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Chapter Summary

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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.

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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 (Figure 9-1). 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.

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Figure 9-1.

Well-publicized pedigree of hemophilia in the royal families of Europe.

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In the case of sex-linked hemophilia, genetics and history are clearly intertwined. A law in the Jewish Talmud, dating from about AD 600, implicitly recognizes the biological associations for this trait by allowing male children to be excused from ritual circumcision based upon having relatives with a bleeding disease. In the case of the descendants of Queen Victoria, hemophilia had serious consequences, at least indirectly, for the stability of the Russian royal family, the Romanovs, which ended with the death of Tsar Nicholas II and his family. His young son Alexei, a great-grandchild of Queen Victoria, suffered from hemophilia. The story of this family's isolation from the problems facing the Russian peasants and the powerful influence of the monk Rasputin form a sad tale of power, conflict, and human weakness. Alexei's medical condition gave Rasputin an influence in political activities that contributed to the overthrow of the Romanovs.

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The Romanov family's execution generated famous rumors, including the supposed survival of the young sister, Anastasia, depicted in numerous books and movies. The story was finally resolved with establishment of the identities of family members taken from a hidden grave and confirmed using conclusive DNA evidence. In fact, the genetic study led to new information. The form of hemophilia seen in the European Royal families has generally been assumed to be the more common form, hemophilia A. But recent sequencing of the loci in preserved ...

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