I believe the implication is meant to be that because he didn't have access to all of his mom's DNA, the transformation only affected his gender somehow? Rather than him turning into a teenage version of his mother, because everything else seems to indicate that the standard transformation should be a full one.
When his father copies his mother's DNA, he looks exactly like her, with the same hair length and everything, and he talks about turning into a middle-aged man at the ramen shop. Even if the latter is just an uniformed hypothetical, at the very least we know that the hair should change to match, which definitely didn't happen to Yuki.
Anyways, I like how his eyes turn into stereotypical all-black alien eyes at times.