matching or near mitochondria, it can be inferred that they share a common maternal-line ancestor at some point in the recent past. Direct paternal lineages Y-Chromosome DNA (Y-DNA) testing involves short tandem repeat (STR) and, sometimes, single nucleotide polymorphism (SNP) testing of the Y-Chromosome, which is present only in males and only reveals information on the strict-paternal line. As with the mitochondria, close matches with individuals indicate a recent common ancestor. Because surnames in many cultures are transmitted down the paternal line, this testing is often used by surname DNA projects. 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 offer 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) varies within a normal distribution 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 encounter an increasing number of ancestors from whom they