A Short Essay on Surathi Genetics
A full explanation of the heredity of Surathi traits is beyond the scope of the main Urbis world description. Nevertheless, some DMs might find further information on this topic useful, and for them this section tries to explain the basics.
Without going into genetics too deeply, surathi traits can be thought of as recessive genes. Recessive genes don't express themselves unless they are inherited from both parents. So if someone with the full expression of the gene has offspring with someone who doesn't have the gene at all, the children will be carriers of the gene, but the gene won't express itself. Someone who only carries the recessive gene has a 50% chance of passing the gene to his children. So if two people who were both carriers of a gene had offspring, their children would have a 25% chance of having the fully expressed gene (i.e., they inherited it from both of their parents), a 25% chance of not having the gene at all, and a 50% of having only the recessive gene (i.e., they inherited it from one parent, but not from the other).
The surathi traits are found in a set of three recessive genes, all of which are passed to offspring seperately. The thin-blooded are merely carriers of these genes - in other words, they have one to three of these genes, but they didn't inherit any of them from both of their parents. This is enough to allow them to learn psychic powers, but doesn't grant them anything else. Lesser brethen have one fully expressed gene (they inherited one of the three genes from both of their parents), favored ones have two, and pure ones all three. This explains why the offspring of true surathi (lesser brethen and up) will always be at least thin-blooded - since at least one surathi gene is fully expressed in true surathi parent, the child will at least have it as a recessive gene. It also explains why any couplings between true surathi and non-surathi will result in thin-blooded - the children will inherit the gene(s) from one parent, but not from the other, and thus merely become carriers.
So, what does all this mean for the game? Well, surathi genes might slumber in almost anyone - or anything. There's always a chance that parents who are completely unaware of their surathi heritage and their status as thin-blooded (and there are many of those) will give birth to a child with animal traits, bringing shame and tragedy to the family. In older times, these children were often immediately killed by the local community, but in these modern times, these children are often merely abandoned instead (and taken in by surathi agents, who often run orphanages...). And any psychic PC is likely to have surathi genes as well - they might not be aware of this, and it won't necessarily shape their character - but they might be confronted by their tainted heritage some day, especially if they have children with another of the thin-blooded...