TOTIPOTENCY AND DETERMINATION
During sexual reproduction, a sperm cell and an egg cell unite to form a one-celled fertilized egg. This cell is totipotent, meaning it has the potential to give rise to any and all human cells, such as brain, liver, blood, or heart cells. The first few cell divisions in embryonic development produce more totipotent cells. After four days of embryonic cell division, the cells begin to specialize.

During early embryogenesis, cells divide and gradually become committed to specific patterns of gene activity through a process called cell determination. Specific genes are associated with the determination event. Because the daughter cells of each "determined" cell have the same limited potential as their parent cell, determination is considered heritable. Determination is permanent under normal conditions but it is possible to reverse the process experimentally.