Every human has the same genes, but what ultimately makes them different are the different variations of base pairs, which are called SNPs (single nucleotide polymorphism; see SNPedia below). Only 0.1% of human base pairs occur as SNPs – the significant remainder is uniform when comparing people from a genetic point of view. In a genetic test (DNA test), the SNPs are genotyped and the individually different nucleic bases are determined. The structure of a SNP consists of rs, which means Reference SNP cluster ID, and a number selected by randomization. However, some SNPs start with i, which means Internal ID. A single nucleic base (adenine, guanine, thymine, and cytosine) in the SNP is called an allele (caveat: the term allele may only be used for base pairs that “represent” a SNP – not for the remaining 99.9%). Here is an example for a SNP: Rs1815739, the number sequence stands for the position in the gene. There are three different possible combinations of the nucleic bases (allele constellation): CC, CT and TT. For the previously mentioned SNP it is known that people with the nucleic bases CC and CT can run faster, but people with the nucleic bases TT have a better endurance. Here, allele C is dominant. If it were recessive, individuals with the base variants (nucleic bases present in the SNP) CT would not be able to run fast either. Other examples of recessivity and dominance are hereditary disorders. In this case, the alleles within the rash-causing SNP act recessively or dominantly depending on the recessive hereditary condition (e.g., cystic fibrosis) or dominant hereditary condition (e.g., hypercholesterolemia). To carry an allele of a recessive hereditary disease means to be a carrier. A SNP can either be homozygous, i.e. have the same allele twice (in this case CC or TT), or heterozygous, i.e. have two different alleles in the SNP (i.e. CT). The non-mutated variant of the SNP is referred to as the wild type. The mutation within a SNP can have both protective and bad effects, but in some cases the wild type is the type of SNP with the bad effect. For example, two ApoE4 alleles are in SNP rs429358 of the ApoE gene, but the wild type provides an extremely high risk of Alzheimer’s disease. In some cases, deletion of an allele occurs, with loss of portions of the DNA sequence. A deletion is abbreviated emit D. Examples of deletions are the absence of the Rhesus factor (in this case both alleles of the responsible SNP are no longer functional) or severe deletions that can lead to hereditary deafness (e.g. hereditary sensorineural hearing loss), which is most often the case. The opposite of a deletion is an insertion (abbreviation I), in which there is a “new gain” (often in the form of duplication of the previous sequence) of the DNA sequence. As with a deletion, this can result in harmless changes, but just as often in hereditary disorders. Examples of hereditary disorders caused by insertion are Tay-Sachs syndrome in most cases. In certain cases, confusion occurs between a DNA test and the dbSNP (Standard SNP Register), resulting in the so-called ambiguous flip, which is an allelic exchange due to ambiguity. For example, the DNA test 23andme sees A as the risk allele, but dbSNP sees T as the risk allele. The reason for this is that SNP can refer to both the plus and minus strands of DNA. Companies such as 23andme refer exclusively to the plus strand, but the standard register dbSNP varies between the plus and minus strands. The rule is that an A in the plus strand is a T in the minus strand or a T in the plus strand is an A in the minus strand and C in the plus strand is a G in the minus strand or a G in the plus strand and a C in the minus strand. A database listing human SNPs is SNPedia, which refers to Pubmed for the listed studies of SNPs. Furthermore, a list of FAQs (answers to frequently asked questions) can be found there.
