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## How Degraded DNA Works: The Technical Challenge
The lock of hair that Horace Deeble took in 1890 was five to six centimeters long — a small piece. Hair, as a biological material, is not ideal for long-term DNA preservation. The DNA in a hair sample is primarily in the root, where cells with intact nuclei are attached, rather than in the shaft, which is primarily keratin. A lock of hair that has been cut or pulled may have some root material, or it may not. Storage conditions matter enormously: cold and dry conditions slow DNA degradation; room temperature and any humidity accelerate it.
For more than a century, the lock of Sitting Bull’s hair was held in the Smithsonian’s storage at conditions that were not designed for genetic preservation. The DNA inside it degraded continuously over that period. By the time Willerslev’s team began working with the material, they were dealing with fragments — short stretches of the original genomic sequence, broken and scattered, with much of the original material irretrievably degraded.
The problem this creates for kinship analysis is primarily statistical. When you compare two people’s DNA to determine their degree of relationship, you look for long shared segments — stretches of the genome where both people carry the same version of the DNA sequence, inherited from a common ancestor. The longer the shared segments, the more recently those two people shared that ancestor. A parent and child share many long segments. Grandparents and grandchildren share fewer and shorter ones. At the great-grandparent level — three generations of separation — you expect to share roughly twelve to thirteen percent of your autosomal DNA, but the exact proportion varies considerably because of the random nature of DNA shuffling during reproduction.