Much of the repeated sequence data in the human genome is not functional; it does not specify protein structure. It has been called “junk DNA,” though it may have functions that are not yet clear to scientists. About 20 percent of human DNA is functional, in the sense that it codes for protein. And, within that 20 percent, there is considerable similarity in sequence among different people. This is exactly what would be expected; the structure of functional DNA would be conserved. Any major alterations in the sequence of the functional DNA would lead to problems with the specification of protein structure and would likely cause problems for the individual.
Changes in one or a few bases in DNA are called mutations, and they do occur. Study of variations in functional proteins, such as hemoglobin, makes it clear that some mutations are innocuous. Scientists know this because these mutated versions of hemoglobin are found in living people. In other words, there has been a mutation in DNA, and it has caused a change in the hemoglobin protein, but the person with the mutation is alive and well. The mutation did not therefore affect the functionality of the hemoglobin. But many mutations do cause problems with protein functionality. These problems often lead to serious medical problems. Many mutations probably lead to early death—so early in the development of an embryo that the mother may not yet even realize she is pregnant. Scientists will never see the mutations that prevent such an embryo from surviving.
In the 80 percent or so of DNA that does not specify protein structure, there is enormous sequence and repeat-sequence polymorphism. For the most part it does not seem to have any negative consequences for the individual and can be exploited for purposes of identification, as has been done by forensic scientists.
