Chapter 25: Evaluation of the Strength of Forensic DNA Profiling Results

25.1: A Review of Basic Principles of Genetics

Mendelian Genetics

• Mendel’s first law: The principle of segregation of alleles. Each pair of alleles segregates from others in the formation of gametes.
• Mendel’s second law: The principle of independent assortment of alleles.
  • The segregation of each pair of alleles is independent of the segregation of other pairs during the formation of gametes.
• Gametes are formed during a process known as meiosis, in which cells with haploid chromosome numbers (23 in humans) are produced by the division of cells with diploid chromosome numbers (46 in humans).
• Linked Genes: Genes residing very closely together on the same chromosome are usually inherited together.
• The Mendelian inheritances of genes can often be measured using probabilities.
  • Probability: A ratio of the number of actual occurrences of an event to the number of possible occurrences.

Population Genetics

• Allele Frequency: It can be calculated directly by counting the number of alleles of one type at a given locus and dividing it by the total number of alleles at that locus in a sampled population.
• Genotype Frequency: It can be calculated by dividing the number of individuals with one genotype by the total number of individuals in a sampled population. Each genotype at the locus can be calculated separately.
• Heterozygosity: The proportion of alleles, at a given locus, that are heterozygous.
• Hardy–Weinberg Principle: It allows predictions of genotype frequencies to be made based on allelic frequencies.
• Observed Genotype Frequencies: Are calculated by dividing the number of individuals with one genotype by the total number of individuals in the population sampled.
• Population Match Probability: The probability of having a matching genotype between two randomly chosen individuals.

25.2: Statistical Analysis of DNA Profiling Results

Genotypes

• Likelihood Ratio: This method is an alternative for evaluating the strength of a match.
• The method allows the calculation of the probability of the DNA profile under two hypothesis:
  • Hypothesis 1 — The evidence and suspect profiles originated from the same source.
  • Hypothesis 2 — The evidence and suspect profiles did not originate from the same source.

Haplotypes

• The term haplotype was first used to describe very closely linked polymorphic loci.
• During meiosis, alleles at neighboring loci cosegregate because of the close linkage of loci.
• Linkage Disequilibrium: Where recombination is very rare, certain allelic combinations occur in populations much more frequently than would be expected.
• The two methods for evaluating the strength of a match between haplotypes are:
  • Mitotype Frequency
  • Likelihood Ratios

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