May 27, 2004

When "small" means "large"

Reuters headline: "Small Changes Separate Man from Ape, Study Shows."

Headline of Science Update piece about the study in question: "Chimp chromosome creates puzzles: First sequence is unexpectedly different from human equivalent."

Science Update explains in more detail that

83% of the 231 genes compared had differences that affected the amino acid sequence of the protein they encoded. And 20% showed "significant structural changes". ... The researchers also carried out some experiments to look at when and how strongly the genes are switched on. 20% of the genes showed significant differences in their pattern of activity.

Note that this is true despite the fact that only 1.44% of the individual base pairs are different. If this seems puzzling to you, then you need to open up a calculator window and determine the value of .9856^N (the probability that all base pairs are the same for a gene with N base pairs, assuming independence) and .0144*N (the average number of base pairs that are different for a gene with N base pairs, assuming ditto), for values of N corresonding to the number of base pairs in a gene (say 1,000 to 100,000).

Perhaps this is the line of reasoning that the Reuters' headline writer had trouble with -- or maybe (s)he never got past the first three words of Maggie Fox's story ("Tiny genetic changes..."). Other headline writers did better, though: the Korea Times has "Big Genetic Gap between Chimpanzee, Human Being"; Xinhua has "Genetic study shows chimps are less human"; and so on.

The full description of the Nature article is:
"DNA sequence and comparative analysis of chimpanzee chromosome 22", by The International Chimpanzee Chromosome 22 Consortium, Nature 429, 382 - 388 (27 May 2004).

Reuters correspondent Maggie Fox actually got the story right -- her first sentence continues "(Tiny genetic changes) add up to huge differences when human DNA is compared to that of chimpanzees" -- it's the headline writer who did her in.

[Thanks to Keith Ivey for correcting my math.]

Posted by Mark Liberman at May 27, 2004 12:40 PM