Mendelian Genetics, Chromosomal Inheritance, DNA Replication, and Protein Synthesis

Ch. 14: Mendelian Genetics

Mendel’s Theory and Application with Pea Plants

Gregor Mendel’s experiments with Pisum sativum (pea plants) established the foundational principles of inheritance. By controlling pollination and tracking traits across generations, Mendel deduced how traits are passed from parents to offspring.

• Key Terms: Gene (heritable factor), allele (variant of a gene), trait (observable characteristic).

• Experimental Approach: Mendel used true-breeding plants and performed cross- and self-pollination to observe inheritance patterns.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation (applies to genes on different chromosomes).

• Example: Crossing purple-flowered and white-flowered pea plants yields all purple F1 offspring, but F2 generation shows a 3:1 ratio of purple to white flowers.

Generations: P, F1, F2, and Beyond

• P Generation: Parental generation (true-breeding individuals).

• F1 Generation: First filial generation (offspring of P cross).

• F2 Generation: Second filial generation (offspring of F1 self- or cross-pollination).

• Example: P (purple x white) → F1 (all purple) → F2 (3 purple : 1 white).

Dominant and Recessive Traits; Homozygous and Heterozygous Genes

• Dominant allele: Expressed in phenotype when present (e.g., purple flower color).

• Recessive allele: Masked by dominant allele; only expressed when homozygous (e.g., white flower color).

• Homozygous: Two identical alleles (PP or pp).

• Heterozygous: Two different alleles (Pp).

Punnett Squares: Predicting Genotypes and Phenotypes

Punnett squares are used to predict the probability of offspring genotypes and phenotypes from parental crosses.

• Steps:

  1. List possible gametes from each parent.

  2. Fill in the grid to show all possible combinations.

  3. Count genotype and phenotype ratios.

• Example: Cross between two heterozygotes (Pp x Pp):

	P	p
P	PP	Pp
p	Pp	pp

• Genotype ratio: 1 PP : 2 Pp : 1 pp

• Phenotype ratio: 3 purple : 1 white

Inheritance Patterns: Sex Linkage, Codominance, Incomplete Dominance

• Sex Linkage: Genes located on sex chromosomes (often X-linked); males (XY) are more likely to express recessive X-linked traits.

• Codominance: Both alleles are fully expressed in heterozygotes (e.g., AB blood type).

• Incomplete Dominance: Heterozygote phenotype is intermediate between homozygotes (e.g., red x white flowers yield pink offspring).

Ch. 15: Chromosomal Basis of Inheritance

Thomas Hunt Morgan’s Fruit Fly Experiments

Thomas Hunt Morgan used Drosophila melanogaster (fruit flies) to demonstrate that genes are located on chromosomes and to study sex-linked inheritance.

• Wild Type: Most common phenotype in a population (e.g., red eyes in fruit flies).

• Mutant: Alternative phenotype due to mutation (e.g., white eyes).

• Sex-Linked Traits: Traits associated with genes on sex chromosomes (e.g., white eye gene on X chromosome).

• Example: Cross between red-eyed female and white-eyed male yields different results in male and female offspring due to X-linkage.

Gene Linkage

• Linked Genes: Genes located close together on the same chromosome tend to be inherited together.

• Recombination: Crossing over during meiosis can separate linked genes, producing recombinant phenotypes.

Chromosomal Mutations: Translocation and Syndromes

• Translocation: Segment of one chromosome attaches to a nonhomologous chromosome.

• Other Chromosomal Abnormalities: Deletions, duplications, inversions, and nondisjunction (failure of chromosomes to separate properly).

• Examples of Syndromes: Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY).

Inheritance Patterns and Pedigrees

• Autosomal Inheritance: Traits inherited via non-sex chromosomes.

• Sex-Linked Inheritance: Traits inherited via X or Y chromosomes.

• Mitochondrial Inheritance: Traits inherited via mitochondrial DNA (from mother only).

• Pedigree Analysis: Diagram showing inheritance of traits across generations; used to determine inheritance patterns.

Ch. 16: Molecular Basis of Inheritance

DNA Replication in Prokaryotes vs. Eukaryotes

• Prokaryotes: Single origin of replication, circular DNA, replication proceeds bidirectionally.

• Eukaryotes: Multiple origins of replication, linear chromosomes, more complex machinery.

DNA Replication: Enzymes and Functions

• Helicase: Unwinds DNA double helix.

• Single-Strand Binding Proteins (SSBPs): Stabilize unwound DNA.

• Topoisomerase: Relieves supercoiling ahead of replication fork.

• Primase: Synthesizes RNA primer.

• DNA Polymerase III (prokaryotes): Synthesizes new DNA strand by adding nucleotides to primer.

• DNA Polymerase I (prokaryotes): Removes RNA primers and replaces with DNA.

• Ligase: Joins Okazaki fragments on lagging strand.

Semi-Conservative Replication

• Each new DNA molecule consists of one parental and one newly synthesized strand.

• Alternative Models: Conservative (parental strands stay together), Dispersive (mixed segments).

Leading and Lagging Strand Synthesis

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, later joined by ligase.

Ch. 17: Protein Synthesis

Central Dogma: DNA → mRNA → Protein

• Genetic information flows from DNA to RNA to protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a polypeptide (protein).

Transcription

• Location: Nucleus (eukaryotes), cytoplasm (prokaryotes).

• Enzyme: RNA polymerase synthesizes RNA from DNA template.

• Product: Single-stranded mRNA complementary to DNA template strand.

Translation

• Location: Ribosome (cytoplasm or rough ER).

• Components: mRNA, ribosomes, transfer RNA (tRNA), amino acids.

• Process: tRNAs bring amino acids to ribosome, matching codons (mRNA) with anticodons (tRNA).

• Product: Polypeptide chain (protein).

Codons, Anticodons, and Amino Acid Sequences

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

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

• Genetic Code: Universal, redundant (multiple codons for same amino acid), non-overlapping.

• Example: mRNA codon AUG codes for methionine (start codon).

Types of Mutations

• Point Mutation: Change in a single nucleotide (substitution, insertion, deletion).

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Introduces a stop codon, truncating protein.

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

Additional info: For Punnett squares, codominance, and incomplete dominance, students should practice with sample crosses. For DNA replication, remember that eukaryotes have more complex regulation and multiple origins. For protein synthesis, refer to the genetic code table for codon-amino acid assignments.