Undue importance is often given to the supposed influence of remote ancestors. It is hoped that the following notes will correct some of these misconceptions. However, the influence of distant ancestors must not be completely ignored since cultural inheritance is equally as important as heredity. Genealogy is a part of social history, especially records of family size and occupations. Many of the skills and trades followed in the 19th century have now disappeared. Illegitimacy was a serious social stigma and the identity and occupation of an illegitimate child's father was rarely entered on the birth certificate. A detailed knowledge of our ancestors, therefore, not only gives us an idea of where we come from but also an insight into our cultural background.
The number of genes we hold in common with our ancestors is halved as we go back through the generations. A great-grandparent only holds about 1/8 of his/her genes in common with one of his/her great-grandchildren. This is another way of saying we only inherit 1/8 of our genes from a single great-grandparent. The remaining 7/8 comes from the other 7 great-grandparents. If we go back 10 generations (about 300 years) the contribution is only (1/2)10 = 1/1024, although by this time sampling variation would probably have reduced it to zero.
To give a quantitative assessment of the closeness of relationship between individuals, I have included sections on how to calculate the coefficient of relationship. Although this is not an essential tool for a genealogist, it adds further meaning to the records. Similarly, methods can also be found for working out the coefficient of inbreeding of an individual, which enables one to predict the increased likelihood of inheriting harmful genetic conditions following cousin marriages.
I have also included sections on forbidden marriage laws, which were first formulated in the reign of Elizabeth I in 1560, but have since been modified several times; and current laws for the inheritance of wealth and property in the U.K.
The following sections, which only require a limited knowledge of genetics, examine various genetic and quantitative aspects of human relationships and genealogy.