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 biological. Gender is not a social construct, there are males and females, and there are in-betweens, but there is nothing social about it. There are social and cultural implications of gender roles in society, which are a social construct, however the genders themselves are completely biological.