Sequential syllable structure for USVs of B6, 129, and F1 mice. (a) Syllable repetition in B6, 129, and F1 mice. The number of events of differing length was normalized with the total number of syllables. B6 and 129 mice emitted different amounts of short repetitions and chevron repetitions. B6-like phenotypes were dominant. * p < 0.01. (b) Proportions of animals with syllable repeats containing at three to six syllables.