Summarizing Family Bloodline
Genetics is about family histories. Whether you are doing experimental breeding of mice or are exploring diversity in the human population, genetic traits are passed in family lineages. But when the focus is on a molecular or a developmental question, that relationship is easily taken for granted. On the other hand, pedigrees can be valuable when taking a broader view of gene expression, phenotypic variability, and patterns of transmission. Pedigrees can yield insights that single mating examples fail to provide. That is especially true for human genetics, where experimental matings typical of model organism studies cannot be performed.
Pedigrees are a simple way to summarize a lot of information about genetic relationships. One of the most famous pedigrees is that for hemophilia in the royal families of Europe. The most common form of this blood clotting condition is hemophilia A, a sex-linked trait associated with a defect in clotting factor VIII. Indeed, it was the first human genetic trait to be found to follow a sex-linked inheritance pattern. Other forms of hemophilia include hemophilia B affecting clotting factor IX, which is also sex-linked, and hemophilia C coding factor XI, which is autosomal. Although hemophilia A is more common, this type of trait heterogeneity can obviously complicate the genetic analysis of a pedigree if one carelessly ignores alternative explanations.
A pedigree is really nothing more than a series of Mendelian genetic crosses involving relatives. But we often find that seeing the patterns of expression in a pedigree can yield important clues about a genetic condition that the study of one isolated patient or family cannot.
Pedigrees are organized by generation. Here is a useful hint: to begin to interpret an inheritance pattern, reverse the way you normally think about gene transmission. Rather than looking for the appearance of a trait among the progeny of a family, look from the progeny generation backward toward the parents. In other words, begin by looking at transmission patterns by moving your attention up the pedigree, not down it. If, for example, a child shows a dominant trait, then you expect one of the parents to show it. The other direction is not as certain. Just because a parent has a dominant trait does not mean that one of their few children will necessarily inherit it. Examples of this logic are explored in the next section.
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