If you have already explored our website, you will have noticed that the studies we offer about your DNA are based on SNPs, but do you know what a SNP is?
It all starts with the chromosomes
Well, to answer this question let's start at the beginning. As you surely already know, within our cells (at the core) is our DNA. DNA is divided into different chromosomes (23 to be exact). Each of these chromosomes has another similar to him which receives the name of homologous chromosome (Except in men, in which the X chromosome has the Y as a homologous, two very different chromosomes). Therefore they are actually 23 couples chromosomes that come one from our father and another from our mother.
Genes are the next level
Within each of these chromosomes we find DNA regions that make up genes. Each gene will code for a phenotype concrete, that is, a physical, physiological, psychological appearance, etc. concrete. Many times (almost always in fact) it is the action of several different genes that modulates a particular phenotype.
The point is that these genes are almost identical in all humans, they only differ in small punctual variations. These point variations are changes in a nucleotide (one of the four types that make up DNA: TO, T, C and G). It is these variations in a single nucleotide that we call SNPs (Single Nucleotide Polymorphism) or polymorphisms.
SNPs and allelic variants. Lactose as an example
Therefore, if we look at the case of the lactose intolerance, a simple example because it is composed of a single gene, all human beings present the gene for lactase (the enzyme that metabolizes lactose). This gene is exactly identical in all of us, except for (let's imagine) a specific nucleotide. A change in this nucleotide will cause us to be able to metabolize lactose or not, and this will be the SNP or polymorphism of lactose intolerance.
In other cases (the vast majority), several SNPs (of the same or different gene) are required to obtain a specific phenotype. It is the composition of these SNPs which gives us a concrete result. For example, in the case of a predisposition to some disease, the sum of all SNPs will determine which way the balance leans the most, towards the side of little predisposition to suffer it or to that of a lot predisposition.
If you remember, at the beginning I told you that we have pairs of homologous chromosomes, each one coming from one of our parents. This means that, for each gene, we will have two variants (one on each chromosome) and these variants are called alleles. Each of these alleles will represent a specific SNP.
Therefore, in the example of lactose intolerance, we will have two lactase genes (one on each homologous chromosome). If we have the two risk allelic variants (i.e. those that cause a defective lactase) we will probably be lactose intolerant. On the other hand, if we have the two normal allelic variants, the probabilities are greatly reduced. Finally, if we have one yes and one no, we will find ourselves in an intermediate case in which we will have a slightly higher probability.