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Genetic Variation


Most of any one person's DNA, about 99.9 percent, is exactly the same as any unrelated person's DNA. Differences in the sequence of DNA among individuals are called genetic variation.

Genetic variation explains some of the differences among people, such as:

  • Blood group
  • Eye color
  • Skin color
  • Hair color
  • Higher or lower risk for getting particular diseases
    • Cystic fibrosis
    • Sickle cell disease
    • Diabetes
    • Cancer
    • Stroke
    • Alzheimer's disease
    • Parkinson's disease
    • Depression
    • Alcohoism
    • Heart disease
    • Arthritis and
    • Asthma

Simply speaking, variation is difference. It's what makes you "you". Genetic variation is a difference in the DNA sequence. The "letters" of DNA are molecules called nucleotides: adenine, cytosine, guanine, and thymine (A,C,G,T) strung together in long chains called sequences.

The occasional single-letter differences that distinguish DNA among people are called single-nucleotide polymorphisms (SNPs), pronounced "snips" (more about this below).

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Single Nucleotide Polymorphisms (SNPs)

Single nucleotide polymorphisms, frequently called SNPs (pronounced "snips"), are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA building block, called a nucleotide. For example, a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA.

SNPs occur normally throughout a person's DNA. They occur once in every 300 nucleotides on average, which means there are roughly 10 million SNPs in the human 36 genome. Most commonly, these variations are found in the DNA between genes. They can act as biological markers, helping scientists locate genes that are associated with disease. When SNPs occur within a gene or in a regulatory region near a gene, they may play a more direct role in disease by affecting the gene's function.

SNPs begin their existence as point mutations (a change in one DNA base pair - a misspelling, that results in the substitution of one amino acid for another in the protein made by a gene), and they eventually become established in a population. This nucleotide substitution must occur in a significant proportion (more than 1%) of a large population for it to be called a SNP.


Here is an example:


In the DNA sequence TAGC, a SNP occurs when:

The G base changes to a C, and the sequence becomes TACC.

When SNPs occur within a gene, the protein that results usually remains somewhat functional.

A great deal of research is currently aimed at mapping all SNPs to see which ones are associated with a greater likelihood for getting cancer.

Image courtesy of the National Cancer Institute


Use and importance of SNPs:

Currently, much of medical practice is based on "standards of care" that are determined by averaging responses across large groups (cohorts). The theory has been that everyone should get the same care based on clinical trials.

SNPs are now thought to be key enablers in realizing the concept of personalized medicine. As seen in the image below, doctors may one day obtain a test that shows an individuals SNP profile. From this profile, they may know which drug an individual needs and will respond to.

Armed with data from the SNP Map, cancer researchers across the country are looking for correlations between:

  • SNPs and precancerous conditions

  • SNPs and drug resistance in chemotherapy

  • SNPs and cancer susceptibility

  • SNPs and drug response.
Image courtesy of the National Cancer Institute

There is much work ahead, but there is also much hope that these and other research
findings will result in improved health care for all.

CISN Summary:

1. All individuals are 99.9 percent the same with respect to their DNA sequence. Variations in genetic sequence may affect single nucleotides; these are known as single nucleotide polymorphisms (SNPs).

2. SNPs are the most common type of change in DNA.

3. SNPs occur when a single nucleotide (building block of DNA) is replaced with another.

4. These changes may cause disease, and may affect how a person reacts to bacteria, viruses, drugs, and other substances.



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