Genes: the unit of heredity that determines a particular characteristic. Dominant gene: expressed in the offspring whenever it is present. Recessive gene: expressed only when it is matched with a similar gene from the other parent. Genotype: the genetic constitution determined at the moment of conception. Phenotype: observable physical characteristics that result from both genetic and environmental influences. Monozygotic twins: identical twins, result of one zygote dividing into two. Dizygotic twins: fraternal twins, result of two separately fertilized eggs. Heritability: statistical estimate of the portion of observed variation in a population that is caused by differences in heredity. Variation: the measure of the overall amount of difference among people. Neuron: communication; receive, integrate, and transmit information in the nervous system through chemical signals. Dendrites: extensions of the neuron that detect information from other neurons. Cell body: information is collected and integrated. Axon: where information is transmitted to other neurons.