Rules of Inheritance
1) Child has two alleles for each autosomal marker (one from mother and one from biological father)
2) Child will have mother's mitochondrial DNA haplotype (barring mutation)
3) Child, if a son, will have father's Y-chromosome haplotype (barring mutation)
markers are examined. Thus, the outcome of parentage testing is simply inclusion or exclusion. Paternity testing laboratories often utilize the same short tandem repeat (STR) multiplexes and commercial kits as employed by forensic testing laboratories (see Chapter 5). However, rather than looking for a complete one-to-one match in a DNA profile, the source of the non-maternal or 'obligate paternal allele' at each genetic locus is under investigation.
The basis of paternity comes down to the fact that in the absence of mutation a child receives one allele matching each parent at every genetic locus examined (Figure 23.2). Thus, parents with genotypes 11,14 (father) and 8,12 (mother) may produce offspring with the following types: 8,11 8,14 11,12 and 12,14. Inversely, if the mother's genotype is known to be 8,12 and the children possess alleles 8, 11, 12, and 14, then we may deduce that the father contributed alleles 11 and 14 -provided of course that the same individual fathered all of the children.
The obligate paternal allele for each child in this example is shown in Figure 23.2. In this particular example, the parents had non-overlapping alleles. Paternity testing becomes more complicated when mother and father share
(a) Mendelian inheritance patterns with a mother possessing alleles A and B contributing one of them to the child while the father who possesses alleles C and D also contributes one of his alleles to the child.
(b) An example pedigree for a family where the parents possess different alleles enabling identification of the obligate paternal allele in each of the children. This scenario can become more complicated to interpret if mutations occur or maternal and paternal alleles are shared.
alleles, but the logic remains the same in calculating exclusion probability and the paternity index likelihood ratio described below.
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