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 units of inheritance, now known as genes.

• Key Point 2: He observed that traits are inherited in predictable patterns, leading to the formulation of the Law of Segregation and Law of Independent Assortment.

• Example: Mendel crossed pure-breeding tall and short pea plants and observed all tall offspring in the F1 generation, but a 3:1 ratio of tall to short in the F2 generation.

Generations: P, F1, F2, and Beyond

Genetic crosses are tracked across generations to observe inheritance patterns.

• 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 individuals crossed with each other.

• Additional info: Further generations (F3, F4, etc.) can be analyzed for extended inheritance studies.

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., T for tall).

• Recessive Allele: Expressed only when two copies are present (e.g., t for short).

• Homozygous: Both alleles are the same (TT or tt).

• Heterozygous: Two different alleles (Tt).

Punnett Squares: Predicting Genotypes and Phenotypes

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

• Key Point 1: Each parent’s possible gametes are listed along the top and side of the square.

• Key Point 2: The resulting boxes show all possible combinations in the offspring.

• Example: Crossing Tt x Tt yields a 1:2:1 genotypic ratio (TT:Tt:tt) and a 3:1 phenotypic ratio (tall:short).

Application: Predicting Outcomes of Crosses

Understanding Mendelian inheritance allows prediction of trait distribution in offspring.

• Key Point: Used in agriculture, medicine, and breeding to anticipate trait inheritance.

Inheritance Patterns: Sex Linkage, Codominance, Incomplete Dominance

Not all traits follow simple dominant-recessive patterns.

• Sex Linkage: Genes located on sex chromosomes (e.g., X-linked color blindness).

• Codominance: Both alleles are fully expressed (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.

• Key Point 1: Discovered sex-linked inheritance patterns (e.g., white-eyed mutation on X chromosome).

• Key Point 2: Identified wild type (normal) and mutant alleles.

• Example: Crossing red-eyed females with white-eyed males produced different results depending on the sex of the offspring.

Gene Linkage

Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as linkage.

• Key Point: Linked genes do not assort independently unless crossing over occurs during meiosis.

Application: Predicting Outcomes of Crosses

Understanding linkage and recombination frequencies helps map gene locations on chromosomes.

• Key Point: Recombination frequency is used to estimate genetic distance between genes.

Chromosomal Mutations: Translocation and Syndromes

Structural changes in chromosomes can lead to genetic disorders.

• Translocation: A segment of one chromosome attaches to another, non-homologous chromosome.

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

• Other Issues: Deletions, duplications, inversions can also cause syndromes (e.g., Down syndrome from trisomy 21).

Inheritance Patterns and Pedigrees

Pedigrees are diagrams that track inheritance patterns across generations.

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

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

• Autosomal Inheritance: Traits associated with genes on non-sex chromosomes.

Inheritance Pattern	Transmission	Example
Autosomal Dominant	Both sexes, every generation	Huntington's disease
Autosomal Recessive	Both sexes, may skip generations	Cystic fibrosis
X-linked Recessive	More males affected	Hemophilia
Mitochondrial	All children of affected mother	Leber's hereditary optic neuropathy

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. There are key differences between prokaryotic and eukaryotic replication.

• 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

Several enzymes coordinate the accurate and efficient replication of DNA.

• 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.

• 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 old (parental) strand and one newly synthesized strand.

• Key Point: Supported by the Meselson-Stahl experiment.

$\text{Parental DNA} \rightarrow \text{First Generation: 1 old + 1 new strand}$

Leading and Lagging Strand Synthesis

DNA polymerase can only synthesize DNA in the 5' to 3' direction, resulting in continuous and discontinuous synthesis.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments.

Ch. 17: Protein Synthesis

Central Dogma: DNA to mRNA to Protein

The central dogma of molecular biology describes the flow of genetic information within a cell.

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

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

Transcription and Translation: Processes and Components

Transcription and translation are the two main steps in protein synthesis.

• Transcription: Occurs in the nucleus (eukaryotes); RNA polymerase synthesizes mRNA from the DNA template.

• Translation: Occurs in the cytoplasm at ribosomes; mRNA is decoded to build a polypeptide chain.

• Components: mRNA, tRNA, rRNA, ribosomes, amino acids.

Reading Codons, Anticodons, and Amino Acid Sequences

The genetic code is read in sets of three nucleotides (codons), each specifying an amino acid.

• Codon: Three-base sequence on mRNA.

• Anticodon: Complementary three-base sequence on tRNA.

• Key Point: The sequence of codons determines the amino acid sequence of the protein.

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

Types of Mutations

Mutations are changes in the DNA sequence that can affect protein synthesis.

• Point Mutation: Change in a single nucleotide (e.g., sickle cell anemia).

• Insertion/Deletion: Addition or loss of nucleotides, potentially causing frameshifts.

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Introduces a premature stop codon.