AN: Harry Potter and canon belong to JK Rowling.
Upon his return to Britain, Tom moved back into the muggle flat he had lived in during his mastery studies. His work as an unspeakable first started in curse reversal research. There, it was his job to study the new curses that were constantly being developed and find counter-curses. This was easy for him since spell creation was something he had a lot of experience in. His favorite thing about returning to London was the ability to talk to Johan face to face; he had missed his sole family member and had often found himself reminiscing about his childhood, when Johan provided all the advice and support he needed. Now, as an adult, it went against his sense of independence to depend on Johan to that degree. Tom was also happy to meet up with his old school friends – Eileen was now a well-respected potion mistress, while Alfie was being groomed to take over his family's financial interests. Cas was now a senior auror, and Ernie had taken up his uncle's law practice. They were uninterested in actually travelling themselves, but were awed by Tom's eclectic career path. Needless to say, it was that variety in his work experience that had landed him the job as an unspeakable.
After a few years, Tom had gained the respect of his superior, enough for him to undertake his own research on the topic of his choice. Tom decided to study where muggleborns came from. For three years straight, he would set up a booth in Diagon Alley in August offering free inheritance tests for muggleborns in return for the right to use the information for research. Many adult muggleborns, and parents of underage muggleborns, agreed, giving Tom a sample size of several hundred to study. In the end, he was able to publish a paper showing that muggleborns were born when two people with magical ancestors within the last five generations had children. This not only gave muggleborns a claim to magical heritage, but also proved that magic truly was in the blood.
Tom was then asked to study squibs in a similar way, which took him longer as he had to individually identify and contact squibs and ask them to take part in his research. He asked for not only inheritance tests, but also medical history and any memorable events from their childhood. He was only able to get a sample size of seventy, but his research did show a pattern. There appeared to be three reasons for why a person was a squib. The most common was when a magical married a muggle with no recent family history of magic, giving their offspring only a fifty percent chance of being born with magic. The second reason was the result of inbreeding, and not for the reason people thought. Magic was in the blood, but the body had a maximum concentration of magical blood it could take before being overwhelmed and leaving the magic blocked up in the child. These squibs had very potent blood for any blood rituals, and could do random bits of magic under pressure, but they could not use their high concentration of magic reliably and would never be able to use wands. The third types of squibs (and the rarest) was when a pregnant woman was magically attacked within a month of when her child was due. This interferes with the child's magical development and their magic never recovers.
Again, this had significant impact on magical society. Pureblood families changed their view of their squib children – rather than useless, they could still breed magical children and take part in family rituals. It also meant that some pureblood families (not all) started encouraging their children to marry half-bloods and muggleborns to reduce the chances of squibs being born.
AN: This chapter ends in 1968. Eileen had met and married a fellow portioner by the name of Damocles Belby and together they were working on a cure for lycanthropy. They had an eight year old son named Severus Belby who seemed intent on following in his parent's footsteps. So there, I still gave you guys Severus, am I not generous?