Perfect Pitch

Musicians, especially singers and string players, are trained to recognize and correct pitch in order to play within the Western 12-tone scale (when playing Western music, obviously). There are different types of pitch-training skills.

Relative pitch is what most of musicians are working with: given a reference note (tuning fork, pitch pipe, other parts of a composition), we can identify pitches relative to that reference note. We can tell when we are singing flat or sharp even when the rest of the a cappella group is singing different notes than we are. We can hear when the piano is out of tune because of how the tones relate to each other.

Absolute pitch, also called "perfect" pitch, is the ability to identify a note by name without any reference pitch being given. Some research suggests that this ability is universally inborn, but disappears in childhood without musical training. Other sources demonstrate that absolute pitch tends to run in familes and among siblings. Studies state that the incidence of absolute pitch in adults is anywhere from 1 in 35 to 1 in 10,000.

What I learned today, which I found utterly fascinating, is that native speakers of certain languages (Vietnamese and Mandarin, among others) have a higher incidence of absolute pitch. It is believed that this is because tonal languages, in which changing the pitch of a spoken word changes its meaning, develops the same ear for perfect pitch as musical training. In one study, native tonal language speakers (without musical training, for the most part) were asked to read a list of Vietnamese or Mandarin words. The subjects of the study all spoke these words at virtually the same pitch. Another study (uncited, unfortunately - from an article in August 2006 Psychology Today) finds that absolute pitch is much more prevalent (32% vs. 7%) in conservatory students who are native speakers of tonal languages.

More details here.
And links associated with the University of California Genetics of Absolute Pitch Study.