2014-10-30
Explanation
The Joke
The comic retells the nursery rhyme "Jack Sprat could eat no fat / His wife could eat no lean" but adds a scientific twist: "These traits were autosomal dominant." A Punnett square shows the genetic cross between Jack (Nf) and his wife (nL), where N = no fat and L = no lean. The final panel reveals that "Li'l Sprat got each bad gene" -- their child inherited both conditions, meaning the child can eat neither fat nor lean (i.e., the child cannot eat anything). The child is shown at a doctor's office receiving an unfavorable diagnosis.
The Humor
The comic takes a lighthearted nursery rhyme and subjects it to rigorous genetic analysis, revealing a dark consequence that the original rhyme glossed over. If Jack Sprat's inability to eat fat and his wife's inability to eat lean are both autosomal dominant traits, then their offspring has a significant chance of inheriting both dominant alleles -- leaving the child unable to eat any food at all. The humor comes from the collision between the cheerful, sing-song nursery rhyme world and the grim reality of Mendelian genetics. It is a classic SMBC move of applying scientific rigor to something whimsical and arriving at a horrifying conclusion.
References
"Jack Sprat" is a traditional English nursery rhyme dating back to at least the 17th century. The Punnett square is a standard tool in genetics for predicting the probability of offspring inheriting particular trait combinations, named after Reginald Punnett. Autosomal dominant inheritance means only one copy of a gene variant is needed for the trait to be expressed.