Original: When in 1633 Duke John Casimir died without issue, the Wettin line in Saxe-Coburg died out, and John Ernest inherited it. Nevertheless he himself also died heirless in 1638, and his territories of were split between Saxe-Weimar and Saxe-Altenburg, which had itself been separated from Saxe-Weimar in 1603.
Simplified: In 1633, the Wettin line died out, and Johann Ernst inherited it, but he died heirless in 1638, and the territories of Saxe-Gotha and Saxe-Eisenach were split between Saxe-Weimar and Saxe-Altenburg.