Genetics: Core Concepts, Molecular Mechanisms, and Inheritance Patterns

Genetic Fundamentals

Key Definitions and Concepts

Genetics is the study of heredity and variation in living organisms. The following are foundational terms and concepts:

• Genes: Physical units of heredity, originally described by Mendel, now defined as DNA sequences. Genes are transmitted to offspring in predictable patterns.

• Chromosomes: Large molecules of double-stranded DNA and protein, containing genes.

• Homologous pairs: Chromosomes carrying genes for the same traits, found in sexually reproducing organisms.

• Mitosis: Asexual reproduction resulting in identical daughter cells.

• Meiosis: Sexual reproduction producing gametes with half the chromosome number.

• Gametes: Sperm and eggs in animals; pollen and eggs in plants.

• Phenotype: Observable traits of an organism.

• Genotype: Genetic constitution of an organism.

• Alleles: Alternative (variant) forms of a gene.

• Genome: Complete set of genetic information carried by a species.

• Deoxyribonucleic acid (DNA): Hereditary material in all organisms.

• Ribonucleic acid (RNA): Used by some viruses and in gene expression.

• DNA replication: Process that duplicates the DNA duplex prior to cell division.

• Transcription: DNA strand is used to direct RNA synthesis.

• Messenger RNA (mRNA): Undergoes translation to produce proteins at ribosomes.

Molecular Genetics

Structure and Function of Nucleic Acids

• Molecular genetics: Studies inheritance and variation of nucleic acids, proteins, and genomes.

• Chargaff's rule: Nucleotides are arranged as complementary base pairs (A with T, C with G).

• DNA nucleotides: Composed of a deoxyribose (5-carbon) sugar, phosphate group, and one of four nitrogenous bases (Adenine, Guanine, Thymine, Cytosine).

• Hydrogen bonds: Form between complementary base pairs.

• Semiconservative replication: Each new DNA duplex contains one parental and one newly made strand.

Central Dogma of Biology

The central dogma describes the flow of hereditary information:

• DNA transcription → RNA translation → Protein

• Reverse transcription (in some viruses): RNA → DNA

Types of RNA

• mRNA: Translated into proteins.

• rRNA: Forms part of the ribosomes.

• tRNA: Carries amino acids to ribosomes.

• MicroRNAs: Small RNA molecules regulating gene expression.

Genetic Code and Translation

• Genetic code: mRNA specifies amino acid sequence using triplet codons.

• There are 64 possible triplet codons; 61 specify amino acids, 3 are stop codons.

• There are 20 common amino acids.

Gel Electrophoresis

Used to assess nucleic acid and protein variation.

• Types: Agarose and polyacrylamide gels.

• Separates molecules based on size, shape, and charge.

Blotting Techniques

Technique	Target
Southern blotting	DNA transfer
Northern blotting	RNA transfer
Western blotting	Protein transfer

Genomics, Proteomics, and Transcriptomics

Genomics

• Focuses on sequencing, interpretation, and comparison of genomes of different organisms.

Proteomics

• Studies the complete set of proteins encoded in a genome.

• Examines protein function, localization, regulation, and interaction.

Transcriptomics

• Studies the complete set of genes undergoing transcription in a cell.

• Metabolomics: Studies chemical processes involving metabolites in a specific cell, tissue, organ, or organism.

Evolution and Population Genetics

Evolutionary Principles

• Life is not static; DNA acquires mutations and evolves.

• Biochemical processes that replicate DNA and express genetic information are universal.

Darwin's Theory of Evolution

• Species have come and gone; changes occur through evolution.

• All organisms are related by common ancestry and have diversified over time.

• Natural selection is the process by which advantageous traits become more common.

Four Processes Leading to Changes in Allele Frequencies

Process	Description
Natural Selection	Different reproductive success due to possession of adaptive traits
Migration	Movement of individuals between populations
Mutation	Addition of new allelic variants
Genetic Drift	Random change of allele frequencies

Phylogenetic Tree

Diagram depicting evolutionary relationships among organisms.

Mendelian Genetics

Mendel's Experiments and Laws

• Mendel tested the blending theory of heredity using pea plants.

• Pure breeding strains: Strains that constantly produce the same phenotype.

• Crosses:

  • Replicate crosses: Repeating the same cross several times.

  • Reciprocal crosses: Same genotypes crossed by the sexes of the parents are reversed.

  • Test crosses: Used to determine the unknown genotype.

• Homozygous: Same alleles from each parent.

• Heterozygous: Different alleles from each parent.

• Theory of particulate inheritance: Plants carry two discrete hereditary units for each trait.

• Alleles: Hereditary particles referred to in the theory.

• Law of segregation (Mendel's first law): Units of heredity separate into gametes, and random union of gametes into progeny is in predictable proportions.

Dihybrid and Trihybrid Crosses

• Dihybrid crosses: Study the simultaneous transmission of two traits.

• Trihybrid crosses: Study three involved traits.

• 9:3:3:1 ratios illustrate Mendel's second law (independent assortment).

Probability in Genetics

• Conditional probability: Involves questions asked after a cross has been made.

• Chi-square (χ²) test: Used to quantify how closely an experimental observation matches the expected outcome.

• Degrees of freedom: Number of outcome classes minus one.

Autosomal Inheritance

• Transmission of genes carried on autosomes; chromosomes found in both males and females.

• Humans have 22 pairs of autosomes and one pair of sex chromosomes (X and Y).

Pedigrees

• Family trees showing inheritance of traits in humans and animals.

• Standard notation is used to indicate males, females, and relationships.

Autosomal Dominant and Recessive Inheritance

• Autosomal dominant: Trait appears in every generation; males and females have trait in equal frequencies.

• Autosomal recessive: Trait may skip generations; often seen among siblings.

Molecular Genetics of Mendel's Traits

• Transmission of alleles is equated with transmission of variable DNA sequences.

• Phenotypic variation results from differences in structure and function of protein products of alleles.

• Molecular analysis leads to identification of DNA sequence differences between alleles and their consequences.

Cell Division: Mitosis and Meiosis

Mitosis

• Produces two identical daughter cells that are exact genetic replicas of the parental cell.

• Most body cells are somatic cells (nonreproductive).

• Diploid number (2n) of chromosomes.

Meiosis

• Produces gametes with half the number of chromosomes as the original cell.

• Gametes are not genetically identical to one another.

Sex Chromosomes

• Determine sex and differ between sexes.

• Discovery of genes on the X chromosome supported the chromosome theory of heredity.

Cell Cycle

• Cycle of DNA replication and division.

• Three principal phases:

  • G1 phase: Gene expression occurs at a high level.

  • S phase: DNA is replicated.

  • G2 phase: Cells prepare for division.

• M phase: Mitosis and cytokinesis.

Mitosis Stages

• Prophase: Chromosomes condense; centromeres migrate.

• Prometaphase: Nuclear envelope breakdown; microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope reassembles; cytokinesis divides the cytoplasm.

Centromeres and Spindle Fibers

• Centromeres: Specialized DNA sequences where sister chromatids are joined.

• Spindle fibers: Microtubules responsible for chromosome movement and cell stability.

• Types:

  • Kinetochore microtubules: Attach to centromeres.

  • Polar microtubules: Extend toward the opposite pole.

  • Astral microtubules: Grow toward the cell membrane.

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Tables were reconstructed to summarize key comparisons and classifications.

• Equations and ratios (e.g., 9:3:3:1) were described in text for Mendelian inheritance.