Human beings are complex living organisms that can be characterized by their appearance and behavior ateach point in the life cycle. Many of these characteristics are uniquely human, such as the array of languages that facilitate interpersonal communication and permit a meaningful interplay of ideas and emotions. Other characteristics, such as affection and aggression, are less distinctive and place our species as one among many that populate the earth. Scientific advances over the past 150 years clearly indicate that hereditary factors are transmitted from generation to generation and account for much of the observed variation among and within species. Although the complexities of human existence cannot be reduced simply to the effects of genes, it is inescapable that genetic factors provide the biological basis for many of our potentialities and vulnerabilities as human beings .