Identifying Genes and Defining Alleles
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Identifying Genes and Defining Alleles
Mutant Hunt - independently isolate number of mutants
with identical phenotypes - verify mutant phenotype is recessive - establish pure-breeding strain for each
How many genes are involved? The same gene for all strains? Different genes for different strains?
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Identifying Genes and Defining Alleles
Mutant Hunt Ex. White flowers in plant species with purple flowers Mutant strain 1 - isolated in Australia Mutant strain 2 - isolated in Pennsylvania
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Identifying Genes and Defining Alleles
Biochemical basis for white flower color
If only one gene involved: (A or a alleles) Enzyme AWhite pigment Purple pigment
If two different genes involved: (Aa and Bb) Enzyme A Enzyme BWhite White Purple pigment
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Identifying Genes and Defining Alleles
Complementation Test - One gene or Two genes?
Cross recessive pure-breeding strains with same(or related) phenotype to each other.
If F1 progeny are all mutant = one gene (two alleles)
If F1 progeny are wild type = two different genes
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Identifying Genes and Defining Alleles
Complementation Test - One gene or Two genes?
Alleles of the same gene
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Identifying Genes and Defining Alleles
Complementation Test - One gene or Two genes?
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Complementation Analysis
Independently isolated mutants - all same phenotype Cross in all possible combinations
+ wild-type offspring (complementation) - mutant offspring
How many genes?
Which mutantsare defective in same gene?
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Multiple Alleles
Many different forms of the same gene
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Multiple Alleles
Example
Cross A x BAnything possible
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Multiple Alleles
Example
w gene
wild-type,white,eosinalleles
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Multiple Alleles
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Multiple Alleles
Humans are highly polymorphic Ex. >200 different alleles for cystic fibrosis gene Ex. >390 alleles for human leukocyte antigen (HLA)
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Dominance of Alleles
Complete Dominance / Complete Recessiveness
Phenotype: Dominant RecessiveGenotype: AA, Aa aa
Haplo-Sufficient
Loss ofFunction
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Dominance of Alleles
Incomplete Dominance (Semidominance)
Haplo-insufficient
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Dominance of Alleles
Co-dominance
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Dominance of Alleles
Sickle cell anemia
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Lethal Genes
Dominant lethal: L- (LL or Ll) doesn’t survive, rare
Ex. Huntington chorea - neurodegenerative, late onset
Recessive lethal: ll homozygotes die
Ex. Achondroplastic dwarfism
a+a+ normal a+ ad dwarf ad ad die in utero
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Examples of Recessive Lethal Genes
Creeper Chickens: Autosomal lethal
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Examples of Recessive Lethal Genes
2:1 ratio
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Subvital Genes
Survival of genotype is not as good as normal
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Gene Interactions & Modified Ratios
Variations of Mendelian Dihybrid Ratios: Two genes involved
A- B- aaB- A-bb aabb
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Gene Interactions & Modified Ratios
Comb shapes
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Gene Interactions & Modified Ratios
Bateson & Punnett crossed purebreeding chickens
How many genes are involved?
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Gene Interactions & Modified Ratios
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Gene Interactions & Modified Ratios
9:3:3:1
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Gene Interactions & Modified Ratios
Flower Color in Sweet Peas - Complementation
9:7 ratio
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Gene Interactions & Modified Ratios
Fruit shape in summer squash
9:6:1 ratio
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Epistasis
One gene masks the expression of another gene
aa B- A- B-
Recessive Dominant
Gene masking other = epistatic
Gene being masked = hypostatic
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Recessive Epistasis
Ex. Coat color in mice
C- color, cc none A- pattern, aa none
9:3:4ratio
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Recessive Epistasis
Ex. Coat color in Labrador retrievers
EeBb x EeBb 9/16 black: 3/16 brown:4/16 yellow
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Dominant Epistasis
Ex. Fruit color in summer squash
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Dominant Epistasis
Ex. Fruit color in summer squash
Hypothetical pathway
ww
ww
Y-
yy
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Dominant Epistasis
Ex. Graying in horses
4 years
7 years
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Gene Interactions: Eye Color in Drosophila
bw+ bw st+ st w+ w
bw+ bw st+ st w+ w
bw+ - st+ - w+ -
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Gene Interactions: Eye Color in Drosophila
bw+ bw st+ st w+ w X bw+ bw st+ st w+ w
bw+- st+- w+-
bw+- st+- ww
bw+- stst w+-
bwbw st+- w+-
bw+- stst ww
bwbw st+- ww
bwbw stst w+-
bwbw stst ww
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Suppression
Second gene blocks mutant phenotype caused by first gene
Normal plant - no malvidin; K- malvidin, kk none; D- suppresses K-, dd no suppression
13:3 ratio
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Modifier Gene
Second gene affects degree of expression of first gene
Ex. dark color versus light color
B- black, bb brown D- intense color, dd dilute color
9:3:3:1 ratio
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Duplicate Genes
Both genes control the same cellular activity
Ex. A1- or A2 - round fruit a1a1 and a2a2 narrow fruit
Enz A1narrow round
Enz A2
A1a1 A2a2 x A1a1 A2a2
9/16 A1- A2-: 3/16 A1- a2a2: 3/16 a1a1 A2-: 1/16 a1a1 a2a2
15 : 1 ratio of round : narrow
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Pleiotropic Genes
One gene has many effects on the phenotype
Ex. Cystic fibrosis - recessive allele, autosomal gene defective calcium transportbreathing difficultiesdigestive problemsreproductive deficienciesreduced immunity
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Penetrance
Percentage of individuals with certain genotype who express the expected phenotype.
brachydactyly
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Expressivity
Degree or extent to which a given genotype is expressed.
Variations may result from:environmentgenetic backgroundother factors
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Variable Expressivity
Spotting in dogs
All have the same genotype
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Variable Expressivity
Neurofibromatosis
café au lait spots
freckling
neurofibromas
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Penetrance and Expressivity
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Monogenic vs Quantitative Traits
Discontinuous traits Continuous traits
AA Aa aa aabbccdd AABBCCDD As gene number increases, phenotype distribution approaches normal curve
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Quantitative Genetics
Polygenic - Many genes affect one aspect of phenotype
Quantitative traits - each allele of each gene contributes equally
Ex. height, weight, skin color
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Quantitative Genetics
Two genes contributing to phenotype quantitatively
F2 ratio
1:4:6:4:1
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Quantitative Genetics
Inheritance of ear length in corn
F1 mean =intermediate
More variabilityin F2