In the original comic book series, every animal with a Y chromosome instantaneously dies at the same time, including most mammals. (Birds, reptiles, amphibians, fish, insects, arachnids, crustaceans, and mollusks are unaffected because they mostly use different sex chromosomes.) While transgender people did not feature heavily in the original comic, it was mentioned that trans men survived due to not having Y chromosomes.
The producers aimed to update and expand on the ramifications of this, to reflect changing societal attitudes toward gender in the almost two decades since the comic began. The showrunners specified that trans women with Y chromosomes also died, as well as women with androgen insensitivity who were unaware they possessed Y chromosomes. The showrunners also added a trans male character to the regular cast (Sam Jordan, played by Elliot Fletcher), who does not have a direct counterpart in the original comic, to expand on this world-building.
Speaking at the August 2021 TCA panel, showrunner Eliza Clark said: "Yorick's maleness is not what sets him apart in this world — it's his Y chromosome that sets him apart. Gender is diverse and chromosomes are not equal to gender. And so, in our world — in the world of the television show — every living mammal with a Y chromosome dies. Tragically, that includes many women; it includes non-binary people; it includes intersex people ... We are making a show that affirms that trans women are women, trans men are men, non-binary people are non-binary, and that is part of the richness of the world we get to play with."[6]
Cast member Fletcher, who like his character is trans, said that "In this world post the event, gender is somewhat irrelevant. I think one of the hilarious things about this show is that post the event, Yorick can walk around without a mask on because he's assumed to be trans,
