People gather at a rally for sexual minority rights at Washington Square Park in New York over the weekend after a report that the Trump administration may roll out a new proposal to define sex at birth.

A new report that the Trump administration hopes to legally define a person’s sex at birth — an unchangeable condition determined by genitalia — has prompted an outcry among the transgender community.

But beyond the political implications — and fears that any such change could ultimately mean the end of civil rights protections for transgender people — the proposal raises fundamental questions about something else: biology.