Section 1: genetic influences on development: genetic basics. Dna composed into segments called genes (basic unites of hereditary information) genes contain sequences of chemicals called nucleotides. There are 23,000 genes in out 46 chromosomes, the total human genome (entire store of human organism"s hereditary information) Genotype the totality of an individuals genes/ organisms unique individual features. Phenotype the persons actual characteristics derived from genotype. Example born with exceptional musical ability is your genotype, but with no access to musical instruments this genotype might not be apparent in your phenotype. Allele pair of chromosomes, each of two forms of a gene (can have dominant and recessive forms) Most characteristics in human development are not determined solely by a single pair of genes, however sometimes single pairs of genes play a crucial part such as the sickle-cell anemia. Polygenic inheritance- interaction of multiple genes rather then just one/ expression of phenotypic characteristics due the inheritance of multiple genes: sex chromosomes.