Mendelian Genetics, Chromosomal Inheritance, Molecular Basis of Inheritance, 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 Point 1: Mendel identified discrete heritable units, now called genes, that determine traits.

• Key Point 2: He observed that traits do not blend but are inherited as distinct units.

• Example: Crossing pure-breeding purple-flowered and white-flowered pea plants resulted in all purple F1 offspring, but the white trait reappeared in the F2 generation.

Generations: P, F1, F2, and Beyond

Mendel’s experiments tracked traits through several generations:

• P Generation: Parental generation; true-breeding individuals.

• F1 Generation: First filial generation; offspring of the P generation.

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

• Key Point: The reappearance of recessive traits in the F2 generation supported the concept of dominant and recessive alleles.

Dominant and Recessive Traits; Homozygous and Heterozygous Genes

Alleles are alternative forms of a gene. The combination of alleles determines the organism’s genotype and phenotype.

• Dominant Allele: Expressed in the phenotype even if only one copy is present (e.g., "A").

• Recessive Allele: Expressed only when two copies are present (e.g., "a").

• Homozygous: Two identical alleles (AA or aa).

• Heterozygous: Two different alleles (Aa).

Punnett Squares: Predicting Genotypes and Phenotypes

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

• Key Point: Each box represents a possible genotype for an offspring.

• Example: Crossing Aa x Aa yields genotypes AA, Aa, and aa in a 1:2:1 ratio; phenotypic ratio is 3:1 if A is dominant.

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: Heterozygotes show an intermediate phenotype (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.

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

• Mutant: Alternative phenotypes due to mutations (e.g., white eyes).

• Sex-Linked Traits: Morgan’s crosses showed that the white-eye mutation was X-linked.

Gene Linkage

Linked genes are located close together on the same chromosome and tend to be inherited together.

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

Chromosomal Mutations: Translocation and Syndromes

Chromosomal abnormalities can result from structural changes such as translocations, deletions, duplications, and inversions.

• Translocation: A segment of one chromosome attaches to a nonhomologous chromosome.

• Example: Chronic myelogenous leukemia (CML) is associated with a translocation between chromosomes 9 and 22.

• Syndromes: Down syndrome (trisomy 21), Turner syndrome (XO), and Klinefelter syndrome (XXY) are caused by chromosomal number abnormalities.

Inheritance Patterns and Pedigrees

Pedigrees are diagrams that trace inheritance patterns through families.

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

• Sex-Linked Inheritance: Traits inherited on sex chromosomes (X or Y).

• Mitochondrial Inheritance: Traits inherited through mitochondrial DNA, passed from mother to all offspring.

Ch. 16: Molecular Basis of Inheritance

DNA Replication in Prokaryotes vs. Eukaryotes

DNA replication is the process by which a cell copies its DNA before cell division. The basic mechanism is conserved, but there are differences between prokaryotes and eukaryotes.

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

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

Enzymes Involved in DNA Replication

• Helicase: Unwinds the DNA double helix.

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

• Primase: Synthesizes RNA primers.

• DNA Polymerase: Adds nucleotides to the growing DNA strand (different types in prokaryotes and eukaryotes).

• Ligase: Joins Okazaki fragments on the lagging strand.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

Semi-Conservative Replication

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

• Key Point: This model was confirmed by the Meselson-Stahl experiment.

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

The central dogma of molecular biology describes the flow of genetic information:

• DNA is transcribed into mRNA, which is then translated into a protein (polypeptide).

Transcription and Translation

• Transcription: Synthesis of mRNA from a DNA template. Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• Translation: Synthesis of a polypeptide using the mRNA template. Occurs on ribosomes in the cytoplasm.

• Key Molecules: mRNA, tRNA, rRNA, ribosomes, amino acids.

Codons, Anticodons, and Amino Acid Sequences

The genetic code is read in triplets called codons on mRNA, each specifying an amino acid. tRNA molecules have anticodons that pair with codons during translation.

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

Types of Mutations

• Point Mutation: Change in a single nucleotide (e.g., silent, missense, nonsense mutations).

• Frameshift Mutation: Insertion or deletion of nucleotides that alters the reading frame.

• Key Point: Mutations can affect protein structure and function, sometimes causing disease.

Table: Comparison of DNA Replication in Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Origin of Replication	Single	Multiple
DNA Shape	Circular	Linear
Location	Cytoplasm	Nucleus
Enzymes	Fewer types	More complex, multiple polymerases

Key Equations and Concepts

• Probability of Genotype (Punnett Square):

$P = \frac{\text{Number of boxes with genotype}}{\text{Total number of boxes}}$

• Central Dogma:

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

• Semi-Conservative Replication:

Each daughter DNA molecule contains one parental and one new strand.

Additional info: This guide expands on the provided outline with definitions, examples, and a comparison table for DNA replication. For more details, refer to figures and diagrams in your textbook chapters.