Adaptations: allow organisms to meet recurring environmental changes to their survival, thereby increasing their reproductive ability. Adoption study: a research method in behavior genetics in which adopted people are compared on some characteristic with both their biological and adoptive parents in an attempt to determine how strong a genetic component the characteristic might have. Alleles: the two genes, one on each chromosome, that control the same trait. Altruism: occurs when one individual helps another, but in so doing accrues some cost. Biologically based mechanisms: mechanisms that receive input from the environment, process the information, and respond to it. Chromosome: tightly coiled strand of deoxyribonucleic acid (dna) and protein that contains the genes. Concordance: the likelihood that two people share a particular characteristic. Cooperation: situation in which one individual helps another and in so doing gains some advantage. Domain-specific adaptations: adaptations designed to solve a particular problem. Dominant gene: a gene, which when present, will produce a particular characteristic.