Forensic statistics

Question 1

What is the analytical threshold?

Answer 1

Below this value, observed peaks cannot be relaibly distinguised from instrument noise (baseline signal); most commonly 50 RFU (relative fluorescence units)

Question 2

What is the limit of linearity?

Answer 2

Above this value, the CCD camera can become saturated and peaks may not accurately reflect relative signal quantities (e.g. flat-topped peaks), and lead to pull-up/bleed-through between dye color channels

Question 3

what is the stochastic threshold?

Answer 3

Above this peak height value, it is reasonalbe to assume that allelic dropout of a sister allele of a heterzygote has not occured at that locus; single alleles above this value in single-source samples are assumed to be heterozygous (e.g. 250RFU)

Question 4

What is the stutter threshold?

Answer 4

Below this value, a peak in reverse (or forward) stutter position can be designated as stutter artifact with single-source samples or some mixtures (oftne higher with lower DNA amounts); e.g. 15%

Question 5

What is the peak height ratio?

Answer 5

Above this value, two heterozygous alleles can be grouped as a possible genotype (often lower with lower DNA amounts); e.g. 60%

Question 6

What is Major/minor ratio?

Answer 6

When the ratio of contributors is closer than this value in a two-person mixture, it becomes challenging and often impossible to correctly associate genotype combinations to either the major or minor contributor (e.g. 4:1)

Question 7

What is the law of segregation?

Answer 7

Medelian genetics

Two members of a gene pair segregate from each other duing se-cell formation (meiosis), so that one-half of the sex cells carry one member of the pair and the other one half of the sex cells carry the other member of the gene pair - you always get 1 from mother and 1 from father

Question 8

What is the law of independent assortment?

Answer 8

mendelian genetics

Different segregating gene pairs behave independently due to recombination when genetic material is shuffled between generations - you’ll never know which allele will be from mom and dad after mitosis

Question 9

Which genes are in linkage equilibrium?

Answer 9

Genes that are randomly associated

Question 10

Which genes are in linkage disequilibrium?

Answer 10

The ones that are inherited together

Question 11

what is the allele frequency?

Answer 11

The numbers of alleles in tested group of individuals divided by the total number of all alleles observed in this population. Since there are two copies of each allele per individual, when there are N individuals in a population there are 2N alleles

Question 12

What is the genotype frequency?

Answer 12

Refers to the number of individuals with a particular genotype divided by the toal number of individuals examined. The sum of genotype frequencies always sums up to 100%

Question 13

Name the four primary assumptions to DNA profile RPM calculations

Answer 13

1. Two alleles inherited at a locus from an individual’s parent are independent. The locus is in Hardy-Weinberg equilibrium (HWE)
2. Alleles at different loci are independent of another. The loci are in linkage equilibrium, therefore genotype frequencies can be multiplied across independent loci to arrive at a profile frequency estimate
3. The true perpetrator is not a relative of a suspect - calculation reflects the possibility of matching an unrelated individual
4. The appropriate population data are used for genotype frequency estimates and there are no significant subpopulation differences in allele frequencies used to compute a profile frequency estimate. A population substructure correction factor (theta) is commonly used to account for potential coancestry of alleles.

Question 14

What is the likelihood ratio (mathematical)?

Answer 14

The probability of the evidence given the hypothesis of the prosecution divided by the probability of the evidence given the hypothesis of the defense