1. DNA Fingerprinting

True or False:

• twins have the different finger prints,

• but the same DNA fingerprints

• true

How much of our DNA is the same as everyone else?

99.9% is the same,

• still a 3 million base pair difference

• this is what we use to create DNA finger prints

Why would it be problematic to have a volunteer basis for giving DNA samples to a crime data base?

• why would someone who committed the crime willing give it up

• invasive and the same as not having probable cause

• may be charged for unrelated crimes

What are potential uses for DNA fingerprinting?

• paternity testing

• prove innocence

• immigration

• unsolvable crimes

What are Short Tandem Repeats (STRs)?

Are they coding or non coding?

How many STR loci are there?

What does it mean if a person has the genotype 5 , 6 for STR #1?

• repeating units of about 4-5 nucleotides

• one person may only have 2 repeats at a specific STR loci, but another may have 3 repeats of this sequence

• these are non coding regions (90% of DNA doesn’t code for anything)

  • you pass only one on to your kids, there are many that exist like how there are different alleles for the same gene

• 30STR loci + 13 of which are used by FBI system

• a person may have the genotype 5 , 6 for STR #1

  • this means one allele has 5 repeats and the other has 6

  • there are two because humans are diploid

How is a DNA fingerprint created?

1. amplify DNA fingerprint with STR region

   • heat and cool many times over, throw in extra bases to pair us with template strands

2. fragments are separated by size using electrophoresis, run gel

   • negatively charged DNA move toward the positively charged opposite end

   • Smaller pieces move farther than the larger pieces.

3. STR regions are stained to become visible

   • florescent or radio activity

Suppose there are 10 alleles for STR 1, each with the same frequency in the population.

What is the likelihood that a random person has the same genotype as the perpetrator’s DNA found at a crime scene?

1/100

• because a person has two alleles

• there is a 1/10 chance that the first allele matches and a 1/10 that the second allele matches

• 1/10 × 1/10 = 1/00

see image for quesiton

no, because mendel’s law of segregation

• they would inherit a 10 from mother, but a 12 or 14 from father

• neither result would be homozygous

see image for question

4

What is an STR loci?

• its a place where there is a short tandem repeat,

• there are many, on another chromosome perhaps

• has nothing to do with the other one loci

• the repeating nucleotides could be different as well as how many times they repeat

Suppose there are 10 alleles for STR 1, each with the same frequency in the population.

And for STR 2 there are also 10 alleles, each with the same frequency.

What is the likelihood that a random person has the same genotype as the perpetrator’s DNA found at a crime scene?

1× 10 = 1/100, but we have two loci we looking at so do this twice

1/100 × 1/100 = 10,000

How might a lawyer get their client off is they have a 1/8 in LA of matching? How might the persecutor respond.

• wow, thats a lot of people within driving distance

• Witnesses saw it was a male, cuts it in half

• he ran off very fast, so is young adult

• now between only probably 1 person in LA

What do the bands on the electrophoresis represent?

The bands are each allele

What are challenges of STR?

• They can overlap, are they really the same or similar

• how many should we compare

  • US law says you have to 13 STRs in court

• jury doesn’t always understand statistics of its enough to convict someone

• some subpopulations all have one or the other of a specific allele for an STR, even though 10 may exist worldwide

• human mistake through the process

  • could intentionally plant DNA evidence

How often are eyewitness accounts wrong when they see a crime?

about 30% of the time