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.
Sex is thus biologically determined, whereas gender is socially constructed. We are socialized to expect that gender category (man/boy or girl/woman) follows sex, and in turn, to infer that sex follows the perceived gender of a person. However, as the rich diversity of gender identities and expressions makes clear, gender does not necessarily follow sex in the ways we are socialized to expect.