The
table found above 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.
Data from Butler et al. 2003. J Forensic Sci. 48(4): 1-4.
a. What is the probability of a Caucasian American having a 16, 17 combination for D3S1358?
(0.200) x (0.281) = 0.0562 = 5.62%
b. What is the probability of an African American having a 16, 17 combination for D3S1358?
(0.248) x (0.242) = 0.0600 = 6%
c. What is the probability of a Caucasian American having a 16, 17 combination for D3S1358, and a 5, 9 combination for TH01?
(0.0562) x (0.002) x (0.114) = 0.0000128 = 1.28e-5
d. 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?
(0.0000128) x (0.017) x (0.137) = 0.0000000298 = 2.98e-8
e. Based on your answers to a, d and e, why does forensic analysis use many CODIS markers (and not just one or two)?
As you add more markers the probability becomes less.
f. 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.
There is no probability that this man could have stolen the car because there is no chance that a Hispanic can have a TH01 marker with a length of 5.
Data from Butler et al. 2003. J Forensic Sci. 48(4): 1-4.
a. What is the probability of a Caucasian American having a 16, 17 combination for D3S1358?
(0.200) x (0.281) = 0.0562 = 5.62%
b. What is the probability of an African American having a 16, 17 combination for D3S1358?
(0.248) x (0.242) = 0.0600 = 6%
c. What is the probability of a Caucasian American having a 16, 17 combination for D3S1358, and a 5, 9 combination for TH01?
(0.0562) x (0.002) x (0.114) = 0.0000128 = 1.28e-5
d. 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?
(0.0000128) x (0.017) x (0.137) = 0.0000000298 = 2.98e-8
e. Based on your answers to a, d and e, why does forensic analysis use many CODIS markers (and not just one or two)?
As you add more markers the probability becomes less.
f. 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.
There is no probability that this man could have stolen the car because there is no chance that a Hispanic can have a TH01 marker with a length of 5.
About Population Genetics
Population genetics is a field of biology that studies the genetic composition of biological populations, and the changes in genetic composition that result from the operation of various factors, including natural selection. Population geneticists pursue their goals by developing abstract mathematical models of gene frequency dynamics, trying to extract conclusions from those models about the likely patterns of genetic variation in actual populations, and testing the conclusions against empirical data. A number of the more robust generalizations to emerge from population-genetic analysis are discussed below.