Unit 4 Review: Mendelian Genetics, Gene Expression, and Mutations

Mendelian Genetics and Inheritance

Definition of a Gene

A gene is a segment of DNA that includes coding regions, regulatory sequences, and sometimes non-coding regions, which together determine the gene's function. The most inclusive definition considers transcription, translation, and regulatory elements.

• Key Point: Genes include promoters and regulatory regions, are transcribed, may be translated, and have a biological function.

• Example: A gene for hemoglobin includes coding exons, introns, and regulatory sequences that control its expression.

Alleles and Dominance Hierarchies

Alleles are alternative forms of a gene. Dominance relationships determine which allele's trait is expressed in heterozygotes.

• Complete Dominance: One allele completely masks the effect of another.

• Dominance Hierarchy Example: For feather color: A1 (Green) > A2 (Red) > A3 (Blue) > A4 (White).

• Genotype-Phenotype Relationship: A bird with red feathers must have A2 as the most dominant allele present (e.g., A2A3, A2A4, A2A2).

Pedigree Analysis and Probability

Pedigrees are diagrams that show inheritance patterns across generations. Probability calculations help predict offspring phenotypes.

• Dominant Trait Example: Orange color in hamsters is dominant. If two hamsters (one AA, one aa) mate, all offspring will be Aa (orange).

• Recessive Trait Example: Orange color in ferrets is recessive. Only aa individuals are orange; heterozygotes (Aa) are not.

• Probability Calculation: Use Punnett squares to determine the likelihood of a particular genotype or phenotype in offspring.

Dihybrid Crosses and Independent Assortment

Tomato Color and Texture

Two genes control tomato traits: one for color (R/r) and one for texture (S/s). Dihybrid crosses illustrate independent assortment.

• Gene R: RR = Red, rr = Green, Rr = Brown

• Gene S: SS/Ss = Smooth, ss = Rough

• Dihybrid Cross: Crossing a brown rough tomato (Rrss) with a dihybrid (RrSs) yields a 9:3:3:1 ratio in offspring.

• Calculation: To find Red and Smooth, multiply probabilities for each trait.

Fruit Fly Wing Color and Number

Two autosomal genes control wing traits in fruit flies. Complete dominance applies.

• Gene 1: Two wings or four wings

• Gene 2: Yellow or translucent wings

• Dihybrid Offspring: All offspring with two translucent wings suggest parental genotypes are both heterozygous for both genes.

Broccoli Trait Inheritance

Three genes (A, B, C) control taste, color, and size in broccoli. Dominant alleles produce desirable traits.

• Cross: AaBbCc x aabbcc

• Calculation: Probability of Delicious, Green, and Big = 1/8 per trait, so expected number = (1/8) * 160 = 20.

Gene Expression and Central Dogma

Transcription and Translation

Gene expression involves transcription (DNA to RNA) and translation (RNA to protein).

• Transcription: RNA polymerase synthesizes mRNA from DNA template.

• Translation: Ribosomes read mRNA codons to assemble amino acids into polypeptides.

• Example: Coding strand 5'-GCGTAAATGAAAGCAGGTATTAACCC-3' is transcribed and translated into a polypeptide sequence.

Order of Events in Eukaryotic Gene Expression

Several processing steps occur before translation in eukaryotes.

• Order: Transcription begins → RNA gets 5' cap → RNA receives Poly-A tail → Introns are spliced → Transcript exits nucleus → Translation begins

• Additional info: These modifications stabilize mRNA and facilitate translation.

Translation: mRNA to Polypeptide

mRNA codons are translated into amino acids using the genetic code.

• Start Codon: AUG (Methionine)

• Stop Codons: UAA, UAG, UGA

• Example: mRNA sequence AUG-AGU-UUG-AAA-UCG-UGG-UUU-GUA translates to Met-Ser-Leu-Lys-Ser-Trp-Phe-Val

Mutations and Their Effects

Types of Mutations

Mutations are changes in DNA sequence that can affect gene function.

• Missense Mutation: A single nucleotide change results in a different amino acid.

• Example: Before: ATG GAA; After: ATG GAC (Glu → Asp)

• Other Types: Nonsense (introduces stop codon), Silent (no amino acid change), Frameshift (insertion/deletion alters reading frame)

Genotype Determination from Crosses

Test Crosses and Genotype Analysis

Test crosses with homozygous recessive individuals reveal the genotype of unknown parents.

• Example: Black long-haired hamsters crossed with white short-haired (double recessive) yield offspring ratios that indicate parental genotypes.

• Complete Dominance: Dominant alleles mask recessive alleles in heterozygotes.

Tables

Summary Table: Types of Mutations

Mutation Type	DNA Change	Effect on Protein
Missense	Single base substitution	One amino acid changed
Nonsense	Single base substitution	Premature stop codon
Silent	Single base substitution	No change in amino acid
Frameshift	Insertion/deletion	Multiple amino acids changed

Summary Table: Gene Expression Events (Eukaryotes)

Step	Description
Transcription begins	RNA polymerase synthesizes pre-mRNA from DNA
5' Cap added	Protects mRNA and aids in ribosome binding
Poly-A tail added	Stabilizes mRNA and facilitates export
Introns spliced	Removes non-coding regions
Transcript exits nucleus	mRNA moves to cytoplasm
Translation begins	Ribosome assembles polypeptide from mRNA

Key Equations

• Probability of genotype in dihybrid cross: $P = \text{probability of trait 1} \times \text{probability of trait 2}$

• Expected number of offspring: $\text{Expected} = \text{Total offspring} \times P$

Additional info:

• Gene expression in prokaryotes does not involve mRNA splicing or a 5' cap/Poly-A tail.

• Pedigree analysis is a key tool in human genetics for tracking inheritance of traits and diseases.

• Test crosses are used to determine unknown genotypes by analyzing offspring ratios.