## Answers

- How many different alleles does it have?

It can repeat anywhere from 5 to 16, that means it has 12 alleles which are the following: 5 repeats, 6 repeats, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16 repeats.

- Circle each tetramer.

- How many repeats are present?

15

- What is the probability that a person would inherit this allele?

This cannot be known. I think your teacher amde a mistake thinking that we could suppose all the alleles have the same probabilities of occurring, but that is wrong, different alleles may have different alleles frequencies, and we cannot know that if we are not given with either genotype frequencies or directly allele frequencies.

But if your teacher is assuming all the alleles have the same frequencies, then this allele occurrance probability is 1/12 = 0.083, or 8.3%

- Is any more or less likely to occur than the others?

Following the same logic from the previous question's discussion, the answer would be: it has the same probability of occurring.

- How many copies of gata locus does each human have?

Each human has 2 copies, because we are diploid organisms.

- How many bands would be visible after electrophoresis?

Two bands, one that runned further in the gel for the 8 gata repeats, and another that runned slower for the 15 repeats.

- Which fragment length would end up closest to the well where the DNA was loaded?

The fragment of 15 repeats in length. That is because electroforesis works by providing a molecular net that will get more easily stuck the larger molecules, while the smaller molecules will pass easier through it reaching larger distances. The 15 repeats allele is bigger, so it will not be able to travel as far as the 8 repeats.

- What is the probability that a person would inherit the second allele?

Okay, again we have a problem here, because we have to assume wrong things about allele frequencies in the populations, but let's keep going with our assumption about equal allelic frequencies.

The probability of inheriting this allele is again 1/12 = 0.083

- What is the probability that a person would end up with this DNA fingerprint?

The probability of 2 events ocurring is the product of multiplying both independent probabilities:

(0.083)(0.083) = 0.006889

- What would be the probability that a person's DNA would match the crime scene evidence for just one STR locus?

Okay, now the probability for each allele would be 1/10 = 0.1, and the fingerprint (both allele copies) would be (0.1)(0.1) = 0.01

- What would be the probability of a suspect's DNA matching all 13 loci?

Now we have to multiply the 13 probabilities of 0.01, that's to say, elevate 0.01 to the 13th potency:

0.0113 = 1x10-24, that is a very low probability

- Would you convict him or her for the crime?

Yes, because that number means only 1 person out of 1,000,000,000,000,000,000,000,000 (24 zeros) has such fingerprint, the probability of conviting an inocent is very very low