Biologists could use experimental breeding to directly study heredity in plants and animals. However, since they could not experiment with humans, eugenicists had to infer hereditary relationships from living family members and reports of their relatives. A pedigree chart was used to track the presence or absence of a given trait (phenotype) through two or more generations of a family. By examining the pattern of appearance of the phenotype through several generations, it is sometimes possible to infer the genetic constitution (genotype) of individuals.
All pedigree charts use certain conventions. Arabic numerals identify individuals, and Roman numerals identify members of the same generation that are displayed in a horizontal row. Squares represent males, and circles represent females. Lines join parents and children. Filled-in squares and circles indicate individuals displaying a certain trait (phenotype).
There are a number of problems with drawing genetic conclusions from pedigree charts. A good pedigree study requires that the presence or absence of the trait be determined unambiguously for as many family members as possible. However, the status of deceased family members or ones who live far away cannot be directly observed, and are often "guessed" from the reports of other family members. In addition, many traits eugenicists were interested in — including alcoholism, manic depression, musical ability, scholarship, or interest in carpentry — are extremely subjective behaviors that cannot be measured in any meaningful way. Another major problem was frequent haphazard collection of data — many questionnaires were improperly completed by the family members themselves, and second or third-hand reports were trusted.
By using the pedigree charts, eugenicists gave the impression that vague behaviors are well-established genetic traits. However, because of their anecdotal nature, most pedigrees made by the eugenicists showed nothing about biological heredity. Most patterns in these "data" are due to learned behaviors shared by family members. A visiting scientific committee to the Eugenics Record Office in 1935 concluded that virtually all pedigree information collected over the preceding quarter-century was worthless for genetic purposes.
Though pedigree charts are still used in genetic studies today, DNA analysis can directly ascertain genotypes for family members. However, a precise definition and rigorous scoring of the phenotype are still essential to establish a hereditary relationship.