Genetics and Gene Expression: Study Notes for General Biology

Genetics: Mendelian Inheritance and Alleles

Recessive Alleles and Probability in Offspring

Inheritance of traits often follows Mendelian patterns, where alleles can be dominant or recessive. The probability of offspring expressing a particular trait depends on the genotype of the parents.

• Recessive Allele: An allele that is only expressed when two copies are present (homozygous recessive).

• Example: If two solid-colored dogs (both must be heterozygous if they produce a spotted offspring) mate, the chance their next puppy is solid-colored is 75%.

Punnett Square for Two Heterozygotes (Aa x Aa):

• Genotypes: AA, Aa, Aa, aa

• Phenotypes: 3 solid-colored (AA or Aa), 1 spotted (aa)

Probability: $\frac{3}{4} = 75\%$ chance of solid-colored offspring.

Single-Gene Disorders: Cystic Fibrosis Example

Many genetic disorders, such as cystic fibrosis, are caused by recessive alleles. Carriers are heterozygous and can pass the allele to offspring.

• Carrier: An individual with one normal and one mutant allele (heterozygous).

• Gamete Probability: A heterozygous individual (Aa) has a 50% chance of passing the recessive allele to a gamete.

Probability: $\frac{1}{2} = 50\%$

Alleles and Blood Group Determination

Genes can have multiple alleles in a population, but each individual only carries two alleles for each gene (one from each parent).

• Blood Group Gene: Three alleles exist (IA, IB, i), but each person has only two alleles for the gene.

• Answer: 2 alleles per person.

Sex-Linked Traits and Inheritance Patterns

Sex-linked traits are often carried on the X chromosome. Females have two X chromosomes, while males have one X and one Y.

• Heterozygous Mother (XAXa) and Dominant Father (XAY):

• Half of sons will inherit the recessive allele and express the trait (since males only have one X).

• All daughters will inherit one dominant and one recessive allele, making them heterozygous.

Summary Table:

Child	Genotype	Phenotype
Son	XAY	No trait
Son	XaY	Recessive trait
Daughter	XAXA	No trait
Daughter	XAXa	No trait (heterozygous)

Blood Type Inheritance

Blood type is determined by the combination of alleles inherited from parents. Type O is recessive (ii), while A and B are codominant (IA, IB).

• Mother: O (ii)

• Son: B (IBi)

• Possible Father Phenotypes: B (IBIB or IBi), AB (IAIB)

• Type O and A are not possible for the father in this scenario.

Gene Expression and Protein Synthesis

Codons and Anticodons

During translation, mRNA codons are recognized by tRNA anticodons, which bring the correct amino acid to the ribosome.

• Codon: Sequence of three nucleotides in mRNA that specifies an amino acid.

• Anticodon: Sequence of three nucleotides in tRNA complementary to the mRNA codon.

• Example: mRNA codon 5' AGU 3' pairs with tRNA anticodon 3' UCA 5'.

tRNA and Amino Acid Pairing

tRNA molecules carry specific amino acids and have anticodons that base pair with mRNA codons.

• Anticodon 3' ACC 5' pairs with codon 5' GGU 3', carrying Glycine.

Gene Expression Regulation

Gene expression can be regulated at multiple levels, including transcriptional and translational control.

• Transcriptional Control: Regulation of gene expression at the level of mRNA synthesis.

• Translational Control: Regulation at the level of protein synthesis from mRNA.

• Translational control allows the most rapid response to environmental change.

Operons and Regulation in Prokaryotes

Operon Structure and Function

Operons are clusters of genes under the control of a single promoter and operator, allowing coordinated regulation in prokaryotes.

• Repressible Operon: Usually on, can be turned off by a repressor (e.g., trp operon).

• Inducible Operon: Usually off, can be turned on by an inducer (e.g., lac operon).

• Example: Suc operon in E. coli is repressed when sucrose is low; it is an inducible operon.

lac Operon Regulation

The lac operon is regulated by both negative (repressor) and positive (activator) mechanisms. Glucose abundance affects transcription.

• Negative Regulation: Lac repressor binds operator in absence of lactose, preventing transcription.

• Positive Regulation: CAP protein activates transcription when glucose is low.

• When glucose is abundant: CAP is inactive, transcription decreases even if lactose is present.

Mutations and Their Effects

Types of Mutations

Mutations are changes in DNA sequence that can affect protein structure and function.

• Missense Mutation: Changes one amino acid in the protein to a different one.

• Nonsense Mutation: Creates a premature stop codon, truncating the protein.

• Silent Mutation: No change in amino acid sequence.

• Frameshift Mutation: Shifts the reading frame, altering all downstream codons.

Example: A missense mutation most likely changes one amino acid to a different one.