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Genomics Enables Scientists to Study Genetic Variability in Human Populations

An illustration shows a human male and human female figure standing side-by-side in silhouette. They are standing with their backs facing the viewer. Both figures are in the anatomical position with their arms resting at their side, palms facing forward.

Thinking about population genetics often brings to mind visions of animals in the wild being swept along by the tide of natural catastrophes, soil depletion, or predation. However, over the past ten years the field of population genetics has undergone major renovations because of recent advances in gene sequencing and screening technologies. These technological innovations have allowed scientists to tackle bigger and broader questions related to population trends, and to study genetic variation on a much broader scale than ever before possible with older methods, such as test crosses, random sampling, and field work. Today, discoveries can be facilitated by the ever-expanding field of genomics, which is the use of large databases for the purpose of studying genetic variation on a large scale across many different organisms.

What is genomics?

A horizontal bar graph shows the size of the genome as a blue bar beneath the total number of genes in an organism's DNA as a red bar for four taxa. The taxa, in order of largest to smallest genome size, are: human, mouse, fruit fly, and E. coli. Humans have 24,000 genes and 3223 million base pairs. Mice have 26,762 genes and 2627 million base pairs. Fruit flies have 13,525 genes and 170 million base pairs. E. coli have 4,289 genes and 5 million base pairs.
FIgure 1: Genome comparison chart
Genome size is the total number of base pairs in an organism. While the number of genes in an organism's DNA (red bars) varies from species to species (numbers at right), it is not always proportional to genome size (blue bars, in millions of base pairs). Note how many genes a fruit fly can squeeze out of its relatively small genome.

Until recently, the term genome was used to describe the complete set of chromosomes that made up a given species. Today, scientists use the term genome to refer to the complete set of DNA sequences derived from each chromosome of a given species. Genomics is a relatively new and ever-expanding field dedicated to the study of defining genomes in this more specific way.

The direct analysis of the genome of an organism, or the genomes of a group of organisms, is now possible through advances in the efficiency of DNA sequencing and large-scale genetic screening. These new high-throughput methods allow researchers to collect vast amounts of information about genetic variation in very short periods of time.

Genomics has also shown that the size of a genome (i.e. the number of nucleotide pairs it contains) is not necessarily proportional to the number of genes contained within it. Some organisms, like the fruit fly, fit a considerable number of genes into a relatively small genome, whereas humans and mice possess many extra "unused" nucleotide pairs that do not encode genes (Figure 1).

See how human genomes compare to others

How many genes does it take to build a human being?

Although early reports suggested that human chromosomes might contain as many as 100,000 different genes, we now know that the 24 different human chromosomes altogether contain 20,000-25,000 different genes. However, it is likely that many of those genes are not absolutely required.

How can we study human genetic variation?

Humans can also be the focus of population genetics studies, as they too have been subject to the forces of change over long periods of time. Recently, the DNA sequence of the entire human genome was deciphered in a massive effort called The Human Genome Project (HGP). This project sequenced the DNA of each human chromosome from end to end, determined the DNA sequence of every human gene, and mapped the precise location of every human gene to a particular region of a human chromosome.

With this information in hand, scientists now have a baseline definition of every human gene. With this baseline, they are beginning to study how the DNA sequences of human genes can vary among individuals and populations. In fact, scientists can currently study the variability of those genes (i.e. all allelic forms) in different populations around the globe. Early results from these studies indicate that humans are identical over the vast majority of their genome. The apparently striking phenotypic variation among human beings around the world can be accounted for by only an exceptionally small number of genetic differences. Genes that code for skin color, facial features, or body size represent a small fraction of the DNA that comprises the total human genome.

Variation in the human genome: SNPs

After the completion of the HGP in 2003, researchers began to pinpoint locations within the genome that varied among individuals. These scientists discovered that the most common type of DNA sequence variation found in the human genome is the single nucleotide polymorphism (SNP, pronounced "snip"). There are approximately 10 million SNPs in the human genome.

A worldwide effort known as the HapMap Project is mapping SNPs and other genetic variants in human populations around the world. By mapping the distribution of SNPs among different human populations, researchers can begin to learn which types of variation are most common in certain regions of the world. This information will help explain human origins and disease risks as well as how they relate to environmental conditions, both past and present. To date, the HapMap project has identified over 3.1 million SNPs across the human genome that are common among individuals of African, Asian, and European ancestry.

The HapMap database has also helped foster a new type of research in personalized medicine called the genome-wide association study (GWAS). With these studies, the distribution of SNPs is determined in hundreds, or even thousands, of people with and without a particular disease. Comparisons between diseased and non-diseased groups of individuals help determine which SNPs co-occur with disease symptoms. With this information in hand, scientists can carry out statistical analyses to help predict whether a certain SNP is associated with a specific disease, with the hope of identifying individuals who may be at risk.

For example, in a recent study conducted in the United Kingdom, researchers genotyped 2,000 individuals who had one of seven common disorders. Next, those individuals were compared to 3,000 genotyped control individuals who did not have the common disorders. With these comparisons, the researchers identified new genetic markers associated with increased risk for disorders such as heart disease and diabetes. In the future this study will be expanded to include 36,000 more individuals, and it will focus on 14 more health-related disorders as well as individual responses to certain drugs. Using these types of studies, scientists can sample large numbers of people and make meaningful predictions regarding disease risk for individuals based on the presence or absence of genetic markers within their genome.

Genomics and biological discovery

Genomic data can support discovery in diverse areas of biology, including medicine, systematics, and conservation biology. Like many histories, the history of genomics is fraught with conflict, disagreement, and excitement. The personalities and ideas that have shaped genomics even included a race between publicly funded and corporate genome sequencing groups that resulted in a tie at the finish line. Several subspecialty areas of genomics are also expanding as the community of scientists within them grows. These relatively new areas of genomic investigation include: epigenomics (the study of DNA modification), transcriptomics (the study of cellular RNA content), and proteomics (the study of proteins that characterize a particular cell).

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