All non-related humans are by 99.9 % identical in DNA-sequence. It appears a scientific challenge with vast medical significance to clarify the influence of the remaining 0.1 % on disease predisposition and progress, and sensitivity or resistance to toxins, environmental stress or pathogens.


Eukaryotes, including mankind of course, harbour two copies of their genome (called diploid), one inherited from each parent, and upon mating, each individual passes on to the progeny one of its set of chromosomes, which may recombine with that of the mating partner.
Mutations inherited to the next generation lead to variations within a species' gene pool. Variants of a single gene are known as alleles. If an organism carries two identical alleles of one gene it is called homozygous, if the alleles of one gene differ it is called heterozygous.
A gene's most common allele is called the wild type allele, and rare alleles are called mutants. However, this does not imply that the wild-type allele is the ancestor the mutants are descended from. Mutated alleles lead to polymorphism in the gene pool, which enables a population to adapt to changing environments, especially in case of heterozygousity. Roughly spoken, a person's individuality can be determined by less than 100 polymorphisms, which may be associated with diverse disease predispositions.

Learn more about genetic polymorphisms.


Frequency of DNA sequence variation

Modern molecular biology and medical research, focussing on haematology, oncology and cancer, investigate minimal genetic differences between individuals.
It was found out that DNA-sequence variation is normal case rather than exception and is assumed as playing a key role in congenital sensitivity to disease and drug side effects. The majority of these variations are single nucleotide changes, so called single-nucleotide polymorphisms (SNPs) or point-mutations.
The probability of SNPs in a genome is estimated to about 0.5-1 x 10-3 per nucleotide. Statistically, two human beings differ in roughly 1.5 to 2 million nucleotides.

In accordance with recent reports one single gene variation predisposes the development of intolerable incompatibility against the antidepressant Paxil (brand name of paroxetine). Humans with a single homozygous mutation of a gene called HTR2A bear a risk to suffer from this incompatibility three times higher than persons without or heterozygous mutation. The symptoms can be that severe, necessitating entrainment. For further information: Genome News Network.



As mentioned above, eukaryotes are diploid, i.e. their genome contains two sets of chromosomes, each set called a haplotype. Large genomes like those of humans harbour many polymorphic sequences. Some of them are frequently coupled with other polymorphisms, thus, dendrogrammic relationships between individuals can be generated.
To identify haplotypes, partial sequencing of different human genomes is required. In the scope of a study called the HapMap project, human haplotypes are identified and compared to neighbouring haplotypes. Ideally, certain haplotypes are associated with the appearance of special diseases or phenotypes.
Within this project, the SNP pattern of patients (e.g. macula degeneration) are compared with healthy persons. Pharmaceutical research focuses attention on such projects expecting 1) improved effect of medicates with less knock-on effects and 2) an individualisation of treatments (e.g. EGFR Receptor). Key instruments of HapMap and pharmacogenetics are gene chips, a novel technology allowing quick testing of millions of polymorphisms.