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

# 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