Shared Environment - Different Genes: Speech-Language Development in a Pair of Dizygotic Twins with and Without Mutation.

This retrospective study compared the speech-language development of a pair of dizygotic twin girls during the first 2 years of life: one with typical development (Twin A) and one with atypical development (Twin B), who was later diagnosed with Rett syndrome (RTT). Audio snippets were extracted from home videos, with nearly equal representation from each child. The audio analysis focused on articulatory complexity, voice characteristics, and linguistic variability. Despite sharing the same social-communicative environment, the twins' speech-language development diverged. From the first to second year of life, articulatory complexity and variability increased in the typically developing Twin A. In contrast, Twin B produced a high number of vocalizations in the 7th month, including canonical sounds with substantial variability. However, her vocalization quantity, complexity, and variability subsequently decreased, resulting in only sporadically discernable canonical vocalizations during her second year. This developmental trajectory points to very early stagnation and regression in Twin B, occurring earlier than typically observed. While Twin B displayed a range of typical vocalization features, deviations in the density and distribution of inspiratory and high-pitched vocalizations during the first year further suggest early speech-language abnormalities in RTT, preceding frank developmental stagnation and regression. As the study relied on limited retrospective data, the findings should be interpreted with caution, and further investigation is needed. Nevertheless, this twin study provides a unique perspective that deepens our understanding of early speech-language developmental profiles in RTT, especially in light of the intertwinement of genetic, individual, and contextual factors.