DNA comparison & match statistics

Law of segregation

A person has two alleles at each locus, and only one allele is passed on to offspring

Law of independent assortment

Different STR loci are usually inherited independently, which allows us to combine probabilities across loci

Allele frequencies

Measures how common each allele is in a population

Crossing over during meiosis

Homologous chromosomes can exchange pieces of DNA (independent assortment)

Highest, lowest

Labs typically report the most conservative statistic… the ______ random match probability (RMP) or the _______ likelihood ratio (LR)

Genotype frequencies

Estimates how common a genotype is

Hardy-weinberg equilibrium

Helps us predict genotype frequencies from allele frequencies

1. Infinite size

2. No mutation

3. No migration

4. No selection

5. Random mating

Hardy-Weinberg assumptions

1. Probabilities range from 0 to 1

2. Mutually exclusive events are additive

3. Independent events are multiplicative

3 laws of probability

Random match probability

The chance that a random, unrelated person would have the same DNA profile. Found by multiplying each genotype frequency together. Expressed as… “The probability that a randomly selected, unrelated person from the population would have the same DNA profile is 1 in X”

Likelihood ratio

Expresses how many times more probable the evidence is under one hypothesis than another. Compares the prosecution hypothesis versus the defense hypothesis. The top is always 1 in single source cases where the profile matches. Expressed as “The DNA evidence is X times more likely if the suspect is the source than if an unknown, unrelated person is the source”.

False

True or false: an inclusion means certainty