Population Genetics
The study of allele frequencies in a given population is population genetics. Natural selection, genetic drift, gene flow, and mutations influence a population's allele frequency. Short Tandem Repeats (STR) aids in identifying similarities from the evidence found by authorities and potential suspects. Mathematics can help determine frequencies by finding the probabilities of possessing a certain trait in a specific population. The frequencies can be multiplied by each other to find the overall probability of having a specific match. When the number of STR increases, the probability's accuracy increases.
Sample Questions
The table below shows the frequencies for STR lengths (repeat number) in different U.S. populations. You can determine the probability of a particular combination of STRs by multiplying the frequencies.
What is the probability of a Caucasian American having a 16, 17 combination for D3S1358?
0.200 x 0.281 = 0.0562
5.62% chance that a Caucasian American with have a 16,17 combination for D3S1358.
What is the probability of an African American having a 16, 17 combination for D3S1358?
0.248 x 0.242 = 0.06
6% chance that a African American will have a 16,17 combination for D3S1358.
What is the probability of a Caucasian American having a 16, 17 combination for D3S1358, and a 5, 9 combination for TH01?
0.200 x 0281= 0.0562
0.002 x 0.114 = 0.000228
0.0562 x 0.000228 = .00001281
0.00128% chance that a Caucasian American will have a 16,17 combination for D3S1358 and a 5.9 combination for TH01.
What is the probability of a Caucasian American having a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51?
0.200 x 0.281= 0.0562
0.002 x 0.114 = 0.000228
0.017 x 0.137 = 0.002329
0.0562 x 0.000228 x 0.002329 = 2.98 e-8
0.00000298% chance that a Caucasian American will have a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51.
Based on your answers to a, d and e, why does forensic analysis use many CODIS markers (and not just one or two)?
The probabilities significantly decreased when the number of CODIS markers the person needed to possess increased.
A female eyewitness has identified a Hispanic male as a having stolen her car. The eyewitness noted that the man who stole her car was bleeding profusely from a head wound. Her car was recovered and male blood with a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51 was found on the drivers seat and steering wheel. Does this finding call the eyewitness evidence into question? Explain your answer.
The female eyewitness was wrong, a Hispanic male did not steal her car. The probability that a Hispanic male will have a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51.
0.286 x 0.204 = 0.058
0 x 0.150 = 0
0.011 x 0.139 = 0.0015
0.058 x 0 x 0.0015 = 0
0.200 x 0.281 = 0.0562
5.62% chance that a Caucasian American with have a 16,17 combination for D3S1358.
What is the probability of an African American having a 16, 17 combination for D3S1358?
0.248 x 0.242 = 0.06
6% chance that a African American will have a 16,17 combination for D3S1358.
What is the probability of a Caucasian American having a 16, 17 combination for D3S1358, and a 5, 9 combination for TH01?
0.200 x 0281= 0.0562
0.002 x 0.114 = 0.000228
0.0562 x 0.000228 = .00001281
0.00128% chance that a Caucasian American will have a 16,17 combination for D3S1358 and a 5.9 combination for TH01.
What is the probability of a Caucasian American having a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51?
0.200 x 0.281= 0.0562
0.002 x 0.114 = 0.000228
0.017 x 0.137 = 0.002329
0.0562 x 0.000228 x 0.002329 = 2.98 e-8
0.00000298% chance that a Caucasian American will have a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51.
Based on your answers to a, d and e, why does forensic analysis use many CODIS markers (and not just one or two)?
The probabilities significantly decreased when the number of CODIS markers the person needed to possess increased.
A female eyewitness has identified a Hispanic male as a having stolen her car. The eyewitness noted that the man who stole her car was bleeding profusely from a head wound. Her car was recovered and male blood with a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51 was found on the drivers seat and steering wheel. Does this finding call the eyewitness evidence into question? Explain your answer.
The female eyewitness was wrong, a Hispanic male did not steal her car. The probability that a Hispanic male will have a 16, 17 combination for D3S1358, a 5, 9 combination for TH01 and an 11, 14 combination for D18S51.
0.286 x 0.204 = 0.058
0 x 0.150 = 0
0.011 x 0.139 = 0.0015
0.058 x 0 x 0.0015 = 0