What Is a SNP? A Plain-Language Guide

If you've opened a raw genome file or a MyGeneLog result and felt like you needed a biology degree to read it, this is the page to start with. Four words do almost all the work: SNP, allele, genotype, and rsID.

SNP: one letter that sometimes differs

Your DNA is a sequence of about 3 billion letters (A, C, G, and T). At the overwhelming majority of positions, every human being on Earth has the same letter. A SNP (pronounced "snip," short for single nucleotide polymorphism) is one of the small number of positions where the letter genuinely varies from person to person — some people have a C there, others have a T, and both are normal, common variation, not a mutation in the alarming sense of the word.

Allele: which letter you actually have

At a given SNP position, the two (or occasionally more) possible letters are called alleles. For the lactose-tolerance SNP rs4988235, the two alleles are C and T.

Genotype: your two copies

You inherit one copy of each chromosome from each parent, so at almost every position you actually carry two alleles — one from your mother, one from your father. That pair is your genotype, written like C/T. You could be C/C, C/T, or T/T at that position, and which one you are is exactly what a MyGeneLog result reports.

rsID: the SNP's actual name

Every well-studied SNP has a reference number from dbSNP, the public NIH database that catalogs them — written like rs4988235. This is the identifier MyGeneLog, published research, and your raw genome file all use to refer to the exact same position, so it's the most reliable way to look a specific result up.

Putting it together

A MyGeneLog finding is really just: "at position rsID, you have genotype X/Y, and published research associates that genotype with trait." Every variant page on this site follows that exact shape.