SNPs and Alleles

Any two people share about 99.9% of the same DNA. That tiny remaining 0.1% is responsible for many of the differences we see between individuals—things like appearance, metabolism, and disease risk. These DNA differences are called genetic variants, or simply variants.

What is an allele?

An allele is a specific version of a genetic variant. At a given position in your DNA, different people may carry different versions (alleles) of the same genetic code.

What is a SNP?

Sometimes a genetic difference comes down to a single DNA building block being changed. For example, one person may have an A at a specific position in their DNA, while another person has a C at that same position.

These single-letter differences are called single nucleotide polymorphisms, or SNPs.

Because DNA is made up of four bases—A, T, C, and G—there are technically four possible alleles at any SNP location. In practice, though, most SNPs only have two common alleles.

Types of alleles

Major and minor alleles

Scientists often describe alleles as major or minor, based on how common they are in a population:

  • Major alleles are the more common version.
  • Minor alleles are less common.

Whether an allele is considered major or minor depends on the population being studied. An allele that's minor in one ethnic group may be major in another.

Example: If a gene has two alleles that influence eye color—one associated with brown eyes and one with blue eyes—the brown-eye allele may be the major allele in China, while the blue-eye allele may be the major allele in Sweden.

Effect alleles

An effect allele is the allele associated with an increased likelihood of a specific trait or outcome.

  • Effect alleles can be major or minor.
  • They're more often minor, but not always.

When an effect allele increases the likelihood of developing a disease or condition, it's often called a risk allele.

Risk alleles

A risk allele is an allele that scientific research has linked to an increased likelihood of a specific trait or health condition.

In many cases the risk allele is the minor allele (the less common version of a SNP), but this isn't always true. An allele can be considered a risk allele regardless of how common it is in the population.

Important: Having a risk allele does not mean you will develop a disease. Most health outcomes are influenced by:

  • Multiple genetic variants (not just one SNP)
  • Lifestyle factors (diet, exercise, sleep, stress)
  • Environmental exposures
  • Overall health and medical history

How SelfDecode uses risk alleles

SelfDecode doesn't evaluate risk based on a single SNP alone. Instead, we:

  • Identify risk and protective alleles across many SNPs
  • Weight them based on the strength and quality of scientific evidence
  • Combine them into scores, insights, and recommendations
  • Provide actionable guidance focused on reducing risk and improving outcomes

This is why you may see:

  • Risk alleles that don't result in high scores
  • Protective factors that offset genetic risk
  • Recommendations focused on modifiable lifestyle changes

Remember: Genetics provides context, not a diagnosis.

Putting it all together

  • SNPs describe where a genetic variation occurs.
  • Alleles describe which version of that variation you have.
  • Major, minor, effect, and risk alleles describe how common an allele is and how it influences traits or disease risk.

Understanding these concepts helps make sense of how genetic data is analyzed and how your unique genetic profile contributes to health-related insights.

See the SelfDecode Glossary for more genetics terms and definitions.

How SelfDecode uses SNPs and alleles in your reports

SNPs and alleles are the foundation of how we generate your personalized genetic insights. Your DNA data is analyzed across hundreds of thousands of SNPs. For each SNP, we identify which alleles you carry and compare them to findings from large-scale scientific studies. Those studies help determine whether certain alleles are associated with increased or decreased likelihoods of specific traits, biological pathways, or health-related outcomes.

Rather than looking at individual SNPs in isolation, SelfDecode often combines the effects of many SNPs together to generate more meaningful insights. This accounts for the fact that most traits and conditions are influenced by multiple genes acting together, not a single genetic variant.

We then pair these genetic insights with actionable lifestyle, nutrition, and supplement recommendations, so you understand not just what your genetics suggest, but how you can work with them.

Remember that genetics represents predispositions—not destinies. Environment, lifestyle, and other factors play a major role in how your genes are expressed.

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