Genetic testing facts

People are almost identical to each other

Like all living creatures on Earth, humans are also the result of evolutionary development that has been going on for millions of years. The hereditary material that determines our development and appearance is transmitted from generation to generation – that is why most people have two legs and hands, five toes on each limb, a lung and a heart. These are just some of the basic properties which are common to all of us as two separate individuals actually share as much as 99.9% of the same hereditary material.

Statements like “like father, like son” are very much true as the genetic composition of an individual is a unique combination of the DNA material provided by both the mother and the father, and the numerous features and similarities of this composition are visible on the outside. Gregor Mendel discovered this as early as the 19th century, after having noted that biological differences are inherited from parent organisms as specific identifiable characteristics. For his research he used a pea plant, and demonstrated that the inheritance of certain traits of the plant follows particular patterns.

But we are still different …

Genetic differencesGenetic composition determines much more than just the structure of a pea plant, or the colour of our eyes and hair, as it has a significant impact on the development of the entire body and it arranges our response to environmental stimuli. After a more detailed examination we find that we are in fact very different – some eat vast amounts of food and never gain weight, others are exceptionally talented in certain sports, while others still drink a cup of coffee and spend a sleepless night. These and many other individual traits are largely a consequence of the differences within the remaining 0.1% of our hereditary notation.

 

Every cell contains DNA organized in chromosomes

People are unique creatures. We consist of a hundred and thousand billion cells that each contains DNA double helix carrying information for creating an organism. In each cell there are two copies of DNA: one from the father and the other from the mother. Each copy of DNA is arranged in super-bend structures, called chromosomes. In each cell we have 23 pairs of chromosomes. Exceptions are the gametes (eggs and semen) where we have only one copy of DNA; meaning only 23 chromosomes.

DNA is formed from a sequence of four different nucleotides

A molecule of DNA is composed of approximately three billion nucleotides. Each nucleotide consists of a phosphate group, pentose (monosaccharide with five carbon atoms) and a nitrogenous base. The individual nucleotides differ only in their nitrogenous bases. The human DNA contains four different nitrogenous bases and therefore four different nucleotides: cytosine (C), guanine (G), thymine (T) and adenine (A).

The specific sequence of nucleotides creates a gene

Imagine an incredibly long helix (three billion nucleotides) in which the sequence of the letters C, G, T, A is fixed. The individual sectors of this incredibly long helix (parts of the specific sequence of the letters C, G, T and A) carry the information for the creation of protein and consequently create genes. There are over 25 000 of these sectors, meaning that this is the precise number of genes we have.

Genetic sequence

Genes carry the information for the creation of protein

Protein is the foundation of the body. Also all the enzymes, the biological molecules which are the key to the proper operation of our organism are proteins by nature. Proteins are built from smaller molecules, named the amino acids. Genes actually carry the information which tells the amino acids how to create the corresponding sequence. The different amino acids in a specific sequence give rise to the corresponding protein.

SNP makes us unique

Mutation in Latin means change. When we refer to mutation in genetics, we mean the change in the DNA sequence. This can occur in various ways: a nucleotide can be inserted into a sequence, another can be erased, and another still can be replaced by another one. The replacement of one nucleotide by another, which is then inherited by the descendants and kept within the population, is called the SNP. SNPs can cause a protein, encoded by a gene, to change, which means that its function will be changed as well. And even though people are identical in more the 99% of their hereditary notation, the SNPs (and some rare DNA mistakes) make everyone unique.

The SNP is inherited from parents and it determines the genotype

Sex cells, as opposed to body cells, contain only 23 chromosomes. Each parent gives their child 23 chromosomes, which means that the human body contains 46 chromosomes; therefore each of our body cells has two copies of a gene for each characteristic. When the gene copies are identical, they are referred to as homozygous, and when the copies are different, the person is a heterozygote. This gene state (homozygous or heterozygous) is the genotype. The gene copies are different when the nucleotide sequence differs in at least one site. For instance, a gene copy contains the A nucleotide in a specific location, while the other copy has the G nucleotide in the same place. We say that this person has two different alleles in the same location.

The genotype influences the phenotype

The two gene copies influence the final characteristic, the phenotype, together. One of great examples when we inherit lightly tanned skin from one of the parents, and a darker tone from the other, which makes our skin tone somewhere in between. Both body height and weight are also an evident example. The phenotype is the visible characteristic of an individual, like the above mentioned skin coloration or hair colour, or any other characteristic, like the predisposition to higher blood pressure or diabetes.

The importance of association studies

Numerous diseases are influenced by several genes as every gene somewhat contributes to the final risk. The SNP itself does not cause disease; it is only the change in a specific location of the DNA that is related to the risk. The SNP is discovered with the help of association studies in which we compare the genetic composition of people suffering from certain diseases with that of people who are healthy. If we find that certain SNPs are more frequent among people suffering from certain diseases, we can conclude that these SNPs are related to the development of said diseases.

Does genetic analysis tell the absolute truth?

With the analysis of SNPs we can calculate the probability of development of a certain disease usually compared to the average risk within the population. Despite the fact that this is a probability backed with a statistic foundation within scientific research, knowing the SNPs cannot predict with certainty whether an individual will develop particular conditions or not. Various environmental factors can influence the development of disease, as well as possible unexplained genetic factors.

Applicability of the genetic analysis

Genetic analysis is not a crystal ball that predicts the future. It is a scientifically-based starting point for calculating the genetic burden, which shows us the level of probability of developing disease due to our genetic composition. Taking into consideration the fact that both genes and the environment affect us, information of this kind can help us reduce the risk of disease. It encourages us to immediately change the way we live our life (e.g. quit smoking) either by aiding the early discovery of disease through early preventive examinations (e.g. lumps in the case of breast cancer, and enlarged prostate in the case of prostate cancer) or by effectively curing the disease.