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.
And beyond the genes, a personâs sex development can be significantly influenced by environmental factors (including the maternal uterine environment in which the fetus developed). So it is simply incorrect to think that you can tell a personâs sex just looking at whether he or she has a Y chromosome.