Genetics and genomics sound alike and are often used interchangeably, yet important scientific and clinical distinctions exist between these two scientific fields of study.
The classical definition of genetics is the study of heredity, how characteristics and traits (phenotypes) of a living organism are transmitted from one generation to the next. This occurs via deoxyribonucleic acid (DNA), a double helix molecule in the cell’s nucleus that comprises genes—the basic unit of heredity. Many of the early principles and rules of heredity were discovered by an Augustinian monk and scientist, Gregor Mendel. His seminal research with pea plants in the mid-1800’s laid the foundation for modern-day genetics.
Genomics is the next evolution of classical genetics, and became possible only in recent decades due significant advances in DNA sequencing and computational biology. In 1976, Belgian scientists succeeded in fully sequencing the genome of bacteriophage MS2, a bacteria-infecting virus. They identified all 3,569 DNA base pairs, and the 4 nucleotides (Adenosine, Cytosine, Guanine and Tyrosine) that make up the DNA code. The first animal genome was completely sequenced in 1998. Five years later, with more than 1,000 researchers from six countries collaborating on the Human Genome Project, all 3.2 billion DNA base pairs in the human genome were identified.
One of the biggest take-aways from the Human Genome Project was that humans share 99.5% of their genome. But while this means we are far more alike with all the genes we have in common, that 0.5% difference is significant. The small differences account for all the different characteristics and traits (phenotypes) that exist among humans: hair or eye color, height, weight, skin pigment, disease risk, etc.
Genomics is the study of the entirety of an organism’s genes—the genome.
Genomic researchers analyze enormous amounts of DNA-sequence data to find variations that affect health, disease or drug response. In humans, that means searching through about 3.2 billion units of DNA across 23,000 genes.
In a clinical sense, genetics is the study of single genes or parts of genes and their effects on a person’s development, disease risk or response to drugs. This is generally referred to as a “monogenic” approach, since the focus is on a single gene. In contrast, genomics is the study of the function and interactions of all the genes in the genome.
While genetics and genomics are still quite distinct in how they impact health and disease, scientists are starting to view genetics and genomics as part of a continuum. On one end of the spectrum are single gene disorders with high penetrance – meaning if you have the mutation, you get the disease. On other end are genomic SNPs, which are common, low penetrance gene variants from multiple locations interacting with environmental factors, leading to complex diseases. Unlike genetic mutations, SNPs don’t automatically cause disease.
The graphic below summarizes some of the major differences between genetics and genomics.
Differences can be gleaned by going from left to right or top to bottom. The bar at the top depicts the effect of a gene change (genotype) on a person’s phenotype. A mutation, the most severe gene change, is on the left of the graphic. A gene variant or SNP (single nucleotide polymorphism), the most common gene change in the human genome, is represented on the right. A gene SNP can confer a significant advantage or it may be deleterious. The next bar represents the effect of environmental factors on the phenotype of an individual. The most significant interaction occurs on the far right between one or more gene variants and environmental factors.
Reading down the page, we see that a gene mutation can be lethal or create a classic monogenic disorder. Down Syndrome and cystic fibrosis are two of the more commonly known diseases caused by genetic mutations. Environmental factors do not play a role in the majority of inherited, single-gene disorders. As we move across to the right, it gets a little blurry. An example of this is the inherited form of breast cancer caused by BRCA1 and BRCA2 genes. Though these are classically considered genetic mutations, and would belong on the left of the graphic, recent studies suggest that the microenvironment associated with the BRCA1 and BRCA2 breast cancer cells, perhaps in concert with gene SNPs, may play a role in its development. In the near future, we may have to modify this graphic for at least some types of inherited genetic diseases.
Genetics: Black & White
Medical genetics has traditionally focused on health conditions that are due to inherited mutations in single genes (Huntington disease), in whole chromosomes (trisomy 21 or Down syndrome) or are associated with birth defects or developmental disabilities. With a traditional, single-gene disorder such as Tay-Sachs disease, genetic information is obtained from a patient as well as his or her immediate family members and relatives. The information is then used to diagnosis or predict the risk of an inherited genetic disorder passing from one generation to another. For many single-gene disorders, there are very limited medical interventions available. Although genetic disorders are individually rare, they account for about 5% of all human disease.
Genomics: Shades of Gray
The remaining 95% of genetic disorders are associated with common gene variants that interact with environmental factors, increasing or decreasing a individual’s susceptibility. This is the focus of genomic medicine: understanding how gene variants or SNPs at one or multiple locations interact with environmental factors (diet, drugs, infectious agents, chemicals, and behavior) to influence biochemical pathways, metabolic processes, and biological systems, which can lead to the chronic and degenerative diseases associated with aging.
For nearly every complex disease of the 21st century, including heart disease, arthritis and other autoimmune diseases, cancer, obesity, Alzheimer’s disease, autism, osteoporosis, and more—both genes and environment play a role. A person’s susceptibility to a disease depends on the number of gene SNPs involved and degree of environmental exposure. When multiple gene SNPs interact to confer an increase disease risk, it is referred to as polygenic. This is in contrast to a gene mutation, which is monogenic – one gene mutation causing one disease.
The profound impact of being able to “unzip” a person’s DNA to reduce, reverse, or mitigate many chronic diseases was not lost on the early genomic researchers. Using this approach to construct truly preventive medicine programs has been the promise and goal of genomic medicine. During the past 13 years, genomic testing has expanded from the lab to the clinic, and continues to grow at an exponential pace.
It is now possible to use results from clinically-based genomic testing to evaluate a person’s disease susceptibility, and develop evidence-based, personalized intervention strategies to reduce those risks. These strategies include DNA-directed lifestyle modifications, dietary recommendations, nutritional supplements and/or exercise, all of which influence how these genes function to create health or disease. Biomarker testing can then be used to evaluate whether the intervention is efficacious. With this approach, the guess-work and inefficiencies of trial-and-error strategies are greatly reduced, leading to better health more quickly and cost-effectively.
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