Disclaimer: I do not own any recognisable Sherlock characters, they belong to the BBC.
Jenna Heulwen Watson was born on the 7th of July 1980 to James and Naomi Watson. She also has a sister, six years her elder, named Harriet.
As far as she could remember, Jenna had always been tomboyish, climbing trees and running everywhere, to the dismay of her mother and the eternal amusement of her father. This is the reason why she insisted she was to be called 'John' when she was three. James, who had secretly been hoping for a son when his wife got pregnant for the second time was delighted to have someone to tell stories of his days as a soldier in all their gory details. In fact, while Naomi treated her children equally, Jenna was James' favourite and he was his little girl's hero.
Life was good, that is, until James' untimely death in 1996. Naomi slowly developed a drinking problem and Harriet, or Harry as John called her, took to smoking while John threw herself in her studies. When Naomi lost her job as a nurse a few months later, her husband's life insurance money was barely enough to keep the family afloat and her younger daughter started taking small jobs in order to get some money into the house because the elder was barely home anymore.
Two years later, Naomi died in a car accident and while Harry had stopped smoking, she became a heavy drinker. John, who had already turned 18 at that point, left for medical school with the money that her father had left for her in the bank and she gained access to on her birthday, as well as what little money she had managed to save from her odd-ends jobs. Later, she became an army doctor, a mix of her parents' professions, and served in Afghanistan.
When she was back to England, it was with a limp, PTSD, a meager pension, and forced appointments with a therapist. She was just a shell of who she had been when her father was still alive, that is, until she met a man who would forever change her life, Sherlock Holmes.