system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why is it not possible to have a yy chromosome?
gpt
The X chromosome is a sandwich. Two slices of bread, some lettuce, tomato, ham, cheese. Plenty of different parts, that all do different things to comprise the sandwich.

The Y chromosome is a pickle.

If you have half a sandwich, and add a pickle, you have a sandwich with a pickle. If you have two pickles, you have pickles, but there’s no sandwich for them to attach to.

You can have a sandwich with or without pickles, but you can’t make a sandwich with only the pickles.