Detailed characterization of various environmental influences on development. Criticisms richest description of environmental influences suggests interventions for optimizing development at various levels of society generalizability from one environment to next questioned not complete; a complement to existing theories. Interaction of heredity and environment: nature and nurture. Father"s sperm + mother"s ovum = zygote with 46 chromosomes in total. The process by which a cell separates chromosomes in its nucleus into two identical cells. Chromosomes within sperm cells divide in half before they fertilize an ovum or egg cell. Chromosomes with egg cells divide in half before they are fertilized. The randomness creates variation in the resulting embryo. Direct production of amino acids necessary for forming new cells. Guide cells to develop different parts of body. May turn on or turn off other genes. Simple dominant recessive inheritance some alleles are stronger than others. To express a trait one must have a homozygous recessive allele.