TBX6 and skeletal dysplasia: Analogous patterns are well recognized in other genetic conditions such as COL2A1 (Collagen, Type II, Alpha-1)-related skeletal dysplasias and TBX6 (T-Box Transcription Factor 6)-related segmentation defects, where allelic heterogeneity and modifiers account for wide phenotypic variability [77,78,79,80].