Statistics

Question 1

what is allele frequency

Answer 1

• the number of times an allele is observed in a population
  
  • obtained by typing a random group of individuals and observing the genotypes
• used to calculate locus probabilities
  
  • derived from population databases

Question 2

what is in commercial STR kits

Answer 2

contain loci from non-coding regions that were chosen to remain close to hardy-weinberg equilibrium

Question 3

what is the correction factor/coancestry coefficient

Answer 3

• uses theta correction
• often employed to probability calculations of homozygotic loci

Question 4

explain the defendant’s fallacy

Answer 4

• if you have an RMP of 1 in 1 million, the defense will say that LA has a population 5 million people, so there are four other people in LA who have the same profile
• this is wrong because the RMP represents a chance/likeliness that something will happen
  
  • not based on literal numbers or populations

Question 5

what is hardy-weinberg equilibrium

Answer 5

• depends on Mendel’s laws of independent assortment of genes during sex cell formation
• an unachievable mathematical relationship between allele frequencies and genotype frequencies that assumes a perfectly balanced population with constant genetic variation
• hardy-weinberg formula
  
  • p^2 + 2pq + q^2 = 1
  • used to calculate locus probabilities

Question 6

what are the hardy-weinberg requirements

Answer 6

• random mating
  
  • no inbreeding or population substructure
• large population
  
  • so allele frequency doesn’t change through genetic drift
• no mutation
  
  • to avoid introducing new alleles
• no natural selection
  
  • so no alleles are favored over others, changing allele frequencies
• no gene flow
  
  • so variability doesn’t increase

Question 7

explain heterozygotes

Answer 7

• less common in real-life than what would be expected with HWE
• should not be corrected

Question 8

explain homozygotes

Answer 8

• more common in real-life than what would be expected with HWE
• corrected when calculating probabilities
  
  • uses theta correction

Question 9

explain inheritance

Answer 9

• STRs are inherited allele traits
• inherited in pairs

Question 10

what is likelihood

Answer 10

• conditional probability
• the chance an event will occur given knowledge that another event already occurred

Question 11

what is the likelihood ratio

Answer 11

• refers to a stable, independent, random flow of alleles within a population
• directly related to the ability to achieve HWE using the product rule
• ratio of two probabilities of the same event under different and mutually exclusive hypotheses
  
  • specific to observed evidence and related individuals

Question 12

what are the likelihood ratio hypothesis

Answer 12

• hypothesis 1 (prosecution)
  
  • probability of the evidence given a presumed individual is a contributor to the evidence
• hypothesis 2 (defense)
  
  • probability of the evidence given a presumed individual is not a contributor to the evidence

Question 13

what is linkage equilibrium ( LE)

Answer 13

• a genetic system with stable, independent and random flow alleles within a population
• allows for the use of the product rule to combine locus probabilities for autosomal STR typing

Question 14

what is the minimum allele frequency and how is it used

Answer 14

• minimum allowable frequency (MAF) within a population group
  
  • used for unobserved alleles and for raising frequencies that fall below to MAF
• the most equation for MAF is 5/2n
  
  • 5 is the minimum number of times that an alleles should be seen for a reliable frequency
  • 2n is 2 times the size of the database
• the MAF allows for a conservative estimate

Question 15

what is the modified homozygote equation

Answer 15

• p^2 + p(1-p)(theta)
• accounts for statistical uncertainty and does not require the assumption of a population to be in HWE

Question 16

what is a population databse

Answer 16

refers to a collection of observed alleles and specific frequencies for tested populations

Question 17

what is population genetics

Answer 17

principles that are combined with statistics to calculate the statistical significance of STR data

Question 18

explain probability

Answer 18

• DNA statistics are used to determine the probability of a profile in the population
  
  • used to assign statistical significance
• probability is a mathematical relationship between the number of times an event is observed compared to the total number of events possible
  
  • 0 = event will not occur
  • 1 = event will occur
  • between 0 and 1 = may or may not occur

Question 19

what is the product rule

Answer 19

• used to combine independent events
  
  • ex: individual locus probabilities in a DNA profile
• a statistical principle allowing for independent or unlinked events to be combined through multiplication

Question 20

what is the prosecutor’s fallacy

Answer 20

• if you have an RMP of 1 in 1 quadrillion, the prosecutor will say that the world has a population of 7 billion people, and how many worlds it would take the see that profile again
• an RMP is an outcome probability, not an expected probability after multiple trials

Question 21

explain random match probability

Answer 21

• estimated frequency at which a particular STR profile would be expected to occur in a population as determined by the allele frequencies from that population group
  
  • probability of randomly selecting an unrelated individual from the population who could be a potential contributor to an evidentiary profile
  • only the frequency of the random match is reported (1/RMP)

Question 22

what is statistical analysis

Answer 22

• purpose is to perform statistical calculations, with proper interpretation, on evidentiary DNA profiles to provide as assessment of the significance of an inclusion
• it quantifies the evidentiary value of the match

Question 23

explain why we use the theta correction

Answer 23

• p^2 does not best represent the homozygotic locus
• addresses the issue of increased homozygosity and accounts for the difference between population frequency and true HWE
• should be based upon the relative size and substructure of a population
  
  • commonly used values are 0.01 (normal) and 0.03 (incest, small populations)