SNPs and Alleles

SNP vs. Allele

Any two human beings have 99.9% of the same DNA! You can tell just from looking around that 0.01% can make big differences. These differences are sometimes called genetic variants or simply variants.

The different types of genetic variants are called alleles. Sometimes only a single base might be swapped out at the same position in the DNA. For example, instead of an ‘A’, there might be a ‘C’. Researchers call these differences single nucleotide polymorphisms or SNPs.

Because there are 4 different bases (‘A’, ‘T’, ‘C’, or ‘G’), there are four possible alleles at each SNP. In reality, most SNPs usually have only two alleles.

Types of Alleles

Scientists characterize alleles as “major” or “minor” based on how many people have them.

Minor alleles are less common in comparison to major alleles. However, an allele can be minor in one population or ethnicity, but major in another. 

Consider a gene with two alleles that affect eye color. One allele causes brown eyes, while the other causes blue eyes. We would expect the brown eye allele to be major in China, but the blue eye allele to be major in Sweden.

Effect alleles are alleles that increase the likelihood of a trait. The effect allele can be major or minor, but it is more common for it to be minor.

Effect alleles are risk alleles when they increase the risk of a disease.