The late 1950s to early 1960s period forged a mosaic-centric view of disorders of sex development, prioritizing chromosomal constitution and its functional consequences for sexual differentiation. Researchers demonstrated that X-chromosome activity in females is mosaic, supporting the concept of one active X per cell and establishing a foundation for Lyonization ideas that connect chromosomal mosaicism to phenotypic outcomes. Studies also highlighted sex chromosome mosaicism (for example XO/XY patterns) as a core mechanism for sexual ambiguity and gonadal dysgenesis, while gonadal histology and cytogenetic testing became essential diagnostic tools. An evolving, multi-variable approach began to include hormonal context, internal and external anatomy, and psychosocial factors in understanding sex development and sex assignment decisions.