The platysma is a superficial muscle that overlaps the sternocleidomastoid. It is a broad sheet arising from the fascia covering the upper parts of the pectoralis major and deltoid; its fibers cross the clavicle, and proceed obliquely upward and medially along the side of the neck.
For example genetic differences, females inherit X chromosomes from their father giving them a genetic make-up of XX, while males inherit a Y chromosome from their father, thus giving them a genetic make-up of XY.
Gender is partly social construct, partly developmental influences, partly brain biology. The XX and XY that the layman thinks defines your biological sex, well actually it doesn't. Our biological sex is incredibly complex, and that alone produces a vast array of different experiences of gender.