Researchers conducted a genetic study of the toxin transport protein (PA) of Bacillus anthracis, the bacterium that causes anthrax in humans (Price et al., 1999. J. Bacteriol. 181: 2358–2362). In 26 strains, they identified five point mutations—two missense and three synonyms (silent mutations that do not change an encoded amino acid)—among different isolates of the 2294-nucleotide gene. Necropsy samples from an anthrax outbreak in 1979 revealed a novel missense mutation and five unique nucleotide changes in samples collected from ten victims. The authors concluded that these data indicate little or no horizontal transfer between different B. anthracis strains.

(a) Which types of nucleotide changes (missense or synonyms) cause amino acid changes?

(b) What is meant by horizontal transfer?

(c) On what basis did the authors conclude that evidence of horizontal transfer is absent from their data?