Something that men have 24 kinds of whereas women have just 23. Although women have two X chromosomes while men have one X and one Y, one of the women's X chromosomes is "transcriptionally silent" i.e. completely inactive.
The Y chromosome is the sole distinguishing factor between men and women and therefore the home of the genes for rationality and abilities such as parking, understanding maps, inventing humourous jokes or indeed anything useful, using an ATM in 30 seconds or less and not turning into a blubbering mess when something trivial occurs, like a parent's death.
"Have you seen The Sarah Silverman Show? She is one funny ass chick!"
"Yeah but she's literally the ONLY one, makes me kinda suspicious about them chromosomes."