Human Genetic Makeup
The human genome is made of DNA (deoxyribonucleic acid). DNA is composed of two long, entwined chains of sugar, phosphate and four different bases. Due to its spatial structure, it is known as a “double helix”.
The long double helix molecule forms the basic substance of chromosomes, which are also known as “carriers of genetic information”. Humans have 46 chromosomes which always exist in pairs, that is two sets of 23 chromosomes (see picture). For each chromosome pair, one chromosome is inherited from the mother and one from the father.
The genome is identical in all nucleated cells of the human body, for example in the oral mucus membrane cells or the hair root cells.
Picture 1: GTG-band stain, normal male karyotype (46, XY)
Genes, the area of the genome containing information for proteins and therefore certain characteristics of the organism, makeup only a small portion of the entire genetic material (approximately 10%). A major portion of the human genome has another, mostly unknown function. It is here that the microsatellite DNA can be found.
DNA or genotyping of microsatellites (MS) or short tandem repeats (STRs) is recognized worldwide as evidence in forensic investigations, paternity tests and other family relationship DNA analyses. Microsatellites are regions of non-coding DNA, comprising a variable number of repeats of a certain sequence motif (approximately 2 - 7 base pairs long), which can be found scattered over the whole genome.
Each individual carries two copies (alleles) of these microsatellites in every nucleated cell; which depending on origin from either the father or the mother, can show different lengths. The alleles are named according to the variable number of sequence motif repetitions:
Picture 2: Naming of the STR-allele by the number of repetitions of a particular sequence motif; presented here: Allele 6
MS are hypervariable and differentiated by the number of repetitions. It is important that the so-called length polymorphisms are examined. This means that only the number or repeating units and not the DNA sequence is examined.
The pattern of microsatellite length is unique for most people (with the exception of identical twins) and forms the so-called "genetic fingerprint". Through the simultaneous examination of multiple microsatellites, it is possible to create an individual’s DNA profile, which is unique in the corresponding ethnic group with a probability of >99.999999%.
Through the comparison of patterns between people, and the used of recognized bio-statistical procedures, family relationships can be confirmed or excluded with a high degree of certainty.
Picture 3: A representative 3 STR marker scheme is presented. The DNA pattern of the child is a combination of the DNA pattern of both parents, in that 1 allele is inherited from the father and 1 allele is inherited from the mother. In all 3 schemes, 1 of the child’s alleles is from the mother (17, 6, 28). The other alleles must, in each case, come from the father. The MS pattern from putative father number 2 shows alleles that are not detectable in the child (15, 16, 9.3 and 29), or that cannot possibly have been inherited from him (28). He must therefore be eliminated as the father. The examination of a minimum of 15 STR schemes and the presence of four and more exclusion constellations on various chromosomes, allows the assertion that the descent from the putative father is excluded. For putative father number 1, a consistent pattern is present, therefore it is possible he is the father. Through the simultaneous analysis of multiple MS markers, the likelihood of fatherhood with a probability (P-value) of over 99.999% can be determined. A P-value ≥99.9% corresponds to the verbal predicate “relationship hypothesis practically proven”.