DNA variation is exhibited in the form of different alleles, or various possibilities at a particular locus. Two forms of variation are possible at the DNA level: sequence polymorphisms and length polymorphisms (Figure 2.5).
As discussed earlier, a genotype is an indication of a genetic type or allele state. A sample containing two alleles, one with 13 and the other with 18 repeat units, would be said to have a genotype of '13,18'. This shorthand method of designating the alleles present in a sample makes it easier to compare results from multiple samples.
In DNA typing, multiple markers or loci are examined. The more DNA markers examined and compared, the greater the chance that two unrelated individuals will have different genotypes. Alternatively, each piece of matching information adds to the confidence in connecting two matching DNA profiles from the same individual. If each locus is inherited independent of the other loci, then a calculation of a DNA profile frequency can be made by multiplying each individual genotype frequency together (see Chapter 21). This is known as the product rule.
Owing to the fact that it is currently not feasible in terms of time and expense to evaluate an individual's entire DNA sequence, multiple discrete locations are
Two forms of variation exist in DNA: (a) sequence polymorphisms and (b) length polymorphisms. The short tandem repeat DNA markers discussed in this book are length polymorphisms.
(a) Sequence polymorphism
(b) Length polymorphism
Schematic representation of two different STR loci on different pairs of homologous chromosomes. The chromosomes with the open circle centromeres are paternally inherited while the solid centromere chromosomes are maternally inherited. Thus, this individual received the four repeat allele at locus A and the three repeat allele at locus B from their father, and the five repeat allele at locus A and the six repeat allele at locus B from their mother.
evaluated (Figure 2.6). The variability that is observed at these locations is used to include or exclude samples, i.e., do they match or not. Because absolute certainty in DNA identification is not possible in practice, the next best thing is to claim virtual certainty due to the extreme small probabilities of a coincidental (random) match.
DNA searches can be narrowed down by comparing multiple data points in a manner analogous to how the U.S. Postal Service delivers mail. The entire United States has over 290 million individuals but by including the zip code, state, city, street, street number, and name on an envelope, a letter can be delivered to a single, unique individual. Likewise, the use of more and more information from DNA markers can be used to narrow a search down to a single individual. If marker 1, marker 2, marker 3, and so on match on a DNA profile between crime scene evidence and a suspect, one can become more confident that the two DNA types are from the same source. The likelihood increases with each marker match.
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