Molecular Anthropology

The human genome is much too large to sequence all of it to make comparisons, using current technology. Instead, much smaller portions of it are used. One strategy is to compare gene sequences, such as the sequence for hemoglobin. A potential problem with this is that most mutations in such useful genes are harmful, and so the few harmless mutations they accumulate may be similar between two individuals, despite a long evolutionary separation. Nonetheless, gene comparisons are useful for distantly related species, such as humans and yeast.

An alternative is to look at noncoding regions of DNA. These include microsatellite DNA sequences, a type of repetitive DNA element found throughout the genome. Because these sequences do not code for protein, most mutations in them do not affect the viability of the organism in which they occur. Thus they accumulate mutations more quickly. Another option is single nucleotide polymorphisms. These are sequences which differ among individuals or groups by a single nucleotide. There are millions of such sequences in the genome. Because there are so many different forms, these noncoding sequences are especially useful for determining kinship among closely related individuals, such as members of a tribe or extended family.

One potential problem with sequence comparisons is back mutation, in which a base mutates to another, and then reverts to the original (for example, C → T → C). When this occurs, two sequences may appear to be more closely related (less separated in time) than they really are, since the intervening mutation (the change from C to T, in this case) may not be apparent. Because of back mutation, the observed number of differences between sequences represents the minimum actual difference. Correction factors can be applied to estimate the true difference.

Another potential problem with any sequence on a chromosome, whether or not that sequence codes for a protein, is that most chromosomes do not remain intact during meiosis. This is because crossing over occurs, in which homologous chromosomes recombine (exchange segments). After a few generations, it becomes very difficult to track individual sequences and compare them with any confidence to similar sequences in another person. To avoid this problem, molecular anthropologists focus on two sources of DNA that do not recombine: the Y chromosome and mitochondrial DNA.