Patterns of Inheritance, Molecular Biology of the Gene, Gene Regulation, and DNA Technology: Study Guide

Patterns of Inheritance

Key Concepts in Genetics

This section introduces foundational terminology and concepts in Mendelian and non-Mendelian inheritance, essential for understanding how traits are passed from one generation to the next.

• Trait vs. Gene: A trait is a specific characteristic (e.g., flower color), while a gene is a DNA segment that codes for a trait.

• True Breeding vs. Hybrid: True breeding organisms produce offspring identical to themselves; hybrids result from crossing different true-breeding parents.

• Locus vs. Allele: A locus is a gene's location on a chromosome; an allele is a variant form of a gene.

• Homozygous vs. Heterozygous: Homozygous individuals have two identical alleles; heterozygous have two different alleles.

• Autosome vs. Sex Chromosome: Autosomes are non-sex chromosomes; sex chromosomes determine biological sex (e.g., X and Y in humans).

• Genotype vs. Phenotype: Genotype is the genetic makeup; phenotype is the observable trait.

• Monohybrid vs. Dihybrid Crosses: Monohybrid crosses involve one trait; dihybrid crosses involve two traits.

• Dominant vs. Recessive Alleles: Dominant alleles mask recessive ones in heterozygotes.

• Incomplete Dominance vs. Codominance: Incomplete dominance results in intermediate phenotypes; codominance expresses both alleles fully (e.g., AB blood type).

• Polygenic Inheritance vs. Pleiotropy: Polygenic inheritance involves multiple genes affecting one trait; pleiotropy is one gene affecting multiple traits.

• Law of Segregation, Law of Independent Assortment, Linked Genes: The law of segregation states alleles separate during gamete formation; independent assortment means genes on different chromosomes assort independently; linked genes are inherited together due to proximity on the same chromosome.

Punnett Squares and Genetic Ratios

Punnett squares are tools for predicting offspring genotypes and phenotypes from parental crosses.

• Genotypes of Parents: Identify alleles each parent carries.

• Gametes: List possible gametes each parent can produce.

• Phenotypic and Genotypic Ratios: Calculate expected ratios in offspring.

• Example: Crossing Aa x Aa yields genotypes AA, Aa, aa in a 1:2:1 ratio; phenotypes (if A is dominant) are 3:1.

Inheritance Patterns

• Complete Dominance: Dominant allele masks recessive (e.g., Mendel's pea plants).

• Incomplete Dominance: Heterozygotes show intermediate phenotype (e.g., red x white flowers = pink).

• Codominance: Both alleles expressed (e.g., human ABO blood group).

• Sex-Linked Genes: Traits on sex chromosomes, often X-linked (e.g., color blindness).

Pedigree Analysis

Pedigrees trace inheritance patterns in families, helping to determine genotypes and predict genetic disorders.

Genetic Disorders

• Albinism: Lack of melanin due to recessive alleles.

• Sickle-Cell Anemia: Abnormal hemoglobin due to a point mutation.

• Color Blindness: Often X-linked recessive.

• Hemophilia: X-linked recessive disorder affecting blood clotting.

• Huntington’s Disease: Dominant allele causes neurodegeneration.

Additional Key Terms and Concepts

• Carrier: Heterozygous individual carrying a recessive allele.

• Testcross: Used to determine unknown genotype by crossing with a homozygous recessive.

• Sex-Linked Traits: More common in males due to single X chromosome.

• Environmental Influence: Environment can affect gene expression (e.g., temperature-sensitive fur color in rabbits).

Molecular Biology of the Gene

Genes, Chromosomes, and DNA

Genes are segments of DNA located on chromosomes, encoding instructions for proteins.

Chemical Structure of DNA

• Nucleotides: Each contains a phosphate group, deoxyribose sugar, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), guanine (G).

• Double Helix: DNA strands run antiparallel, held together by hydrogen bonds between complementary bases.

Base Pairing Rule: A pairs with T, C pairs with G.

Genetic Code: The sequence of bases encodes genetic information.

DNA Replication

• Semi-Conservative: Each new DNA molecule has one old and one new strand.

• Key Enzymes:

  • DNA helicase: Unwinds the double helix.

  • DNA polymerase: Synthesizes new DNA strands.

  • DNA ligase: Joins Okazaki fragments on the lagging strand.

• Free Nucleotides: Serve as building blocks for new DNA strands.

Equation for DNA Replication:

$\text{(Parent DNA)} \xrightarrow{\text{Replication}} \text{(Parent + New DNA)}$

Gene Mutation

Mutations are changes in DNA sequence, which can alter protein function.

Chemical Structure of RNA

• Nucleotides: Contain ribose sugar, phosphate, and bases: adenine (A), uracil (U), cytosine (C), guanine (G).

• Single-Stranded: RNA is usually single-stranded.

Types of RNA and Their Functions

• mRNA (messenger RNA): Carries genetic code from DNA to ribosome.

• rRNA (ribosomal RNA): Forms part of the ribosome structure.

• tRNA (transfer RNA): Brings amino acids to ribosome during translation.

Protein Synthesis

• Transcription: DNA is used as a template to make mRNA. Involves promoter, RNA polymerase.

• Translation: mRNA is decoded at the ribosome to assemble amino acids into a protein. Involves tRNA, ribosome, amino acids.

Codons and Anticodons

• Codon: Three-base sequence on mRNA specifying an amino acid.

• Anticodon: Three-base sequence on tRNA complementary to mRNA codon.

Introns, Exons, and RNA Splicing

• Introns: Non-coding regions removed from pre-mRNA.

• Exons: Coding regions joined together to form mature mRNA.

• RNA Splicing: Process of removing introns and joining exons.

Types of Mutations

• Point Mutation: Change in a single nucleotide.

• Insertion/Deletion: Addition or loss of nucleotides.

• Frameshift Mutation: Insertion or deletion shifts reading frame, altering downstream amino acids.

Sickle Cell Disease

Caused by a point mutation in the hemoglobin gene, resulting in abnormal hemoglobin structure and sickle-shaped red blood cells.

Gene Regulation

Importance of Gene Regulation

Gene regulation ensures genes are expressed at the right time, place, and amount, critical for development and cellular function.

Lactose Operon (lac operon) in Bacteria

• Regulatory Gene: Codes for repressor protein.

• Promoter: Site where RNA polymerase binds.

• Operator: DNA segment where repressor binds.

• Structural Genes: Code for enzymes to metabolize lactose.

• Repressor Protein: Blocks transcription when lactose is absent; inactivated when lactose is present, allowing gene expression.

Epigenetic Inheritance

Heritable changes in gene expression not involving changes to DNA sequence (e.g., DNA methylation, histone modification).

Methods of Gene Regulation

• DNA Packing (Histones): Tightly packed DNA is less accessible for transcription.

• Methylation: Addition of methyl groups silences genes.

• Barr Body: Inactivated X chromosome in females.

• Transcription Factors: Proteins that regulate transcription initiation.

• Regulatory Proteins: Bind DNA to enhance or inhibit transcription.

• Noncoding RNA (miRNA & siRNA): Regulate gene expression post-transcriptionally.

• RNA Interference: Small RNAs block translation or degrade mRNA.

• Alternative RNA Splicing: Different mRNAs produced from same gene.

• Protein Processing: Modifications after translation affect protein function.

Homeotic Genes

Control the development of anatomical structures in organisms.

Monitoring Gene Expression

• Nucleic Acid Hybridization: Detects specific DNA/RNA sequences.

• DNA Microarray: Measures expression of thousands of genes simultaneously.

Cloning and Stem Cells

• Reproductive Cloning: Producing a genetically identical organism via nuclear transplantation.

• Therapeutic Cloning: Producing embryonic stem cells for medical treatment.

• Embryonic Stem Cells: Can become any cell type; Adult Stem Cells: More limited in differentiation.

Example: Stem cells used to regenerate damaged tissues.

Cancer and Gene Regulation

• Carcinogens: Substances causing cancer by inducing mutations.

• Oncogenes: Mutated genes causing excessive cell division.

• Tumor-Suppressor Genes: Normally inhibit cell division; mutations can lead to cancer.

DNA Technology and Genomics

Key Terms

• Biotechnology: Use of living organisms to develop products.

• Genetic Engineering: Direct manipulation of genes.

• Recombinant DNA: DNA formed by combining DNA from different sources.

Gene Cloning and DNA Technology

• Plasmid: Circular DNA in bacteria used as vectors.

• Restriction Enzyme: Cuts DNA at specific sequences.

• DNA Ligase: Joins DNA fragments.

• Nucleic Acid Probe: Detects specific DNA sequences.

• Reverse Transcriptase: Synthesizes DNA from RNA template.

Gene Editing (CRISPR-Cas9)

Allows precise modification of genes, with potential for treating genetic diseases.

Transgenic and Genetically Modified Organisms (GMO)

• Transgenic Organism: Contains genes from another species.

• GMO: Organism with artificially altered genome.

Applications: Pharmaceuticals (e.g., insulin), agriculture (e.g., pest-resistant crops).

Concerns: Human health risks, environmental impact.

Gene Therapy

Introduction of normal genes to treat genetic disorders.

DNA in Criminal Investigation

• DNA Profile: Unique DNA pattern for identification.

• Polymerase Chain Reaction (PCR): Amplifies DNA sequences.

• Gel Electrophoresis: Separates DNA fragments by size.

• Short Tandem Repeats (STR): Repeated DNA sequences used in profiling.

• Next-Generation Sequencing: Rapid sequencing of entire genomes.

Genomics, Human Genome Project, and Bioinformatics

• Genomics: Study of whole genomes.

• Human Genome Project (HGP): Mapped all human genes; Whole-Genome Shotgun Method: Sequences entire genome by breaking into fragments, sequencing, and assembling.

• Bioinformatics: Application of computational tools to analyze biological data.

• Proteomics: Study of all proteins produced by a genome.