Short-a-chrome 

When you are missing all or part of a least one chromosome. Dumb ass. Stupid. Inept.

How’d that short-a-chrome graduate from high school?

Short-a-chrome by Goofnut March 16, 2023

short a couple chromosomes 

idiom/adjective phrase: to be mentally deficient in a very noticeable manner.

Etymology:

From the fact that many cases of mental retardation involves having an abnormal number of chromosomes.

Notes:
Funnily enough, means the same thing as "to have a couple extra chromosomes". "To be short a couple chromosomes" is perhaps a more natural way of expressing the idea as it plays to the human intuition that "more is better" and also because it is very easy to imagine how someone without the normal number of chromosomes could turn out retarded; however, "to have a couple extra chromosomes" may actually be more accurate in most cases. The most common genetic cause for mental retardation is Down's syndrome, which actually involves having more chromosomes than the normal person (a third copy of chromosome 21, to be precise).

I asked Zoe why her cousin was such a manchild, and she said, "Oh, he's short a couple chromosomes."

short a couple chromosomes by Choosy Floozy November 24, 2019