Genetics is the study of heredity. Biological parents pass down their genetic traits to their offspring. Although children resemble their parents, genetic traits often vary in appearance or molecular function. For example, two parents with normal color vision can sometimes produce a son with red-green colorblindness. Patterns of genetic inheritance will be discussed in a later section. Molecular geneticists study the biological mechanisms responsible for creating variation between individuals, such as DNA mutations (see Chapter 4), cell division, and genetic regulation.
Molecular anthropologists use genetic data to test anthropological questions. Some of these anthropologists utilize ancient DNA (aDNA), which is DNA that is extracted from anything once living, including human, animal, and plant remains. Over time, DNA becomes degraded (i.e., less intact), but specialized laboratory techniques can make copies of short degraded aDNA segments, which can then be reassembled to provide more complete DNA information. A recent example of an aDNA study is provided in Special Topic: Native American Immunity and European Diseases, and aDNA is also explored in Appendix D.
DNA Structure
The discovery, in 1953, of the molecular structure of deoxyribonucleic acid (DNA) was one of the greatest scientific achievements of all time. Using X-ray crystallography, Rosalind Franklin (Figure 3.7) provided the image that clearly showed the double helix shape of DNA. However, due to a great deal of controversy, Franklin’s colleague and outside associates received greater publicity for the discovery. In 1962, James Watson, Francis Crick, and Maurice Wilkins received a Nobel Prize for developing a biochemical model of DNA. Unfortunately, Rosalind Franklin had passed away in 1958 from ovarian cancer. In current times, Franklin’s important contribution and her reputation as a skilled scientist are widely acknowledged.
Figure 3.7: Chemist and X-ray crystallographer Rosalind Franklin. Credit: Rosalind Franklin from the personal collection of Jenifer Glynn by MRC Laboratory of Molecular Biology is under a CC BY-SA 4.0 License.
The double helix shape of DNA can be described as a twisted ladder (Figure 3.8). More specifically, DNA is a double-stranded molecule with its two strands oriented in opposite directions (i.e., antiparallel). Each strand is composed of nucleotides with a sugar phosphate backbone. There are four different types of DNA nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). The two DNA strands are held together by nucleotide base pairs, which have chemical bonding rules. The complementary base-pairing rules are as follows: A and T bond with each other, while C and G form a bond. The chemical bonds between A-T and C-G are formed by “weak” hydrogen atom interactions, which means the two strands can be easily separated. A DNA sequence is the order of nucleotide bases (A, T, G, C) along only one DNA strand. If one DNA strand has the sequence CATGCT, then the other strand will have a complementary sequence GTACGA. This is an example of a short DNA sequence. In reality, there are approximately three billion DNA base pairs in human cells.
If you removed the DNA from a single human cell and stretched it out completely, it would measure approximately two meters (about 6.5 feet). Therefore, DNA molecules must be compactly organized in the nucleus. To achieve this, the double helix configuration of DNA undergoes coiling. An analogy would be twisting a string until coils are formed and then continuing to twist so that secondary coils are formed, and so on. To assist with coiling, DNA is first wrapped around proteins called histones. This creates a complex called chromatin, which resembles “beads on a string” (Figure 3.9). Next, chromatin is further coiled into a chromosome. Another important feature of DNA is that chromosomes can be altered from tightly coiled (chromatin) to loosely coiled (euchromatin). Most of the time, chromosomes in the nucleus remain in a euchromatin state so that DNA sequences are accessible for regulatory processes to occur.
Human body cells typically have 23 pairs of chromosomes, for a total of 46 chromosomes in each cell’s nucleus. An interesting fact is that the number of chromosomes an organism possesses varies by species, and this figure is not dependent upon the size or complexity of the organism. For instance, chimpanzees have a total of 48 chromosomes, while hermit crabs have 254. Chromosomes also have a distinct physical structure, including centromeres (the “center”) and telomeres (the ends) (Figure 3.10). Because of the centromeric region, chromosomes are described as having two different “arms,” where one arm is long and the other is shorter. Centromeres play an important role during cell division, which will be discussed in the next section. Telomeres are located at the ends of chromosomes; they help protect the chromosomes from degradation after every round of cell division.
Special Topic: Native American Immunity and European Diseases—A Study of Ancient DNA
Beginning in the early fifteenth century, Native Americans progressively suffered from high mortality rates as the result of colonization from foreign powers. European-borne diseases such as measles, tuberculosis, influenza, and smallpox are largely responsible for the population collapse of indigenous peoples in the Americas. Many Europeans who immigrated to the Americas had lived in large sedentary populations, which also included coexisting with domestic animals and pests. Although a few prehistoric Native American populations can be characterized as large agricultural societies (especially in Mesoamerica), their overall culture, community lifestyle, and subsistence practices were markedly different from that of Europeans. Therefore, because they did not share the same urban living environments as Europeans, it is believed that Native Americans were susceptible to many European diseases.
In 2016, a Nature article published by John Lindo and colleagues was the first to investigate whether pre-contact Native Americans possessed a genetic susceptibility to European diseases. Their study included Tsimshians, a First Nation community from British Columbia (Figure 3.11a-b). DNA from both present-day and ancient individuals (who lived between 500 and 6,000 years ago) was analyzed. The research team discovered that a change occurred in the HLA-DQA1 gene, which is a member of the major histocompatibility complex (MHC) immune system molecules. MHC molecules are responsible for detecting and triggering an immune response against pathogens. Lindo and colleagues (2016) concluded that HLA-DQA1 gene helped Native Americans adapt to their local environmental ecology. However, when European-borne epidemics occurred in the Northwest during the 1800s, a certain HLA-DQA1DNA sequence variant (allele) associated with ancient Tsimshian immunity was no longer adaptive. As the result of past selective pressures from European diseases, present-day Tsimshians have different HLA-DQA1 allele frequencies. The precise role that HLA-DQA1 plays in immune adaptation requires further investigation. But overall, this study serves as an example of how studying ancient DNA from the remains of deceased individuals can help provide insight into living human populations and historical events.
