Question

1. While walking home late at night, a woman was knocked over and raped. He didn’t leave any hairs at the crime scene but left eyeglasses with unusual frames. An optician helped police locate him. The man also left a half-eaten lollipop at the scene. The DNA taken from the suspect matched that found on the lollipop at four STRs on different chromosomes. Allele frequencies from the man’s ethnic group are listed besides the prolife pattern show here:
   • STR 1         Homozygote     Allele frequency = 0.2
   • STR 2         Allele 1 = 0.3      Allele 2 = 0.7
   • STR3          Homozygote     Allele frequency = 0.1
   • STR4          Allele 1 = 0.4      Allele 2 = 0.2

1. How would the electropherogram for STRs 1 and 3 look different from those for 2 and 4?
2. What is the probability that the suspect’s DNA matches that of the lollipop rapist by chance? Show the calculations
3. The man’s population group is highly inbred. How does this affect the accuracy or reliability of the DNA profile? (P.S. the suspect was so frightened by the DNA analysis that he confessed!)

Answer 1

STR analysis is a genetic profiling tool that makes use of polymorphic repeat sequences present in the genome. It is advantageous to choose a polymorphic loci so that it has more number of alleles distributed in the population. The comparison results with such alleles will have more accuracy.

Part A. STR 1 and STR3 are homozygotes (both alleles are same) so they will give only one peak in the electropherogram. Whereas STR2 and STR4 are heterozygous and have two alleles, which will show as two peaks in the electropherogram due to minor differences in the lengths of DNA fragments. Since electrophoresis will cause differently sized DNA to migrate with different distances this can be observed in the case of heterozygous alleles.

PART B. Assuming population follows Hardy - Weinberg equilibrium; p² + 2pq + q² =1

where p² and q² are the probability of a homozygous genotypes

2pq = probability of heterozygous genotype

Considering this,

pSTR1 = probability of STR1 = p² =(0.2)² = 0.04

pSTR2 = probability of STR2 = 2pq = 2 x 0.3 x 0.7 = 0.42

pSTR3= probability of STR3 =  p² = (0.1)² = 0.01

pSTR4 = probability of STR4 = 2pq = 2 x 0.4 x 0.2 = 0.16

Probability of exact match on all four loci = product of the probabilities of each STR

= 0.04 x 0.42 x 0.01 x 0.16 = 0.00002688

PART C. High inbreeding in a population will lead to increased homozygosity at various genetic loci (including STRs). This will negatively affect the accuracy of the results as many more individuals will share same genotype due to inbreeding.