Genetic Segregation
One of the common technologies of the Universe is the ability to genetically engineer the human form.
It is a matter of social fashion that these variations tend not to be too grotesque - those that can choose, i.e. the richer parts of society - choose offspring that are genetically perfect examples of humankind. The 'Perfects' are healthy, strong, intelligent and good-looking (within small variations). As a result, perfects form virtually the totality of all the areas of responsibility and leadership. All the military are perfects, all government, industry and educationalists are perfects.
The remainder of the poorer part of the population tends to be made up of those with more natural genetic material. The Naturals are prone to the usual range of human abilities - the best can equal the perfects, but might carry injurious recessive genes, but the majority are just not up to the standards of perfection.
On new colonial worlds, there tends to be a higher proportion of naturals, though perfects (or the natural descendants of perfects) still tend to lead and take important jobs. The imperial or colonial powers have very high proportions of perfects.
Discrimination on the grounds of genetic makup is not only legal, it is required in some cases - for example naturals are not normally allowed to serve in the military, police or on the judiciary. This discrimination is regarded as normal - in fact it is seen as reasonable - since unlike 'race' discrimination in ancient times on Earth - the discrimated group is actually inferior (rather than merely inferior by virtue of the dominant social power structures). Arguments that suggest that discrimination should end on the grounds that 'all humans are equal' fail in the light of verifiable biological fact.
The acceptance of the genetic status quo is so widespread that the subject is never even mentioned or considered by those in power - the naturals being, in effect, an invisible part of all human societies of the Universe.
There are small groups of natural activists, some resorting to direct action at times - but they have so far failed to get widespread support, even among the natural population. Most well known of these groups is the feared Democracy And Freedom Today group - which has been responsible for seemingly random acts of terrorism and mayhem throughout human space for many years.
See also Perfects Naturals Democracy And Freedom Today Movement