One of the most important tools used by genetic professionals is the pedigree, a pictorial description of a family tree. A complete pedigree provides information about the biological relationships of individuals in the family, their medical history, the pattern of inheritance of a genetic disorder in the family, variable expression of the disorder, which family members are at risk, fertility of individuals (including pregnancies, miscarriages, and stillbirths), and family members who are dead. Physicians sometimes refer to a pedigree as a "genogram." However, genograms usually contain more social information about family relationships than a traditional pedigree used by geneticists and genetic counselors. For example, a genogram can show a teenage child who has a poor relationship with a parent or an individual estranged from the family.
Use of Pedigrees
Pedigrees can be used in the clinical setting, such as genetic counseling sessions or genetic evaluations, or in genetic research. By analyzing how many family members have a genetic disorder, how these individuals are related, and the sex of the affected individuals, it is often possible to determine the inheritance pattern of the genetic disorder in the family. Together, the inheritance pattern and an accurate diagnosis help the genetic professional provide accurate risk information to the family. This includes risk information for future pregnancies or relatives who are currently unaffected, but who are at risk for developing the disorder based on family history information. Genetic testing options, if available, can then be offered to those at risk.
The pedigree is also a standard tool used by researchers. For example, in studies aimed at identifying genes that cause human genetic disorders, researchers must collect detailed information on relatives participating in the study, particularly those relatives who are affected with the disorder. Researchers compare the genes of affected individuals with the genes of those who did not inherit the disorder to identify the specific genes responsible. In other studies the disease-causing gene is known, and researchers study the gene mutation (s). A pedigree can help identify which family members should be included in mutation analysis, as only those family members who are affected or are at-risk could carry a mutation. Researchers can also pictorially show laboratory data, such as genotypes or haplotypes, on the pedigree.
Terminology
Standard symbols are used to draw pedigrees. For example, males are represented by squares and females by circles. The individual who brought the family to the attention of the medical professional or researcher is called a "proband" and is identified on the pedigree by an arrow pointing towards the symbol for that individual. This individual usually has the disorder of interest to the researcher or physician.
The informant is the individual who was interviewed to obtain the pedigree. The informant may or may not be the same person as the proband. The name of the informant, date the pedigree is drawn, and interviewer are also noted on the pedigree. So, too, is the founder of the pedigree, who is the first family member known to have had the disorder.
Drawing and Recording Pedigrees
Pedigrees are hand-drawn or created using special computer software. The standard pedigree typically includes at least three generations, with each generation arranged horizontally and connected to the other generations by lines. Family members who have the genetic disorder in question are colored-in or shaded. Unique symbols represent carriers, miscarriages, people of unknown sex, twins, and other categories of individuals. Furthermore, different patterns within the pedigree symbol may represent variable expression (variation in symptoms of individuals with the same disorder). For example, in a pedigree showing myotonic dystrophy, a shaded upper-left quadrant of the male or female symbol may represent cataracts, but a shaded right-lower quadrant may represent heart problems.
The process of collecting the family history used to draw the pedigree includes interviewing the informant and asking questions in three general categories. The detailed information about family members is recorded under the symbol for that individual. First, the interviewer asks standard questions about the family members, resulting in a skeleton picture of the family. These questions collect identifying information about each family member, such as their names, names of their parents, siblings, and children, dates of birth, dates of death, cause of death, and pregnancies. Detailed information is also included about general health problems, such as cancer and heart disease, and their specific symptoms, onset, and age at diagnosis. The interviewer may also ask about the ethnic background of family members and any possible consanguineous relationships (relationships between individuals who are related to each other by blood, such as first cousins).
Second, the interviewer may ask general questions to identify common genetic syndromes in the family, such as birth defects, mental retardation, vision and hearing loss, and common genetic disorders found more prevalently in certain ethnic backgrounds. For example, cystic fibrosis is relatively more common in people of Western European decent, whereas sickle cell disease is more common in those of African decent. If any common genetic syndromes are identified, they are then discussed during the genetic counseling appointment.
Lastly, the interviewer may collect targeted information about the specific genetic disease for which the family was referred. Information is gathered on the symptoms of the disease, age of onset, and age at diagnosis. These targeted questions help genetic counselors and researchers identify the typical course of disease in the family, disease severity, possible variable expression, reduced penetrance, or genetic anticipation. Because accurate diagnoses are essential for accurate genetic counseling and research studies, all diagnoses should be confirmed by medical records. Further, pedigree updates can be obtained over time during follow-up genetic counseling sessions or research interviews.
Confidentiality
Pedigrees contain very personal and identifying information about families. It is therefore essential that pedigrees be kept confidential. This is important not only in the genetic counseling clinic or research group, but also within the family. It is unethical to share information given about family members with other family members, insurance companies, or other professionals not involved in the genetic counseling sessions or research study. Information about individuals should only be released with proper written consent by that individual.
Bibliography
Bennett, Robin L., et al. "Recommendations for Standardized Human PedigreeNomenclature." American Journal of Human Genetics 56 (1995): 743-752.
Stroop, Jennifer B. "The Family History as a Screening Tool." Pediatric Annals 29 (2000): 279-282.
Internet Resource
March of Dimes. "Taking a Family Health and Social History." http://www.fullcirc.com/mod/outlines/family/history.htm.
—Elizabeth C. Melvin
