Recommendation 4.4: If the possible contributors of the evidence sample include relatives of the suspect, DNA profiles of those relatives should be obtained. If these profiles cannot be obtained, the probability of finding the evidence profile in those relatives should be calculated...
In this chapter we will examine the situation when DNA samples being compared are from related individuals. Since many alleles will be shared when samples are from related individuals, different statistical equations must be applied. In Chapter 21 equations were introduced to correct for relatedness in determining frequency estimates of genotypes and DNA profiles. Tables 21.6 and 21.7 worked through example data to determine the frequency estimates from full siblings, parents, half siblings, and cousins based on NRC II recommendation 4.4 (see Appendix VI).
Besides its use in criminal investigations, DNA data plays an important role in parentage and kinship testing. Different questions are usually being asked in parentage testing rather than in criminal casework where a direct match is being considered between evidence and suspect. We will consider several applications involving DNA evidence from related individuals including traditional parentage testing that usually involves addressing questions of paternity (i.e., who is the father?) and missing persons and mass disaster investigations that involve reverse parentage analysis (i.e., could these sets of remains have come from a child of these reference samples?). Immigration cases also involve kinship testing to determine if an individual could have a proposed relationship to reference samples. Figure 23.1 illustrates the questions posed with parentage and reverse parentage analysis.
In the case of people who are close relatives, such as parents and offspring, full siblings, half siblings, and cousins, we can use the model of Mendelian segregation (see Chapter 19) to calculate the amount of shared genetic information. Once we establish the expected conditional probabilities between two relatives we can either calculate an exclusion probability, or more precisely a likelihood ratio expressing how much more likely it is that we would see the
Figure 23.1 Illustration of question being ashed with (a) parentage testing and (b) reverse parentage testing. The most common form of parentage testing namely paternity testing, uses results from a mother and a child to answer the question if the alleged father could have fathered the child versus a random man. With a reverse parentage test, DNA types from one or both parents are used to determine if an observed type could have resulted from a child of the alleged father and mother.
DNA evidence under the hypothesis that they came from people with a specific relationship as opposed to the hypothesis that they came from two ostensibly unrelated people.
This approach has a long-standing precedent in the calculations for paternity determination where an exclusion probability can be calculated to express how rare it would be to find a random man who could not be excluded as the biological father of the child. In 1938 the first publication (Essen-Moller 1938) appeared that developed the theoretical framework for calculating what we now refer to as the paternity index (PI). Since that time it has become common to calculate a likelihood ratio that quantifies the DNA evidence under two competing hypotheses.
PATERNITY (PARENTAGE) TESTING
Every year in the United States more than 300 000 paternity cases are performed where the identity of the father of a child is in dispute (The American Association of Blood Banks [AABB] 2003). These cases typically involve the mother, the child, and one or more alleged fathers. In 2002, almost one million samples were run for this purpose in the United States alone (AABB 2003). A number of different laboratories perform parentage testing (see Appendix III).
The determination of parentage is made based on whether or not alleles are shared between the child and the alleged father when a number of genetic
Parentage (Paternity) Testing
Parentage (Paternity) Testing
(B) Reverse Parentage Testing
(Missing Persons Investigation)
Alleged Alleged father mother
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