The joke is very obviously referencing Down Syndrome, with the joke you mentioned in another comment being some sort of subversion of expectations by having the gene duplication mistaken, except there are diseases with gene absence as the root cause.
Punching down at disabled people is some chud shit.
name one person with missing chromosome
Anyone with Turner syndrome (fully or partially missing X chromosome)
https://en.wikipedia.org/wiki/Turner_syndrome
The joke is very obviously referencing Down Syndrome, with the joke you mentioned in another comment being some sort of subversion of expectations by having the gene duplication mistaken, except there are diseases with gene absence as the root cause.
Punching down at disabled people is some chud shit.