Schools

DNA Identification Exercises

Exercise 1 - Biological Relationship Testing
Exercise 2 - DNA Testing at the Scene of a Crime

Exercise 1 - Biological Relationship Testing

If the genotypes of both parents are known, we can use a Punnett Square to predict the possible genotypes of the children. You have probably already practiced doing Punnett Squares using letters to represent alleles inherited from the parents.

We inherit each DNA marker containing a certain number of repeats of letters in the DNA sequence from our parents. By using a Punnett Square for each locus, we can determine which form or allele of the DNA markers each child has inherited from the mother and from the father. There are 4 possible combinations at each locus.

For example, if we draw a Punnett Square using parents called Kate and Ravi, we can see the four possible genotypes for the D3 locus that their children could have:

Kate's pattern
at the D3 locus

Ravi's pattern
at the D3 locus

	13	19
16	16,13	16,19
18	18,13	18,19

Therefore, by examining the genotypes at each of the 10 loci used in DNA identification testing, it is possible to determine how likely it is that people are related. This type of testing is called biological relationship testing and includes determining if a man is the father of a child (paternity testing) or if two people are siblings (kinship testing).

In this exercise, Kate and Ravi have 3 children: John, Paul and Daisy. Paul was adopted. In the table below, the genotypes at 10 markers of each family member have been identified, providing a DNA profile for the parents and the children. By using the Punnett Square system and other information in the table, see if you can determine the identity of children 1, 2 and 3.

DNA marker name	DNA profile from Mum (Kate)	DNA profile from Dad (Ravi)	DNA profile from Child 1	DNA profile from Child 2	DNA profile from Child 3
D3	15,18	15, 17	15, 17	17, 18	18,18
VWA	16,16	16, 16	16, 16	16, 16	16, 16
FGA	19, 24	21, 22	19, 21	21, 24	19, 24
AMEL	X, X	X, Y	X, Y	X, X	X,Y
D8	12, 13	13, 13	13, 13	12, 13	13, 13
D21	29, 31	28, 30	28, 31	29, 30	28, 31
D18	12, 13	12, 17	12, 17	12, 17	12, 15
D5	11, 13	11, 13	11, 13	11, 11	13, 13
D13	11, 11	9, 11	11, 11	9, 11	11, 12
D7	10, 10	8, 10	10, 10	10, 10	8, 10

Exercise 2 - DNA Testing at the Scene of a Crime

A thief wearing a balaclava has robbed a jewellery store. In his haste to escape, he has ripped his balaclava off and thrown it away nearby. It was later retrieved by the police. Several witnesses have also provided a description of the man and the police have identified several suspects with known criminal records based on these descriptions.

Fortunately, some of the criminal's hair was found inside the balaclava. DNA was able to be extracted from the roots of the hair. The police have asked the forensic laboratory to examine this DNA and compare it with the DNA of the 4 suspects.

The results of the DNA investigation are shown below. In this exercise, your job is to see which suspect's DNA most closely matches the DNA found at the crime scene. The same 10 loci used in exercise 1 have been examined. If 2 or more loci do not match for any one suspect, then that suspect can be excluded from suspicion.

DNA marker name	Profile of DNA found at the crime scene	DNA profile from Suspect 1	DNA profile from Suspect 2	DNA profile from Suspect 3	DNA profile from Suspect 4
D3	15,18	15, 18	15, 17	15, 18	18,18
VWA	16,16	16, 16	16, 16	16, 16	16, 16
FGA	19, 24	21, 22	19, 21	19, 24	19, 24
AMEL	X, Y	X, Y	X, Y	X, X	X, Y
D8	12, 13	12, 13	13, 13	12, 13	13, 13
D21	29, 31	29, 31	28, 31	29, 31	28, 31
D18	12, 13	12, 17	12, 17	12, 13	12, 17
D5	11, 13	11, 13	11, 13	11, 13	13, 13
D13	11, 11	11, 11	11, 11	11, 11	11, 12
D7	10, 10	8, 10	10, 10	10, 10	8, 10

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