Humans share 99.5% identity at the genomic sequence level suggesting that the resulting phenotypic diversity comes from the remaining 0.5% difference as well as epigenetic modifications [1-3].

Sequence differences arise due to the presence of a number of short and variable repeats, so-called insertion or deletion polymorphisms called single nucleotide polymorphisms or mononucleotide polymorphisms (Single Nucleotide Polymorphism, SNP’s). Among SNP’s, transitions (A ↔ G or C ↔ T) are more common than transversions (A ↔ C or T and G ↔ C or T) [1-5].

This simplest form of DNA variation between individuals is as described, the replacement of a single nucleotide by another. This type of change is called a “single nucleotide polymorphism” (SNP) and is found to be more common than other types of polymorphisms.

There are at least 10 million SNP’s within the genome, occurring approximately every 100-300 base pairs and with an allele frequency greater than 1%, making them by far the most common type of variation in the human genome [1-3].

Recently, there has been a boom in genome-wide association studies (GWAS), where the prevalence of specific SNP’s is associated with phenotypes or diseases [4].

SNP’s can be observed between individuals in a population and can, for example, affect inducer activity (gene expression), messenger RNA (mRNA) conformation (and stability) and translational efficiency [5].

How SNP ‘s  affect   our genes

Mutations have the potential to alter all steps of gene expression depending on their genomic location. When present within transcriptional regulatory elements, they can affect mRNA expression.

When they occur in genes, SNP’s can affect mRNA splicing , RNA nuclear-cytoplasmic export, stability and translation. When present within a coding sequence and leading to an amino acid change (referred to as a non-synonymous SNP or mutation), they can modify the activity of the protein.

The Encyclopedia of DNA Elements (ENCODE)1 project aimed to identify and record functional elements in the human genome and has been quite useful in understanding the potential impact that sequential variations of  SNP ‘ s have on gene expression and function  [6] . So:  

1. If the  SNP  and  the mutation it causes are synonymous  (ie do not change the nature of the amino acid), then translation rates or mRNA half-life may be affected [7,8].

• If the SNP and the mutation it causes is a premature stop codon , this  can lead to the production  of a  protein product that is smaller in size or a protein with a near – null phenotype due to the arrest of  DNA replication and may  also mediate protein degradation  [7, 8]  (codon: codon, the sequence of 3 DNA bases corresponding to an amino acid).  

• If the SNP and the mutation it causes are nonsynonymous ,  depending on the codon position and the type of substitution, structural and functional changes can occur in the encoded proteins and this can completely change the phenotype and function of the protein [7, 8].

These phenotypic changes may vary from individual to individual depending on the degree of functional changes in the gene product, i.e. the protein encoded by the affected gene.

We should also not forget that phenotypic variations could also be due to prolonged interactions of genetic and environmental factors.

These types of changes in the gene are also responsible  for the evolution of the genome and for every human characteristic (phenotype), such as curly hair, individuality such as obesity, hypertriglyceridemia, hypercholesterolemia, etc., as well as individual differences in drug response. or preparations.

Therefore,  SNP ‘s may be responsible for an individual ‘s susceptibility to many common diseases and metabolism. They can also play a direct role with or without other factors in the phenotypic expression of diseases [ 9 – 19 ]. 

In recent years, the application of genetic knowledge has revolutionized our ability to understand the effects of nucleotide substitutions and the genetic basis of many complex and common disorders.

Thus, in a world in which predictive, preventive and personalized care will be the standard practice, people will not only live longer but will be able to live more fulfilling, productive and active lives  [9-19] .