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 identity is the person's feeling of being male, female, both, a mixture, or neither, which is shown to other people through gender expression (clothes, hairstyles, mannerisms, etc.). It is separate and independent from sexual orientation. Trans people can be lesbian, gay, bi, straight, pansexual, etc.