Paternity Testing

Short tandem repeats (STR)

• Microsatellites

• Repeat units of 1-7 bases

• 5-100 repeats on each satellite

Kinship testing

• Paternity (civil and criminal)

• Human ID in mass disasters, mass graves and compromised remains

• CODIS loci is used for paternity testing (13 STRs are commonly used)

• Two outcomes – the person involved could be biological father OR they could be excluded as the biological father

• If they are excluded, no statistical test is performed

Exclusion

• The mother is heterozygotic

• The child is homozygotic

• The father is heterozygotic

• The allele that is the same between the mother and child must come from the mother

• Match the other alleles to see if they are similar with the father – they are not, so this is an exclusion

• Mutation – man has mutation at 3’ end of primer for a particular STR so allele does not amplify and would be missing therefore, no STR and an exclusion

• Need three exclusions to use as evidence to exclude a man being the biological father

Inclusion

• If there are matching STRs between the profiles, this would be an inclusion

• Must do a statistical test and prove within 99.99% accuracy

Assessment of tests

• The maternity of a child is not in question

• In a paternity test, the tested man can either be excluded as the biological father or not excluded

• At this point of inclusion, a statistical calculation must be performed to assess evidence strength

Paternity index

• Probability of the match if the man is the biological father vs. probability of the match if another unrelated man is the biological father

• For each STR, need to calculate the likelihood ratio: X/Y

  X = chance that the Alleged Father (AF) could transmit the obligate allele

  X = 1 if father is homozygous

  X = 0.5 if father is heterozygous

  Y = chance that some other man of the same race could have transmitted the allele

Maternal index value

Mother is heterozygotic, so maternal allele probability = 0.5

Combined Paternity Index (CPI)

• Multiply the values of all STRs to get the combined paternity index

• Take the big number and put it into the Probability of Paternity (POP)

Probability of Paternity (POP)

• The prior probability is only 0.5

• There are only two possibilities - they are the father or they are not