Genetics: Heredity, DNA, and Genes

• Genetics is the study of heredity and variation in living organisms
• Explores how traits are passed from parents to offspring
• Explores how these traits can differ.

Basic Concepts

• Genes are the fundamental units of heredity
• Genes contain instructions for building proteins or functional RNA molecules, located on chromosomes
• Alleles are different versions of a gene
• Individuals inherit one allele from each parent for each gene
• Genotype refers to the genetic makeup of an organism
• Phenotype refers to the observable characteristics influenced by both genotype and environment.

DNA Structure and Function

• DNA (deoxyribonucleic acid) carries genetic information
• DNA is a double helix with two nucleotide strands
• Each nucleotide includes a deoxyribose sugar, a phosphate group, and a nitrogenous base
• Four nitrogenous bases exist: adenine (A), guanine (G), cytosine (C), and thymine (T)
• Adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C)
• Base sequence in DNA determines the genetic code
• DNA replication ensures each daughter cell gets a complete set of genetic information during cell division.

Gene Expression

• Gene expression synthesizes a functional gene product (protein or RNA) from the information encoded in a gene
• Involves transcription and translation
• Transcription synthesizes RNA from a DNA template, using RNA polymerase
• Translation synthesizes a protein from the information encoded in mRNA
• Ribosomes are the sites of translation
• Transfer RNA (tRNA) molecules bring amino acids to the ribosome and attach to the growing polypeptide chain.

Mendelian Genetics

• Gregor Mendel, the father of genetics, studied pea plants
• His studies led to the formulation of the basic principles of heredity
• Mendel's laws
  • Law of Segregation: Alleles for each trait separate during gamete formation
  • Law of Independent Assortment: Alleles of different genes assort independently during gamete formation
  • Law of Dominance: Some alleles are dominant and mask the expression of recessive alleles
• Monohybrid cross: study of a single trait
• Dihybrid cross: study of two traits
• Punnett square: Predicts genotypes and phenotypes of offspring in a genetic cross.

Chromosomal Basis of Inheritance

• Genes are located on chromosomes
• Chromosomes are structures of DNA and proteins containing genetic information
• Humans have 46 chromosomes, arranged in 23 pairs
• Homologous chromosomes: pairs carrying the same genes
• Sex chromosomes determine sex (e.g., XX for female, XY for male in humans)
• Meiosis: Cell division producing gametes (sperm and egg cells)
• During meiosis, homologous chromosomes pair up and exchange genetic material via crossing over
• Crossing over increases genetic variation in offspring.

Mutations

• Mutation: a change in the DNA sequence
• Can occur spontaneously or be induced by environmental factors (e.g., radiation, chemicals)
• Mutations can be beneficial, harmful, or neutral
• Types
  • Point mutations: Changes in a single nucleotide base
    • Substitutions: Replacing one base with another
    • Insertions: Adding a base
    • Deletions: Removing a base
  • Frameshift mutations: Insertions or deletions altering the reading frame
  • Chromosomal mutations: Changes in the structure or number of chromosomes
    • Deletions: Loss of a portion of a chromosome
    • Duplications: Repetition of a portion of a chromosome
    • Inversions: Reversal of a segment of a chromosome
    • Translocations: Movement of a segment to a nonhomologous chromosome
• Germ cell mutations can be passed to offspring.

Genetic Variation

• Genetic variation: differences in DNA sequences among individuals
• Sources of genetic variation
  • Mutation
  • Sexual reproduction
    • Independent assortment
    • Crossing over
    • Random fertilization
• Genetic variation is essential for evolution
• Allows populations to adapt to changing environments.

Population Genetics

• Population genetics: Study of the genetic composition of populations and how it changes
• Population: A group of individuals of the same species that live in the same area and interbreed
• Gene pool: Total collection of genes in a population
• Allele frequency: Proportion of a particular allele
• Hardy-Weinberg principle describes stable allele and genotype frequencies from generation to generation under certain conditions
• Conditions for Hardy-Weinberg equilibrium: no mutation, random mating, no gene flow, no genetic drift, and no selection
• Deviations from Hardy-Weinberg equilibrium indicate evolutionary forces.

Genetic Engineering and Biotechnology

• Genetic engineering manipulates an organism's genes
• Biotechnology uses biological systems/organisms to develop or modify products
• Techniques
  • Recombinant DNA technology: Combining DNA from different sources
  • Gene cloning: Making multiple copies of a gene
  • Polymerase chain reaction (PCR): Amplifying DNA sequences
  • Gene therapy: Introducing genes to treat disease
  • Genome editing: Precisely modifying DNA using tools like CRISPR-Cas9
• Applications of genetic engineering/biotechnology
  • Producing pharmaceuticals
  • Developing genetically modified crops
  • Diagnosing and treating diseases
  • Producing biofuels.

Genomics

• Genomics studies entire genomes
• Sequencing, mapping, and analyzing the complete DNA set in an organism
• Genomics can
  • Identify genes associated with diseases
  • Understand the genetic basis of complex traits
  • Study evolutionary relationships
  • Develop personalized medicine

Epigenetics

• Epigenetics studies heritable changes in gene expression without DNA sequence alterations
• Epigenetic mechanisms
  • DNA methylation: Addition of a methyl group
  • Histone modification: Changes to proteins around which DNA is wrapped
• Epigenetic modifications passed to daughter cells during cell division
• Epigenetics plays a role in development, aging, and disease.