There is very little biological difference between the humans of the Phoenix Empire and the humans we know today.
Human phenotypes have mostly mingled to the point of insignificance. The only exception are Noble Houses that have over centuries married for a certain cultural and genetic 'look' to improve their branding. One can safely assume that a person from, say, a strong House Wu Tzun background will most probably have more asian influences in his genetic makeup, while a House Medina descendent will look rather arab or dark-skinned.
Also, the population of some planets can lean a little more towards one phenotype or another as the result of cultural selection in favor of a certain phenotype, usually the one preset by the ruling Noble House. This will obviously hold true much longer than the political situation in many cases - former holdings of House Habichtswald still show an above-average population of blond, blue-eyed caucasians, even though the House is extinct since decades.
While the average slave has a life expectancy of a little below 30 years, the average commoner will live up to 80 years, more or less. And even though some nobles are known to have lived for several centuries, the average noble lifespan is about 65 years only. Politics is a deadly game in the Phoenix Empire.
Decorative Genetic ArtifactsEdit
An important distinction is that during the time of the Gilded Republic, humans used genetic modifications as fashion or lifestyle statement, much like we would use hairdye or tattoos. Those modifications were not inheritable per se, but entered the genepool none the less. So while humanity overall looks pretty much the same as in our days, there are always children born to parents where dormant artifacts of modified genes recombine to unexpected and often inhuman results - children with scales, pink hair, striated skin and so on.
The most famous example of those artifact mutations are the purple eyes of Emperor Grigorij Dracon.