General Genetics Study Notes

Introduction to Genetics and Key Terminology

• Genetics: The science dealing with biological heredity and variation among organisms related by descent.

• Heredity and Variation: Heredity is the mechanism by which traits pass from parents to offspring; variation refers to the differences among parents and offspring and within offspring.

• Gene: The basic functional unit of heredity, consisting of a linear sequence of nucleotides located at a specific locus on a chromosome.

• Genotype vs. Phenotype: Genotype is the genetic constitution of an organism ($P = G + E$, where $P$ is phenotype, $G$ is genotype, and $E$ is environment); phenotype is the observable appearance or measurement of a character.

• Homozygote vs. Heterozygote: A homozygote carries identical alleles at a given locus (e.g., $AA$ or $aa$); a heterozygote carries two different alleles at a locus (e.g., $Aa$).

• Allele: One of a pair or series of alternative forms of a gene occupying a specific locus.

• Qualitative vs. Quantitative Traits: Qualitative traits feature discrete phenotypic classes controlled by one or a few major genes; quantitative traits display continuous variation controlled by many additive polygenes and are strongly influenced by the environment.

Cytogenetics and Chromosome Structure

• Cytogenetics: A hybrid science correlating cellular structures and events—especially of chromosomes—with genetic phenomena.

• Chromatin Types:

  • Euchromatin: Uncondensed, active DNA regions undergoing transcription.

  • Heterochromatin: Highly condensed, transcriptionally inactive chromosomal regions.

• Centromere Morphological Types:

  • Metacentric: Centromere is central, dividing the chromosome into two equal arms.

  • Submetacentric: Centromere is off-center, producing distinct short ($p$) and long ($q$) arms.

  • Acrocentric: Centromere is near one end, making the short arm barely discernible.

  • Telocentric: Centromere is located at the extreme end.

  • Acentric: Fragment lacking a centromere.

• Karyotype: A complete set of metaphase chromosomes arranged systematically by size, shape, and centromere position.

DNA Replication and Cell Divisions

• DNA Replication: Occurs semi-conservatively during the $S$ phase of interphase, catalyzed by DNA polymerases.

• Cell Cycle: Consists of Interphase ($G_1$, $S$, $G_2$ gaps) and Cell Division ($M$ phase and cytokinesis).

• Mitosis: Somatic cell division resulting in two genetically identical diploid ($2n$) daughter cells.

  • Stages: Prophase, Metaphase, Anaphase, and Telophase.

• Meiosis: Division in germ cells comprising two nuclear divisions ($M_1$ and $M_2$) resulting in four haploid ($n$) gametes or spores.

  • Prophase I Sub-stages: Leptotene, Zygotene (synapsis and synaptonemal complex formation), Pachytene (bivalent/tetrad formation), Diplotene (chiasmata formation), and Diakinesis (terminalization).

  • Genetic Significance: Halves chromosome number ($2n \rightarrow n$) and generates genetic variability via random orientation at Metaphase I ($2^N$ combinations) and crossing over during Prophase I.

• Gametogenesis:

  • Animals: Spermatogenesis yields four functional haploid spermatozoa; oogenesis yields one functional haploid ovum and three degenerating polar bodies.

  • Angiosperms: Microsporogenesis yields pollen grains containing a tube nucleus and two sperm nuclei; megasporogenesis yields a mature embryo sac with an egg nucleus, polar nuclei, and antipodals. Double fertilization produces a diploid ($2n$) embryo and a triploid ($3n$) endosperm.

Mendelian and Qualitative Genetics

• Law of Segregation (Mendel's First Law): The two alleles for a trait remain distinct and segregate during gamete formation so that each gamete receives only one allele.

  • Monohybrid Cross: Yields an $F_2$ phenotypic ratio of $3:1$ and a genotypic ratio of $1:2:1$

• Law of Independent Assortment (Mendel's Second Law): Alleles of different gene pairs segregate independently during gamete formation.

  • Dihybrid Cross: Yields an $F_2$ phenotypic ratio of $9:3:3:1$

• Modifications to Mendelian Phenotypic Ratios:

  • Incomplete Dominance: Heterozygote shows an intermediate phenotype ($1:2:1$ ratio).

  • Codominance: Both alleles are fully expressed in the heterozygote (e.g., $MN$ or $ABO$ blood groups).

  • Lethal Alleles: Cause lethality in homozygous condition, altering standard ratios (e.g., $2:1$ ratio).

  • Epistasis: Gene interactions where an allele at one locus masks or alters the expression of an allele at another locus (e.g., complementary $9:7$, recessive $9:3:4$, dominant $12:3:1$, additive $9:6:1$, duplicate dominant $15:1$, inhibiting $13:3$).

• Multiple Alleles: Presence of more than two alternative alleles for a gene in a population (e.g., human $ABO$ blood system with alleles $I^A$, $I^B$, and $I^O$).

• Gene Linkage and Genetic Mapping:

  • Linkage: Linked genes reside on the same chromosome and tend to be inherited together.

  • Recombination Frequency ($R$): $\text{Recombination Frequency } (R) = \frac{\text{Total Recombinants}}{\text{Total Progeny}}$. A $1\%$ recombination frequency equals $1\text{\,map unit (mu)}$ or $\text{centimorgan (cM)}$.

  • Interference ($I$) and Coincidence ($S$): $S = \frac{\text{Observed Double Crossovers}}{\text{Expected Double Crossovers}}$ and $I = 1 - S$

Statistical Applications in Genetics

• Descriptive Measures:

  • Mean ($\bar{x}$): $\bar{x} = \frac{\sum x_i}{n}$

  • Sample Variance ($S^2$): $S^2 = \frac{\sum (x_i - \bar{x})^2}{n - 1}$

  • Standard Deviation ($S$): $S = \sqrt{S^2}$. For a normal distribution, approximately $68\%$ of observations lie within $\bar{x} \pm 1S$ and $95\%$ within $\bar{x} \pm 2S$

• Probability Rules:

  • Sum Law: $P(A \text{ or } B) = P(A) + P(B)$ for mutually exclusive events.

  • Product Law: $P(A \text{ and } B) = P(A) \times P(B)$ for independent events.

  • Binomial Permutations: $(a + b)^n$ predicts the combined probabilities of alternative independent outcomes across $n$ trials.

• Chi-Square Test ($\chi^2$):

  • Formula: $\chi^2 = \sum \frac{(O - E)^2}{E}$, where $O$ is observed and $E$ is expected values.

  • Degrees of Freedom ($df$): $df = n - 1$, evaluated against a standard significance threshold of $p = 0.05$

Quantitative Genetics

• Polygenic Inheritance: Continuous variation controlled by multiple genes (polygenes), each having a small, cumulative, and additive effect on the phenotype.

• Nilsson-Ehle Wheat Model: Demonstrated continuous variation in wheat kernel color using additive alleles ($1:4:6:4:1$ for two loci; $1:6:15:20:15:6:1$ for three loci).

• Quantitative Formulae:

  • Fraction of $F_2$ expressing extreme parental phenotype: $\left(\frac{1}{4}\right)^n$, where $n$ is the number of polygene pairs.

  • Number of $F_2$ phenotypic classes: $2n + 1$

  • Number of $F_2$ genotypic classes: $3^n$

• Transgressive Inheritance: Occurrence of $F_2$ progeny exhibiting phenotypes more extreme than either parent due to recombination of complementary additive alleles.

Sex Determination and Sex Linkage

• Sex Determination Systems:

  • $XX-XY$ System: Females are homogametic ($XX$), males heterogametic ($XY$) (e.g., mammals, humans).

  • $XX-XO$ System: Females $XX$, males $XO$ (e.g., grasshoppers).

  • $ZZ-ZW$ System: Males homogametic ($ZZ$), females heterogametic ($ZW$) (e.g., birds, reptiles).

  • Haplo-diploidy: Males are haploid (from unfertilized eggs), females diploid (from fertilized eggs) (e.g., bees, wasps).

  • Environmental Sex Determination: Sex is determined by external factors such as incubation temperature or proximity to females (e.g., turtles, Bonellia viridis).

• Inheritance Classifications:

  • Sex-Linked: Genes located on sex chromosomes showing criss-cross inheritance (e.g., hemophilia, red-green color blindness).

  • Sex-Influenced: Autosomal traits expressed differently in males and females due to hormonal environments (e.g., pattern baldness in humans, horns in sheep).

  • Sex-Limited: Autosomal traits expressed exclusively in one sex (e.g., milk yield, cock-feathering in birds).

Mutations

• Gene Mutations:

  • Point Mutations: Substitutions involving transitions (purine to purine or pyrimidine to pyrimidine) or transversions (purine to pyrimidine or vice versa).

  • Frameshift Mutations: Insertions or deletions of nucleotides not in multiples of three, altering the downstream reading frame.

• Chromosomal Structural Mutations:

• Deletion: Loss of a chromosomal segment (e.g., Cri du chat syndrome from $5p$ deletion).

• Duplication: Presence of an extra chromosomal segment (e.g., Bar eye in Drosophila).

• Inversion: $180^\circ$ rotation of a segment; categorized as paracentric (excluding centromere) or pericentric (including centromere).

• Translocation: Exchange of chromosomal segments between non-homologous chromosomes.

  • Chromosomal Numerical Mutations:

• Aneuploidy: Addition or loss of specific individual chromosomes ($2n - 1$ monosomy, $2n + 1$ trisomy such as Down syndrome / Trisomy 21).

• Euploidy / Polyploidy: Variation in complete sets of chromosomes ($3x$ triploid, $4x$ tetraploid).

• Autopolyploidy: Polyploidy involving chromosome sets derived from a single species.

• Allopolyploidy: Polyploidy resulting from hybridization of two distinct species followed by chromosome doubling (e.g., Triticale, Raphanobrassica).

Molecular Genetics and Protein Synthesis

• DNA as Genetic Material: Confirmed through Griffith's transformation experiment ($1928$), Avery-MacLeod-McCarty experiment ($1944$), and Hershey-Chase bacteriophage experiment ($1952$).

• DNA Double Helix: Modeled by Watson and Crick ($1953$); double-stranded, antiparallel ($5' \rightarrow 3'$ and $3' \rightarrow 5'$) helix with complementary base pairing ($A=T$ and $G \equiv C$).

• Chargaff's Rules: In DNA, purine content equals pyrimidine content ($A + G = T + C$; $A = T$ and $G = C$).

• RNA Types:

  • $mRNA$: Carries transcribed genetic instructions from DNA to ribosomes.

  • $rRNA$: Structural and enzymatic component of ribosomes.

  • $tRNA$: Adapter molecule transferring specific amino acids to mRNA codons during translation.

• The Genetic Code:

• Triplet Code: 64 codons code for 20 standard amino acids.

• Key Features: Degenerate, non-overlapping, universal, comma-less, and non-ambiguous.

• Start Codon: $AUG$ (Methionine).

• Stop Codons: $UAA$, $UAG$, $UGA$

  • Protein Synthesis Steps:

• Transcription: RNA polymerase synthesizes a complementary $mRNA$ copy from the DNA template strand.

• Translation: Ribosomes read $mRNA$ codons to assemble amino acids via charged $tRNA$s in three stages: initiation, elongation, and termination.

Population Genetics

• Core Definitions:

  • Gene Pool: Sum total of all alleles carried in an interbreeding population.

  • Allele Frequency: Relative proportion of an allele type in a population ($p + q = 1.0$ for a two-allele locus).

• Hardy-Weinberg Law: Allele and genotype frequencies remain constant across generations in a large, randomly mating population in the absence of evolutionary forces.

  • Hardy-Weinberg Equation: $p^2 + 2pq + q^2 = 1.0$ (where $p^2 = \text{freq}(AA)$, $2pq = \text{freq}(Aa)$, and $q^2 = \text{freq}(aa)$).

  • Multiple Alleles ($ABO$ System): $(p + q + r)^2 = p^2 + 2pr + q^2 + 2qr + 2pq + r^2 = 1.0$

• Disturbing Forces: Mutation, selection, migration (gene flow), genetic drift, and non-random mating (inbreeding).

Introduction to Genetics and Key Terminology

• Genetics: The branch of biological science dealing with heredity and variation among organisms related by descent, established as a formal discipline following the rediscovery of Gregor Mendel's principles in $1900$.

• Heredity and Variation:

  • Heredity: The biological mechanism by which physical, physiological, and biochemical traits are transmitted from parents to offspring ("like begets like").

  • Variation: The structural or functional differences exhibited among individuals of the same species, arising from genetic recombination, environmental interaction, and mutation.

• Gene: The primary physical and functional unit of heredity, consisting of a specific linear sequence of nucleotides situated at a designated locus on a chromosome.

• Genotype vs. Phenotype:

  • Genotype: The specific genetic constitution of an organism (e.g., $AA$, $Aa$, or $aa$).

  • Phenotype: The observable morphological, physiological, or behavioral expression of a character, governed by the nature-nurture equation $P = G + E$, where $P$ is phenotype, $G$ is genotype, and $E$ is environmental influence.

• Homozygote vs. Heterozygote:

  • Homozygote: An organism carrying identical alleles at a given locus on homologous chromosomes (e.g., $AA$ or $aa$), producing only one type of gamete.

  • Heterozygote: An organism carrying two different alleles at a given locus (e.g., $Aa$), producing multiple gametic types.

• Allele: One of a pair or series of alternative molecular or functional forms of a gene occupying a specific chromosomal locus.

• Qualitative vs. Quantitative Traits:

  • Qualitative Traits: Characterized by discrete phenotypic categories controlled by one or a few major genes, exhibiting minimal environmental influence (e.g., flower color or blood groups).

  • Quantitative Traits: Display continuous phenotypic variation across a population, regulated by multiple polygenes with cumulative additive effects, highly influenced by the environment (e.g., grain yield or human height).

Cytogenetics and Chromosome Structure

• Cytogenetics: A hybrid discipline correlating cellular structures and mechanics—particularly the behavior and organization of chromosomes—with genetic phenomena and gene transmission.

• Chromatin Organization:

  • Euchromatin: Uncondensed, transcriptionally active chromosomal regions that stain lightly during interphase.

  • Heterochromatin: Highly condensed, transcriptionally inactive chromosomal regions that remain tightly coiled and stain intensely.

• Centromere Morphological Types:

  • Metacentric: Centromere is positioned centrally, dividing the chromosome into two equal arms.

  • Submetacentric: Centromere is positioned off-center, producing distinct short ($p$) and long ($q$) arms.

  • Acrocentric: Centromere is located near one terminal end, rendering the short ($p$) arm barely discernible.

  • Telocentric: Centromere is located at the extreme tip, resulting in a single visible arm.

  • Acentric: Chromosomal fragment lacking a centromere, leading to loss during cellular division.

• Karyotype: A standardized visual arrangement of an individual's complete set of metaphase chromosomes ordered systematically by size, shape, and centromeric index. Used to detect structural or numerical chromosomal abnormalities (e.g., Down syndrome, $2n + 1 = 47$).

DNA Replication and Cell Divisions

• DNA Replication: Occurs semi-conservatively during the $S$ phase of interphase, catalyzed by DNA polymerases. Each parent strand serves as a template to form a complementary daughter strand.

• The Cell Cycle: Comprises two primary periods:

  • Interphase: Period of cellular growth and metabolic synthesis, divided into $G<em>1$ (pre-DNA synthesis), $S$ (DNA replication), and $G</em>2$ (post-DNA synthesis).

  • Cell Division: Encompasses nuclear division ($M$ phase) followed by cytokinesis.

• Mitosis: Equatorial cell division resulting in two daughter cells genetically identical to the parent cell ($2n <br>ightarrow 2n$). Subdivided into prophase, metaphase, anaphase, and telophase. Serves as the basis for growth, repair, and asexual reproduction.

• Meiosis: Reductional cell division occurring in germline cells, yielding four genetically diverse haploid gametes or spores ($2n ightarrow n$).

  • Meiosis I: Reductional division separating homologous chromosomes. Prophase I includes five sub-stages: leptotene, zygotene (synapsis forming bivalents via synaptonemal complex), pachytene (crossing over and chiasmata formation), diplotene (repulsion of homologues), and diakinesis (terminalization of chiasmata).

  • Meiosis II: Equational division separating sister chromatids at their centromeres.

  • Genetic Significance: Maintains constant species chromosome number across generations and generates genetic variability via independent assortment ($2^N$ combinations, where $N = 23$ yields $8,388,608$ unique gametic types in humans) and genetic crossing over.

Mendelian and Qualitative Genetics

• Law of Segregation (Mendel's First Law): The two alleles for a character segregate cleanly during gamete formation so that each gamete receives only one allele.

  • Monohybrid Cross: Yields an $F_2$ phenotypic ratio of $3:1$ and a genotypic ratio of $1:2:1$.

• Law of Independent Assortment (Mendel's Second Law): Alleles of different gene pairs segregate independently of one another during gamete formation, provided the loci are on separate chromosomes.

  • Dihybrid Cross: Yields an $F_2$ phenotypic ratio of $9:3:3:1$

• Modifications to Mendelian Ratios:

  • Incomplete Dominance: Heterozygote exhibits an intermediate phenotype ($1:2:1$ phenotypic ratio; e.g., snapdragon flower color).

  • Codominance: Both alleles in a heterozygote are fully and distinctly expressed (e.g., $MN$ and $ABO$ blood groups).

  • Lethal Alleles: Homozygous expression causes lethality, altering classic ratios to $2:1$ (e.g., yellow coat color in mice).

  • Multiple Alleles: Presence of three or more alternative alleles for a single gene locus within a population (e.g., human $ABO$ system with alleles $I^A$, $I^B$, and $I^O$).

  • Epistasis: Inter-locus interactions where an allele at one locus masks or alters the phenotypic expression of an allele at another locus.

  • Duplicate Recessive (Complementary): $9:7$

  • Recessive Epistasis: $9:3:4$

  • Dominant Epistasis: $12:3:1$

  • Additive Epistasis: $9:6:1$

  • Duplicate Dominant Epistasis: $15:1$

  • Dominant and Recessive (Inhibiting): $13:3$

• Gene Linkage and Genetic Mapping:

  • Linkage: Genes located on the same physical chromosome tend to be inherited together as a linkage group.

  • Recombination Frequency ($R$): $ext{Recombination Frequency } (R) = rac{ ext{Total Recombinants}}{ ext{Total Progeny}}$. A $1$% recombination frequency equals $1 ext{ ext{ map unit (mu)}}$ or $1 ext{ ext{ centimorgan (cM)}}$. Maximum recombination for unlinked genes is $50$% ($R = 0.50$).

  • Interference ($I$) and Coincidence ($S$): Coefficient of coincidence $S = rac{ ext{Observed Double Crossovers}}{ ext{Expected Double Crossovers}}$, and Interference $I = 1 - S$.

Statistical Applications in Genetics

• Descriptive Statistics:

  • Mean ($ar{x}$): $\bar{x} = rac{ rac{\sum x_i}{n}}$, representing the central phenotypic value.

  • Sample Variance ($S^2$): $S^2 = rac{\sum (x_i - \bar{x})^2}{n - 1}$, quantifying phenotypic dispersion.

  • Standard Deviation ($S$): $S = rac{\sqrt{S^2}}$. In a normal distribution, $\bar{x} rac{\pm}{} 1S$ includes $rac{68}{}$% of data, and $\bar{x} rac{\pm}{} 2S$ includes $rac{95}{}$% of data.

• Probability Rules:

  • Sum Law: $P(A ext{ ext{ or }} B) = P(A) + P(B)$ for mutually exclusive events.

  • Product Law: $P(A ext{ ext{ and }} B) = P(A) imes P(B)$ for independent events.

  • Binomial Expansion: $(a + b)^n$ calculates combined probabilities of alternative outcomes across $n$ independent trials.

• Chi-Square Test ($rac{\chi^2}{}$):

  • Formula: $rac{\chi^2}{} = rac{\sum rac{(O - E)^2}{E}}$, where $O$ is observed and $E$ is expected values.

  • Degrees of Freedom ($df$): $df = n - 1$, evaluated against significance thresholds (typically $p = 0.05$).

Quantitative Genetics

• Polygenic Inheritance: Continuous variation controlled by multiple additive genes (polygenes), each contributing a small, cumulative portion to the phenotype.

• Nilsson-Ehle Wheat Model: Demonstrated polygenic kernel color in wheat using additive loci ($1:4:6:4:1$ for two loci; $1:6:15:20:15:6:1$ for three loci).

• Quantitative Genetics Formulae:

  • Fraction of $F_2$ expressing extreme parental phenotype: $rac{\left(\frac{1}{4}\right)^n}$, where $n$ is the number of polygenic loci.

  • Number of $F_2$ Phenotypic Classes: $2n + 1$

  • Number of $F_2$ Genotypic Classes: $3^n$

• Transgressive Inheritance: Occurrence of $F_2$ offspring exhibiting phenotypes more extreme than either parent due to recombining complementary additive alleles.

Sex Determination and Sex Linkage

• Sex Determination Systems:

  • $XX-XY$ System: Females homogametic ($XX$), males heterogametic ($XY$) (e.g., mammals, humans).

  • $XX-XO$ System: Females $XX$, males $XO$ (e.g., grasshoppers, roaches).

  • $ZZ-ZW$ System: Males homogametic ($ZZ$), females heterogametic ($ZW$) (e.g., birds, reptiles, butterflies).

  • Haplo-diploidy: Males haploid ($n$) from unfertilized eggs, females diploid ($2n$) from fertilized eggs (e.g., bees, wasps).

  • Environmental Sex Determination: Sex determined by environmental variables like incubation temperature (turtles, alligators) or social context (Bonellia viridis).

• Inheritance Classifications:

  • Sex-Linked: Loci located on sex chromosomes showing criss-cross inheritance (e.g., hemophilia A, red-green color blindness).

  • Sex-Influenced: Autosomal traits whose expression differs between sexes due to hormonal backgrounds (e.g., pattern baldness in humans, horns in sheep).

  • Sex-Limited: Autosomal traits expressed exclusively in one sex (e.g., milk production in cows, cock-feathering in birds).

Mutations

• Gene Mutations:

  • Point Mutations: Single base-pair alterations, including transitions (purine to purine, pyrimidine to pyrimidine) and transversions (purine to pyrimidine or vice versa).

  • Frameshift Mutations: Insertions or deletions of nucleotides not in multiples of three, shifting the downstream translational reading frame.

• Chromosomal Structural Mutations:

  • Deletions (Deficiencies): Loss of a chromosome segment; induces pseudodominance and synaptic looping (e.g., Cri-du-chat syndrome, deletion on chromosome $5p$).

  • Duplications: Doubling of a chromosomal segment; provides evolutionary raw material for gene diversification (e.g., Bar eye mutation in Drosophila).

  • Inversions: Reversal of chromosomal segment orientation by $180^{\circ}$. Classified as paracentric (excluding centromere) or pericentric (including centromere); acts as crossover suppressors.

  • Translocations: Exchange of chromosomal fragments between non-homologous chromosomes (e.g., reciprocal translocations).

• Numerical Chromosomal Mutations:

  • Aneuploidy: Gain or loss of individual chromosomes ($2n \pm k$), including monosomy ($2n - 1$) and trisomy ($2n + 1$), caused by non-disjunction (e.g., Down syndrome, trisomy $21$; Turner syndrome, $45,X$; Klinefelter syndrome, $47,XXY$).

  • Polyploidy: Multiples of complete chromosome sets ($3x$, $4x$, $6x$). Autopolyploids arise within a single species, whereas allopolyploids originate from interspecific hybridization followed by chromosome doubling (e.g., hexaploid bread wheat Triticum aestivum, $2n = 6x = 42$; Triticale, $2n = 6x = 42$).

Molecular Genetics and Protein Synthesis

• Proof of DNA as Genetic Material: Established through Griffith's transformation experiment ($1928$), Avery-MacLeod-McCarty experiment ($1944$), and the Hershey-Chase bacteriophage experiment ($1952$).

• DNA Structure & Chargaff's Rules:

  • Antiparallel double helix ($5' \rightarrow 3'$ and $3' \rightarrow 5'$) modeled by Watson and Crick ($1953$).

  • Chargaff's Rules: In double-stranded DNA, purine content equals pyrimidine content ($A + G = T + C$; $A = T$ and $G = C$).

• RNA Types:

  • $mRNA$: Transcribes genetic instructions from DNA to ribosomes.

  • $rRNA$: Structural and enzymatic component of ribosomes.

  • $tRNA$: Adapter molecule transferring specific amino acids to matching codons via anticodons.

• The Genetic Code & Protein Synthesis:

  • Properties: Triplet code ($64$ codons for $20$ amino acids), non-overlapping, degenerate, non-ambiguous, universal, with initiation ($AUG$) and termination ($UAA$, $UAG$, $UGA$) signals.

  • Transcription: RNA polymerase synthesizes $mRNA$ from the DNA template strand.

  • Translation: Ribosome-mediated synthesis of polypeptide chains comprising aminoacylation, initiation, elongation, and termination steps.

Population Genetics

• Core Definitions:

  • Gene Pool: Sum total of all alleles present within an interbreeding population.

  • Allele Frequency: Relative proportion of a specific allele in a population ($p + q = 1.0$ for a bi-allelic locus).

• Hardy-Weinberg Law: Allele and genotype frequencies remain constant across generations in a large, randomly mating population in the absence of evolutionary disturbing forces.

• Hardy-Weinberg Equations:

  • Bi-allelic Locus: $p^2 + 2pq + q^2 = 1.0$, where $p^2 = ext{ ext{freq}}(AA)$, $2pq = ext{ ext{freq}}(Aa)$, and $q^2 = ext{ ext{freq}}(aa)$.

  • Multiple Allelic Locus ($ABO$ System): $(p + q + r)^2 = p^2 + 2pr + q^2 + 2qr + 2pq + r^2 = 1.0$

• Disturbing Forces: Mutation, natural selection, gene flow (migration), genetic drift (in small populations), and non-random mating (inbreeding).