Paternity Testing

Biology AllianceMay 13

A pair of homologous chromosomes, (one from mom, one from dad)

Gene• unit of DNA information about a trait• passed from parents to offspring• specific location on a chromosome

Fig. 8-1, p.113

Alleles – different versions of a gene

Human Chromosomes

Humans have 23 pairs of homologous chromosomes

Polymerase Chain Reaction (PCR)

• Developed in 1983 by Kary Mullis• Awarded Nobel Prize in 1993

PCR Basics

• Isolate and mass produce a particular DNA fragment– Template DNA– Polymerase – enzyme that replicates

DNA, matches complementary bases– Primers – 10-30 base pairs long,

complementary to ends of fragment

PCR Basics

• Isolate and mass produce a particular DNA fragment– Denature– Anneal– Extend

Short Tandem Repeats

• Short regions of DNA that differ substantially among people– TCAT

• Each person carries a unique combination of repeats

(a) DNA and dye are loaded in a well on a gel, and anelectric field is placed across the gel.

(b) DNA fragments move through the gel, shorter fragments fasterthan longer fragments.

Electrode

GelWellDNA samples from PCR

Direction ofelectric field

Applications?

• Implicating a crime suspect, acquitting the wrongly convicted

• Paternity• Identifying the deceased

Obtaining DNA

• Any biological material– Body tissues– Body fluids– Hair follicles– Dried material

Power of Discrimination

• Ability to discriminate between different individuals

• The larger the number of loci used, the more powerful the ability to discriminate

CODIS

• COmbined DNA Index System• database of DNA obtained from crime scenes

and convicted violent offenders• 13 loci

Alleles

Paternity IndexLikelihood ratio: X/Y

X = chance that the AF could transmit the obligate alleleY = chance that some other man of the same race could have transmitted the allele

X = 1 if father is homozygousX = 0.5 if father is heterozygous

D8 hypothetical example:13 is obligate allele, frequency of 9 = 0.308

PI = 0.5/0.308 = 1.62

Paternity Index

Combined Paternity Index (CPI)

• All PI multiplied by each other

27,746

Probability of Paternity

CPI CPI + (1-prior probability)

27,746 27,746 + (1-0.5)

x 100

x 100 = 99.998%