The seed landed on a 16-year-old boy called Ethan. Ethan was lying in his bed, listlessly browsing his phone with his door locked. The door was locked because of what was on his phone- advice for people questioning their gender. See, since he started puberty Ethan had been uncomfortable about his body. It felt wrong that he was slowly turning into a man. When most teenage boys saw girls' bodies or clothes, they got turned on, but more than that when he saw them Ethan felt weirdly jealous. He wanted to look like they did and wear that stuff, but he knew it wouldn't fit him right and people would shun him if they could tell he wasn't a 'real' woman.
He'd found out after looking up stuff about these feelings that it meant he might be transgender, but that identity didn't feel right to Ethan. He admired trans people who were brave enough to be their real selves, but not only did he feel he could never come out and be accepted (especially with so many people who thought what he was feeling wasn't real or just meant he was a perv), with how expensive and imperfect the process was and how long it took, he felt scared it wouldn't really make him happy with who he was. So nowadays he just looked for advice on how to make his discomfort with his body feel less bad.
The seed fed on that desire, and set about giving Ethan a life he- or rather she- would be happy with. It would transform his body from male to female, give her all the memories she needed to be a girl, and made sure everyone she knew would remember her as a girl. To allow this to happen, it reached right back to moments before Ethan was conceived, and turned the sperm that would fertilize the ovum which would form him from one with a Y chromosome to one with an X chromosome.
A weird sensation swept through Ethan's body as his longtime wish was granted. His skin pricked up all over and he shivered in anticipation. While he couldn't tell what was happening, the seed's magic made sure it felt pleasant. Standing up and going to look at himself in the mirror, he noticed his hair had lengthened on his head and shrunk into his skin across much of the rest of him. He seemed to shrink all over- he became a little shorter, his arms and legs more petite, his hands and feet shrank and his clothes hung off him.
When he pulled off his T-shirt, he saw his skin looked soft and smooth, and slight curves had formed around his torso. No way, this couldn't be...
He groaned and noticed his voice increase in pitch as odd sensations surrounded his head and neck. He screwed his eyes shut, and when he opened them again, he saw a face that was still the same person, but distinctly not. The cheeks were softer and the lips puffier, so it looked more like...
A sharp moan accompanied the sensations moving down into his chest. He saw his nipples grow larger, puffier and more sensitive, and he felt unfamiliar desires well up in his mind. He wanted to have them rubbed, and pinched, and sucked. Curiously, his hands moved to the erect teats and stroked them, surges of pleasure running through him in response. Soon flesh started to sprout around them, and instinctually his hands cupped this, making him feel even more pleasure as he moaned eagerly. By the time he pulled his hands away, he had sprouted pert C-cup breasts.
Just one part of his boy's body had not changed yet, and he pulled his pants off to see a firm erection. Stroking it carefully, he panted as it slowly receded. And as it shrank away, he adjusted mentally. At last Ethan's gender was changing. She replaced he, female replaced male, woman replaced man in her perception of herself. His testicles moved up inside her and became her ovaries, connecting up to a vaginal tract and womb that formed inside her. His penis steadily shrank and shrank until it formed her clitoris. His scrotum sank inwards, sifted and peeled open to form her vagina.
Effie slowly came down from the sensations and looked at her body in amazement. She looked like a girl, and she felt like a girl! She even knew everything she would have to know to be a girl! When she checked her phone, she saw that on her social media she was called Effie, and had always been a girl! She felt like she could cry with happiness.