Definition Of Some Genetic Terms
Allele – one of the variants of genetic material on a specific location (locus) of the chromosome. An individual has a chromosome pair where there are two alleles, which can be identical or not, and this is called homozygosis or heterozygosis. Different alleles in a human population can be the reason for inherited characteristics, such as blood type or hair colour.
Allele (dominant) – a gene variant that dominates in the expression over the other allele and therefore appears more often in the population. According to classical Mendelian genetics, the dominant allele is the reason for a 3-fold larger incidence of one phenotype, which is the consequence of allele forms A/A, A/a and a/A where A denotes the dominant allele.
Allele (recessive) – gene variant that is expressed only if both alleles are recessive. It appears less frequently in the population. According to classical Mendelian genetics, the probability of the presence of a phenotype which results from two recessive alleles (a/a) is 0.25.
Amino acid – a basic structural unit, from which protein is built. Different combinations of amino acids represent different proteins. The exact combination of amino acids is encoded in DNA with three sequential nucleotides: GCU is the code for amino acid alanine, UGU for cysteine, etc.
Association study – an overview of variations in the human genome with the aim of finding the genetic links to human diseases such as asthma, high blood pressure, etc. In order to figure out these links, the genomes of the disease-free individuals must be compared to the genomes of the people who have the disease. In both groups, known sites for SNPs are checked in the genomes in order to see if some SNPs appear more often in the diseased group than in the healthy group. If certain SNPs occur mainly in the patient group, it can be assumed that they are linked to the ocurrence of disease. The strength of this association is revealed by the result of the association study. Given the SNPs in his genome and the incidence of disease in the population, we can use this association to calculate the risk of the disease ocurrance for a random individual whom we assume to originate from the same population. The data of association studies, published in prestigious scientific journals, are used by geneDecode™ to calculate the risk of disease on the basis of personal genotyping.
Caucasians – people of white origin.
Chromosome – a rod-like form of the DNA molecule containing many hundreds or thousands of genes. The nucleus contains 22 autosomal chromosome pairs as well as one pair of sex chromosomes. In addition to the DNA molecule, there are proteins (mainly the histones) around which the DNA wraps. Such wrapping and further transformation result in a tightly formed chromosome taking up much less space than in an unfolded form.
Chromosome (autosomal) – a chromosome where both chromosomes in a pair are similar. One chromosome from a chromosome pair is obtained from the father, the other one from the mother.
Chromosome (sex) – there are two types of chromosomes: X (female) and Y (male). Women have two X chromosomes in a pair (XX) while men have an X and a Y chromosome (XY); the Y chromosome is inherited only from the father and its presence in the fetal development results in the creation of a male child.
DNA molecule – a molecule found in the cell nucleus that carries instructions for the creation of an organism. The human DNA, constructed from four different nucleotides, has a form of a double helix, meaning that two complementary DNA strands wind around each other. Complementary means that nucleotide C always pairs with G and A always with T.
DNA translation – a process of protein formation, in accordance with the genetic code.
Dominant disease – a disease that requires only the presence of one allele in the chromosome pair for its expression, instead of two as is the case with recessive diseases.
Enzyme – a protein, involved in chemical processes of the body. Its aim is to minimize the activation energy needed for the chemical reaction to take place, thus facilitating its course. In this way, the substrate is transformed more rapidly into the product, for instance the transformation of starch into glucose.
Exon – a part of the gene sequence, also termed the coding sequence, which remains in place during DNA transcription. The exons, which are spliced together, are then translated and the corresponding protein is formed.
Gene – the code in a DNA sequence which carries information for protein formation. Genes are passed on from parents to children and give all the information needed for the formation and development of an organism.
Genome – the entire DNA found in the cell nucleus that contains all autosomal chromosomes and both sex chromosomes.
Genotype – both allele forms of the gene of an individual. The genotype can represent all alleles in the cell, but mainly it is used to describe one or more genes exerting a common effect on a trait.
Genotyping – the process of determining an individual’s genotype, whereby the genotype of one or more genes is studied. Genotyping of a disease or association with a certain ethnic group is a process that uses SNP markers. Their appearance is linked to the appearance of a specific gene, and the information about what SNP markers an individual possesses is used to determine the probability of developing a disease.
Haemophilia – a recessive, sex-linked disease, where blood does not coagulate properly. Even small wounds may make the diseased individual bleed to death.
Haplotype – a group of alleles found in the same chromosome. Due to its locus, they are tightly interlinked and thus inherited together.
Heritability – the proportion of phenotypic variation that is attributable to genetic variation. The greater the heritability, the greater the genetic influence on the appearance of a specific disease and the smaller the environmental influence (e.g. smoking and exposure to poisonous substances). Heritability is merely an appraisal; the exact determination would require extremely large population samples.
Heterozygote – the alleles in both chromosomes are different.
Homozygote – the alleles in both chromosomes are identical.
Intron – a part of the gene sequence, also termed the non-coding gene sequence. Introns are found between exons and are spliced out during transcription, resulting in splicing together of exons. Proteins are formed only from exons.
Marker – a known DNA nucleotide sequence, which is located in such close proximity to a specific gene on the chromosome that the marker and the gene are inherited together. The marker is used to follow a certain gene. SNPs are a type of markers used by the geneDecode™ for the services it provides.
Mitochondrion – a cell organelle which produces energy for the cell function and contains its own genetic material.
Mutation – a random change in the genetic code. There are deletions with nucleotides deleted in a part of the genetic material; insertions with nucleotides inserted in a part of the genetic material; and substitutions with nucleotides substituted by other nucleotides
Nucleotide – the basic building block of a gene. An individual unit is composed of a phosphate group, a pentose (a sugar with a five-carbon ring) and a nitrogen base. Only nitrogen bases are different in different nucleotides. In a human DNA there are four different nitrogen bases and consequently four different nucleotides: cytosine (C), guanine (G), thymine (T) and adenosine (A).
Personalized medicine – working with an individual; various tools (genotyping, risk assessment, etc.) are used to determine the individual’s probability of contracting a disease and the types of diseases he or she may contract. Based on the results obtained, preventive measures can be suggested and, in the case of appearance of the disease, the most effective type of treatment can be determined.
Phenotype – the visible characteristics of an individual, such as the hair colour, as well as the presence or absence of disease. The expression of a phenotype is influenced by the genotype as well as the individual’s environment.
Polymorphism – the presence of two or more different alleles of one gene in the population. The consequence is the presence of several different phenotypes. However, the different allele must be present in more than 1% of the population or else it is considered a mutation.
Prevalence – indicates the proportion of people in a population who have a certain sign or disease at the time point of the research, irrespective of when they fell ill or when the signs appeared. In order to be able to compare prevalence in populations, we must also know the population where the proportion of the people was measured in addition to the proportion of the people itself, as well as the time point of the measurement. By measuring prevalence we can follow a decrease of a certain disease due to improved preventive measures (e.g. prevalence of HIV infection) as well as determine the incidence of a certain disease on a lifetime basis, in a certain life period or annually.
Recessive disease – the gene causing the disease is recessive, meaning that two such recessive alleles must be present in a chromosome pair (each on its own chromosome) in order for the disease to be expressed. If only one recessive allele is present on one of the chromosomes, the disease does not develop.
Risk or probability – a statistically calculated data, giving a figure between 0 and 1 (or 0% to 100%), where the figure represents the probability of the event occurring. 0 – it will not occur, 1 – it will occur with certainty. The data can also be expressed in percentages (0% – 100%).
Risk (absolute) – the probability, indicating the possibility of a disease developing in an individual’s lifetime. For instance, if 3 out of 30 people fall ill, the absolute risk of an individual falling ill is 10%.
Risk (relative) – the probability, used to show the possibility of disease development in two different groups of people. If the data above is used, the probability of an individual falling ill is 10%. If the relative probability is 2, it means that the possibility of the second group falling ill is 2 times greater than the possibility of the first group. The second group will thus have twice the number of the diseased, i.e. 20%, which means that 6 out of every 30 people will fall ill.
SNP (Single Nucleotide Polymorphism) – a DNA sequence marker, formed by one nucleotide substitution (e.g. A→ C) at a specific point. It represents a variation in the genetic code which may be different in different people. These small variations, however, may be very numerous for there are 30 million SNPs in a human genome. This very variability dictates the phenotype differences among individual people. Research in this area was fuelled by the finding that certain SNPs are linked with or more prevalent in certain diseases or traits. The results of these studies (association studies) form a theoretical background for the services that geneDecode™ provides.