Below is a schematic representation of one VNTR studied in 20 individuals. Since
ID: 42811 • Letter: B
Question
Below is a schematic representation of one VNTR studied in 20 individuals. Since it is a single locus, each person should have two bands , one for each of the two homologous chromosomes. There are five different size bands (labeled A-E for convenience) see in this sample:
Assuming this small sample is presentative of the population, what are the frequencies of each fragment in the population?
A_______ B________ C________ D________ E_________
There is a crime committed and DNA evidence shows bands A and E. A suspect is tested and found to have the same two bands. He could be the criminal, but he could also be some poor schmuck in the wrong place at the wrong time. What is the chance that a random individual from this population would have exactly those two bands? Would you convict the suspect based on this evidence?
Explanation / Answer
Frequencies of each fragment in the population :
A : 4/40 = 0.1
B : 10/40 = 0.25
C : 6/40 = 0.15
D : 12/40 = 0.3
E : 8/40 =0.2
When profiles from a single VNTR locus from unrelated individuals are compared, the profiles are normally different. However it is possible for two individuals to have same profile at one or two locus by chance.
Random match probability
The probabilities at each site are multiplied together, to obtain the overall probability of a random person having exactly the same alleles at all the sites as the person or fingerprint of interest. This is called multiplication rule and the probability obtained is random match probability.
Here, DNA evidence shows bands A and E,
The probability = frequency of A X Frequency of E
= 0.1 x 0.2 = 0.02
This is a small number, indicating small chance that the DNA profile obtained could belong to anyone other than suspect.
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