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01-11-2017 | Original Article

Likelihood ratio formulae for disputed parentage when the product of conception is trisomic

Authors: Duncan A. Taylor, Oliva Handt, Damien Abarno, John Buckleton

Published in: International Journal of Legal Medicine | Issue 6/2017

Abstract

We present here the derivation of paternity index formulae that covers situations of a disputed paternity trio with a trisomic product of conception. We consider six possible mechanisms for trisomy to occur: dispermy, dieggy, paternal meiosis I or II, and maternal meiosis I or II in the calculation. We also provide a biological explanation for how each of the mechanisms could give rise to a trisomy. The paper is set out in a general manner so that the tables presented can be used on any instance of trisomic offspring. This work is motivated by a case of disputed paternity where the product of conception was trisomic, i.e. the electropherogram of the product of conception possessed three alleles at each locus. The outcome was extremely strong support for the alleged father’s paternity of the product of conception.

Note that we will use the term ‘child’ for the products of conception from this point forward so that the nomenclature is similar to other disputed paternity calculations.

Note here that the solid and hollow diamonds represent allelic place holders whose alleles could be the same (making the individual homozygous at that locus) or different (making the individual heterozygous at that locus).

Note that allelic designations ‘A’, ‘B’, etc. are allelic placeholders and do not signify common alleles between loci. We signify this difference by underscoring the alleles with their corresponding loci.

How close the approximation holds true depends on the proximity of the loci to the centromeres of the chromosomes. In the case of GlobalFiler™, the closest locus is D21S11 which is ~6.3 Mb. These distances can be converted approximately to map units, w (i.e. distance in Morgans), and used in the equation P(c crossovers) = e ^−2w(2w)^c/c!

from [7], and carrying out such an exercise (not shown) shows that for all loci in the GlobalFiler™ system, \( \Pr \left({C}_O^l|{E}_i\right)\approx \Pr \left({C}_E^l|{E}_i\right)\approx 0.5 \).

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Title: Likelihood ratio formulae for disputed parentage when the product of conception is trisomic
Authors: Duncan A. Taylor
Oliva Handt
Damien Abarno
John Buckleton
Publication date: 01-11-2017
Publisher: Springer Berlin Heidelberg
Published in: International Journal of Legal Medicine / Issue 6/2017
Print ISSN: 0937-9827
Electronic ISSN: 1437-1596
DOI: https://doi.org/10.1007/s00414-017-1573-y

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Likelihood ratio formulae for disputed parentage when the product of conception is trisomic

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