How to Calculate Probabilities Using Pedigree Charts

Pedigree charts are diagrams that show the pheno- types and/or genotypes for a particular organism, its ancestors, and descendants. While commonly used in human families to track genetic diseases, they can be used for any species and any inherited trait. Geneticists use a standardized set of symbols to represent an indi- vidual’s sex, family relationships and phenotype. These diagrams are used to determine the mode of inheritance of a particular disease or trait, and to predict the probability of its appearance among offspring. Pedigree analysis is therefore an important tool in basic research, agriculture, and genetic counseling. Each pedigree chart represents all of the available in- formation about the inheritance of a single trait (most often a disease) within a family. The pedigree chart is therefore drawn using factual information, but there is always some possibility of errors in this information, es- pecially when relying on family members’ recollections or even clinical diagnoses. In real pedigrees, further (including age of onset) and variable expressivity of disease alleles, but for the examples presented in this book, we will presume complete accuracy of the ped- igrees – that is, the phenotype accurately reflects the genotype. A pedigree may be drawn when trying to determine the nature of a newly discovered disease, or when an individual with a family history of a disease wants to know the probability of passing the disease on to their children. In either case, a tree is drawn, as shown in Figure 6-2, with circles to represent females, and squares to represent males. Matings are drawn as a line joining a male and female, while a consanguineous mating (closely related, such as siblings or first cousins) is two can arise due to incomplete penetrance The affected individual that brings the family to the attention of a geneticist is called the proband (or prop- ositus). If the individual is unaffected, they are called the consultand. If an individual is known to have symp- toms of the disease (affected), the symbol is filled in. Sometimes a half-filled in symbol is used to indicate a known carrier of a disease; this is someone who does not have any symptoms of the disease, but who passed the disease on to subsequent generations because they are a heterozygote. A circle with a dot in the centre indi- cates female carriers of X-linked traits. Note that when a pedigree is constructed, we often don’t know whether a particular individual is a carrier or not. Not all carri- ers are always explicitly indicated in a pedigree. If you don’t know the exact genotype, don’t guess. If some- one can be a homozygote or heterozygote, indicate the second allele a slash (e.g. A/-). #tickleCellAnemia #pedigree #geneticDisorder #disease #Gene #allele #PedigreeAnalysis #pedigreeAnalysisPractice #pedigreeAnalysisGenetics #pedigreeAnalysisProbability #pedigreeAnalysisAnswers #pedigreeCharts #Genetics #HowToSolvePedigreeProblems #solvePedigreeProblems #solvingPedigreeProblems #pedigreeProblemsAndSolutions #howToAnalyzePedigree #analyzingPedigree #geneticsProblems