By default the genotypes displayed on the 23andMe website refer to the plus (+) strand of the Genome Reference Consortium Human Build 37 (GRCh37 or “Build 37”) genome assembly. In Browse Raw Data, genotypes are also reported on the plus (+) strand of the subsequent GRCh38 (“Build 38”) genome assembly.
A reference genome is assembled by scientists as a representative example of the nucleotide sequence of the genome for a species. The reference human genome was first published in 2004, but it is occasionally revised to account for new discoveries and fix errors. When these regions are updated or corrected, a new version of the genome, known as a “reference assembly,” is released.
Each chromosome consists of two strands of DNA that are complementary to each other. The DNA nucleotide base adenine (A) always pairs with thymine (T) and the base guanine (G) always pairs with cytosine (C) across these two strands. One strand is called the positive (+) strand and the other is called the negative (-) strand. 23andMe always reports genotypes on the positive strand of the specified reference genome assembly.