In forensics the science of investigating crimes paternity t

In forensics (the science of investigating crimes), paternity testing and the identification of human remains, biologists assume that populations are in HW equilibrium. This can become a matter of some significance in high-profile legal disputes. Consider this diagram of DNA fingerprint in which each band represents a specific DNA sequence. The different shading of the bands is used to symbolize 3 different genetic loci that were tested. A genetic locus is like a gene - it has a particular location and sequence on a chromosome but it may not code for a protein. a) Individual A has 2 bands for locus 1 (the solid black bands) while Individual B has only 1 band. Use the terms heterozygous and homozygous to explain this difference. b) Suppose the frequencies of the alleles are measured in a population and these data are obtained: A sample of blood from a crime scene shows the same banding pattern as Individual B. If the population in HW equilibrium, what is the probability that the blood from the crime scene came from Individual B?

Solution

Answer:

7. a. Heterozygous loci have both alleles present at a given time, for example Aa (considering diploid organism), where as homozygous have only one type (AA or aa). Generally, both alleles have different sequence lengths due to deletions, insertions etc. So if any locus is homozygous, it will produce only one length but heterozygous will produce two different lengths. That is why only one band is seen in case of homozygous and two in heterozygous.

Here, A is heterozygous and consists of 2 bands while B is homozygous.

b. B is homozygous for loci 1 but heterozygous for loci 2.

For the statistical analysis, the formula P = 2pq was used for heterozygous loci and for homozygous loci the formula, P = p² was used.

So, for loci 1: P = 0.1 * 0.1 = 0.01 (Slow allele)

loci 2: P = 2 * 0.03 * 0.97 = 0.0582

loci 3: P = 2 * 0.55 * 0.45 = 0.495

Therefore, probability that the blood from the crime scene came from Individual B

= 0.01 * 0.0582 * 0.495 = 0.000288