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Lecture 15
Gamete formation with linked loci
Gametes depend on crossing over
If no crossing over occurs between loci in a meiosis
100% parental, 0% recombinant
If crossing over occurs between loci in a meiosis
50% parental, 50% recombinant
Frequency of meioses with crossing over determines recombinant percentage
What determines frequency of meioses with crossing over between loci?
Physical distance between loci
Large distance, high frequency of crossing over
Small distance, low frequency of crossing over
Lecture 16
Genetic mapping
Use percentage of recombinant gametes to map gene loci
Test crosses
Double heterozygote crossed with double homozygous recessive
Progeny phenotypes reflect gamete genotypes from double heterozygote
Double homozygous recessive is unimportant
Gene nomenclature (fruit flies)
Gene named after abnormal phenotype
Abnormal phenotype dominant: gene name, symbol begin with upper case
Abnormal phenotype recessive: gene name, symbol begin with lower case
Normal (wild type) allele is given a superscript plus
Abnormal allele is not
Lecture 17
Example test cross
Percentage recombinants is the map distance
Coupling and repulsion
Errors result in map distances above 50%
More than one crossover between loci
Two, three, and four strand double crossovers
Average out at 50% recombinants
Hence one or more crossovers gives 50% recombinants
Mapping function
As separation increases, measured map distance approaches 50%
Short distances add up, long distances do not
Use of χ2 test to distinguish unlinked and linked loci
Lecture 18
Mutation
Gene and chromosome
Types of gene mutation
Substitutions
Transitions and transversions
Indels
Causes of gene mutation
Spontaneous
Tautomers, deamination, depurination, miss-paired repeats
Induced
Base analogues, base modifiers, intercalating agents
Ultraviolet light, thymine, high energy radiation
Lecture 19
Effects on gene function
Coding regions
Silent, missense, nonsense, frameshift
Non-coding regions
Reversions and suppressors
Exact and equivalent reversions
Intragenic and extragenic suppressors
Frame shift and nonsense
Organismal effects
Morphological, behavioral, lethal, nutritional, conditional
Penetrance and expressivity
Complementation test
Lecture 20
Chromosome identification
Deletions
Recessive lethal, hemizygosity, suppressed recombination
Duplications
Inversions
Paracentric and pericentric
Altered linkage map (homozygous), suppressed recombination (heterozygous)
Reciprocal translocations
Cross shapes (heterozygous), altered linkage maps, reduced fertility (heterozygous)
Abnormal euploidy
Even n fertile, odd n sterile
Aneuploidy
Lecture 21
Skipped
Lecture 22
Definitions
Population, gene pool
Describing the gene pool
Genotype frequency, allele frequency
Does sexual reproduction alter the gene pool?
Hardy-Weinberg equilibrium
Assumptions: two alleles, random mating, infinite population
Conclusions: gene pool unchanged, genotypes in fixed ratio
Applications of Hardy-Weinberg equilibrium
Determine allele frequencies in dominant-recessive situation
Determine whether population is in Hardy-Weinberg equilibrium
Lecture 23
Mechanisms that change the gene pool
Mutation
Creates and eliminates alleles
Equilibrium reached slowly
Selection
Genotype affects probability of having offspring
Fertility, survival, attractiveness
Relative fitness
Modified Hardy-Weinberg equation
Frequency-independant selection
Heterozygote has intermediate fitness, highest fitness, lowest fitness
Frequency-dependant selection
Lecture 24
Mechanisms that change the gene pool (continued)
Genetic drift
Random events may affect finite populations
Most relevant in small populations
Bottleneck and founder effects
Gene flow
Mating between populations
Depends on amount of mating and difference in gene pools
Sources of Variation
Quantifying variation
Percentage of polymorphic loci
Average heterozygosity