, and love became messy distractions, and they too were machined out, and without romantic love, they developed new reproductive technologies.
-September, Fringe season 5 episode 11, quoted from Transcript : Donald's Apartment - Filling In The Blanks section
This base genetic template is apparently white and male.
So assuming the original template was based on a Norwegian man, it makes sense that they would be white and male. Since race and gender are irrelevant to the pursuit of higher intelligence, there would be no reason to deviate from the template in order to increase variety. Particularly since part of the goal seems to have been Homogenization.