Ty was about to decide on the chromosome of the sperm that had fertilized him, but all of a sudden he realized it already contained one- and he felt it subliminally trying to get him to align with it, faintly sensing it like hearing someone yelling from afar. He was curious how his development would turn out led by this chromosome, and decided to allow nature to take its course.
Once he wished for this, the chromosomes sealed the zygote Ty's fate. The haziness around them cleared, and Ty understood that this time the sperm from which he developed had not a Y chromosome but a second X.
He wouldn't be born male, she would be born female.
As his old self was wiped from his genome to make way for her new self, Ty felt like it was instructing him how to develop into her- and in his head he saw and felt his perception of himself alter to fit her new self.
His hands and feet got more small and petite, his muscles more lithe, his figure more petite and curvy.
His skin smoothed and softened, most of its hair vanishing as he became supple to the touch.
The reflection of his face turned more feminine, with puffier cheeks, a softer jawline, long hair and the bulge in his neck vanishing as his voice turned girlish.
Soft mounds formed on his previously flat chest while his nipples widened and grew puffy, giving him modest B-cup breasts.
Last and most intensely, he saw his manhood turn to womanhood. He watched his crotch start flattening, saw his penis sucking up into his body and felt his testicles get pressed flat to his crotch and pop inside him.
His castration and emasculation made him feel detached from being a man- and so she became a woman. Her clitoris tucked into its hood and her vaginal lips prised apart the sac that had been his scrotum.
Ty slowly felt herself adapt to her reassigned sex. He would not be June and Mark's adoptive son, she would be Amanda and Jacob's daughter- it felt like changing sex was a price worth paying for a loving family. Plus she did feel curious about life as a girl.