User:Thewellman/sandbox

for inclusion in Genetic genealogy under

Uses

Pedigree family trees

Pedigree family trees have traditionally been prepared from recollections of individuals about their parents and grandparents. These family trees may be extended if recollections of earlier generations were preserved through oral tradition or written documents. Some genealogists regard oral tradition as myths unless confirmed with written documentation like birth certificates, marriage certificates, census reports, headstones, or notes in family bibles. Few written records are kept by illiterate populations, and many documents have been destroyed by warfare or natural disasters. DNA comparison may offer an alternative means of confirming family relationships of biological parents, but may be confused by adoption or when a mother conceals the identity of the father of her child.

While mitochondrial and Y-chromosome DNA matching offers the most definitive confirmation of ancestral relationships, the information from a tested individual is relevant to a decreasing fraction of their ancestors from earlier generations. Potential ambiguity must be considered when seeking confirmation from comparison of autosomal DNA. The first source of ambiguity arises from the underlying similarity of every individual's DNA sequence. Many short gene segments will be identical by coincidental recombination (Identical by State: IBS) rather than inheritance from a single ancestor (Identical by Descent: IBD). Segments of greater length offer increased confidence of a shared ancestor. A second source of ambiguity results from the random distribution of genes to each child of a parent. Only identical twins inherit exactly the same gene segments. Although a child inherits exactly half of their DNA from each parent, the percentage inherited from any given ancestor in an earlier generation (with the exception of X chromosome DNA) is normally distributed around a median value of 100% divided by the number of ancestors in that generation. An individual comparing autosomal DNA with ancestors of successively earlier generations will ultimately encounter ancestors from whom they inherited no DNA segments of significant length. Cousins descended from the same ancestor are less likely to have inherited the same DNA segments from that ancestor. All descendants of the same parent or grandparent, and nearly all descendants of the same great-grandparent, will share gene segments of significant length; but 10% of 3rd cousins, 55% of 4th cousins, 85% of 5th cousins, and more than 95% of more distant cousins will share no gene segments of significant length.

The best autosomal DNA method for confirming ancestry is to compare DNA with known relatives. A more complicated task is using a DNA database to identify previously unknown individuals who share DNA with the individual of interest; and then attempting to find shared ancestors with those individuals. The first problem with the latter procedure involves the relatively poor family history knowledge of most database populations. A significant percentage of individuals in many DNA databases have done DNA testing because they are uncertain of their parentage, and many who confidently identify their parents are unable or unwilling to share information about earlier generations. It may be easier to identify a shared ancestor in the fortunate situation of shared DNA between two individuals with comprehensive family trees, but finding multiple shared ancestors raises the possibility that one of those ancestors may have been incorrectly identified. Resolving that ambiguity typically requires finding a third individual sharing both the ancestor and the gene segment of interest.