Heredity is the process of transmission of characteristics from one generation to another. Genes are the suppliers of specific traits. Environment is the sum total of all the external conditions that affects an individual. It includes social, cultural and life experiences. Twin studies explain variations in individual intelligences. Family studies, adoption studies and ethnic differences provide evidences of heritability. The physical and social environments provided by parents, education and schooling influences IQ development. Differential exposure of specific genotypes to specific environment produces different results of variation of IQ on different individuals. Influence of environmental evidences on IQ explains that biologically unrelated children, reared under same environment may show similarities in IQ content. Childhood illness, lead poisoning and other hazardous diseases may be caused both due to genetic and environmental interaction. Both heredity and environment affects development of intelligence and other cognitive abilities. More research work and case studies are required in this field to resolve controversies related to heredity and environmental effects on intelligence among individuals.