Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

A man in bed says to a woman, "Hey baby, wanna do a little horizontal gene transfer?" The woman responds, "But... aren"t horizontal gene transfers between animals usually mediated by viruses?" The man"s expression changes, and she suggests, "You might want to get tested."

The Humor

The joke plays on the double meaning of "horizontal gene transfer" as a pickup line. The man is using the scientific term as a euphemism for sex ("horizontal" suggesting a lying-down position, "gene transfer" suggesting reproduction). However, the woman takes him literally and points out the actual biology: in animals, horizontal gene transfer (as opposed to vertical gene transfer through reproduction) typically occurs through viral infection. This turns his suggestive come-on into an accidental admission that he might have a sexually transmitted infection, prompting her to suggest he get tested. The humor lies in how scientific literacy completely deflates a would-be romantic moment.

References

• Horizontal gene transfer (HGT): Also known as lateral gene transfer, this is the movement of genetic material between organisms through mechanisms other than parent-to-offspring reproduction. While common in bacteria (through conjugation, transformation, and transduction), in animals it most often occurs via viral vectors (retroviruses inserting their DNA into host genomes).