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.

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.

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.

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

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.

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.

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.

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