2. Genetics as a Tool in Genealogical Research

A. LARGE PEDIGREES OF A GENETIC CONDITION

Large pedigrees with a relatively rare genetic condition connecting every generation (in genetic jargon, this is called a dominant fully penetrant genetic condition) immediately have value to a genealogist in that, the genealogical research is confirmed by the genetic transmission of the condition.

An interesting example was the study of hemophilia in Maine by our Center for Human Genetics. Hemophilia was a good disease to study since it was easily diagnosed, and was clearly inherited as an X-linked recessive in the main two forms of the disease, hemophilia A and hemophilia B. With the help of public health nurses, physicians, and other medical professionals, we were able to develop on a confidential basis huge pedigrees showing distant relationships among affected families. Eighty percent of the whole population of Maine afflicted with hemophilia B descended from one couple who migrated to Cherryfield, Maine, in the late 1700s. In one line, it could be shown that a woman must be a carrier of the disorder because her father was affected. Since her father being a male had only one X chromosome, which had the hemophilia variant, he would have had to pass all his damaged X chromosomes to daughters, and all his normal Y chromosomes to his sons. At one time living in the household with this woman was another woman with the same surname but 50 years younger. We had always thought that genealogically there was a high likelihood these women were mother and daughter, but there was no definitive proof, and giving birth at age 50 was unlikely but not impossible. Later we discovered that the younger woman had a son with hemophilia. Thus the likelihood of a mother-daughter relationship was greatly strengthened. In this case we were dealing with hemophilia B, a much rarer condition than hemophilia A, although both are X-linked recessives.

Another conclusion from this study was that the genealogical research connecting the dozens of lines affected families back to this founding couple of the 1700s was sound because it was supported by the genetic evidence. Such confirmation of the connection between long lines, some as far as 10 generations apart (20 generations in all) can come in no other way.

Geneticists would call this couple carrying hemophilia B who came to Cherryfield 'founders,' and the high frequency of hemophilia-B in Maine descending from them a 'founder effect.' These are useful terms for genealogists, too.

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