Topic 4

Mutations

Where does variation come from - 2 factors

• Environmental and Genetic Variation 

Environment Variation

Individuals can possess different phenotypes as a result of exposure to different environments, despite identical genotypes

Genetic Variation

Individuals can possess different phenotypes as a result of genetic differences transmitted from parent to offspring, Ex. fruit fly wings are 100% genetics so homozygous bb = winkles, everything else is normal wings

Genotype due to environmental interaction

• Individuals can possess different phenotypes as a result of the interaction of their genotypes within the environment 

• Ex. sex of a leopard gecko is influenced by temperature during embryo development 

  • Extremes low or high = female

  • Average of around 30 =  males 

Phenotypic plasticity

genetically identical individuals can have different phenotypes in different environmental conditions

Reaction norm

• pattern/range of phenotypes that the same genotype can possess as a result of different environments 

• Mutation

• Ultimate source of genetic variability 

• A change in the base sequence of DNA 

• Raw material of evolution 

• 1900s by TH Morgan - fruit flies

What is mutation + its characteristics

• Any heritable change in the genetic material - DNA 

• Single gene mutations or chromosomal mutations (many genes) 

• Initially any new mutation at a diploid nuclear locus will have low frequency in a population - 1/2N

• On its own, mutations have limited impacts on population allele frequency 

• Needs to be combined with other factors like selection

DNA structure - purine vs pyrimidines

• Purine: 2 rings 

• Pyrimidines: 1 ring 

• AT - 2 HB 

• GC - 3HB

Mutation due to DNA alteration

• Deamination (one change in seq) → replication (change in seq paired with different letter) → completed substitution (new AA - Thymine) 

Mutation due to DNA copying error

• Misalignment with 2 DNA strands (usually happens when there are repeats) 

• Causes single base-pair insertion

Point mutation

• Single base pair changes 

• Results in new alleles 

• Transitions and transversions

Transition vs transversion

•  Transition - purine to purine or pyr to pyrimidine - more common 

• Transversion is purine to pyrimidine/vice versa

What are the 3 potential impacts of point mutations

• Synonymous mutation - no change in AA - due to redundancy in genetic code - wobble 3rd 

• Non-synonymous mutation - change in AA 

• Non sense mutation-premature STOP - change codes for a stop codon/non functional protein

• Insertions or deletions

• insertion/deletion of 1,2,or any non-multiple of 3 nucleotides will result in a shift of the codon reading frame 

• What are the 4 phenotypic changes of mutation 

• Neutral - no impact 

• Deleterious - reduces fitness of individual 

• Beneficial - increase fitness of individual 

• Letal - death prior to reproduction 

Sickle Cell Anemia

• Homozygous mutant allele - sickle cell anemia (Glu to Val) 

• Homozygous normal allele - susceptible to malaria 

• Heterozygous - resistance to malaria (heterozygous superiority maintains mutant allele

Where do new genes come from

• Gene Duplication (2 mechanisms) 

• Derived from scratch 

Gene Duplication via 2 mechanisms

• Duplication by unequal crossing over 

  • Homologous chromosomes align incorrectly, inner chromatids cross over at non-allelic repeat loci 

  • Genes formed have same introns as original genes and occur in tandem/repeat with original genes on same chromosome

• Duplication by retroposition 

  • Transcription → introns spliced out → reverse transcription → integration 

  • New gene reintegrate into chromosome and represents a new copy of existing gene 

  • Pseudogene non functional - no regulatory sequences or may insert by chance near regulatory sequence and become functional 

  • Lack introns and found far from original gene

Derived from scratch

Derived from non-coding regions of DNA