10/18/2015 1 Biology 102 Biology 102 Lecture 9: Genetic Inheritance Lecture 9: Genetic Inheritance • Asexual reproduction = daughter cells genetically Asexual reproduction = daughter cells genetically identical to parent (clones) identical to parent (clones) • Sexual reproduction = offspring are genetic Sexual reproduction = offspring are genetic hybrids hybrids • Tendency to inherit best traits of both Tendency to inherit best traits of both parents parents • Survival advantage against environmental Survival advantage against environmental change, competition, disease, etc. change, competition, disease, etc. Genetic Variability Genetic Variability • Siblings will often look similar, but not identical Siblings will often look similar, but not identical • Each inherits 50% from each parent, but not the Each inherits 50% from each parent, but not the same 50% same 50% • Crossing over Crossing over Genetic Variability Genetic Variability • Ultimate sources of variability Ultimate sources of variability • Mutations Mutations • Crossing over (recombination) Crossing over (recombination) • Independent assortment Independent assortment Genetic Variability Genetic Variability • Problem with inbreeding Problem with inbreeding • Limited number of genes Limited number of genes • Increased chances that deleterious mutations Increased chances that deleterious mutations will show up will show up Genetic Variability Genetic Variability • Remember how mutations affect genes Remember how mutations affect genes • Protein product altered in 1 of 4 ways… Protein product altered in 1 of 4 ways… 1) No effect 1) No effect • Silent mutation Silent mutation 2) Protein is altered, but it doesn’t matter 2) Protein is altered, but it doesn’t matter • Neutral change Neutral change – HAT HAT vs vs CAP CAP 3) Protein loses some or all of its function 3) Protein loses some or all of its function • Deleterious change Deleterious change - HAT HAT vs vs CAT CAT 4) Protein functions better 4) Protein functions better • Example: HIV resistance Example: HIV resistance Mutations Mutations
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•• Decrease in physical coordinationDecrease in physical coordination
•• Mental declineMental decline
•• Behavioral symptomsBehavioral symptoms
•• Symptoms usually do not appear until after age Symptoms usually do not appear until after age 35, after the gene may have been passed on to 35, after the gene may have been passed on to offspringoffspring
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Huntington’s DiseaseHuntington’s Disease
•• Scenario: A male is diagnosed with Huntington’s Scenario: A male is diagnosed with Huntington’s
Disease. His wife is tested for the disease gene Disease. His wife is tested for the disease gene
and has two healthy alleles. They have three and has two healthy alleles. They have three
children.children.
•• Disease is Disease is autosomalautosomal dominantdominant
•• How many disease alleles must be present to How many disease alleles must be present to
cause Huntington’s Disease? cause Huntington’s Disease?
•• Let’s assume he is heterozygous: Let’s assume he is heterozygous: HhHh
•• His wife is homozygous: His wife is homozygous: hhhh
•• What is the probability that any one of their What is the probability that any one of their
children will develop Huntington’s Disease?children will develop Huntington’s Disease?
Maternal allelesMaternal alleles
h h hh
Pate
rnal
allel
esPa
tern
al a
llel
es
H
H
hh
•• Possible offspring genotypes? Phenotypes?Possible offspring genotypes? Phenotypes?
Huntington’s DiseaseHuntington’s Disease
•• Based on this information, the affected Based on this information, the affected
individual’s children decided to be tested, and to individual’s children decided to be tested, and to
have their children testedhave their children tested
•• This information was compiled into a This information was compiled into a pedigreepedigree
•• Step 2: Assign all offspring of healthy Step 2: Assign all offspring of healthy
individuals one healthy alleleindividuals one healthy allele
•• Step 3: Assign all affected individuals one Step 3: Assign all affected individuals one
disease alleledisease allele
•• Step 4: Work from siblings or offspring to fill Step 4: Work from siblings or offspring to fill
in any missing information (if possible in any missing information (if possible –– some some
alleles may remain unknown)alleles may remain unknown)
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Huntington’s DiseaseHuntington’s Disease
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Hh
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hh
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PunnettPunnett SquaresSquares
•• Let’s look at a another human disease : Let’s look at a another human disease : TayTay--
Sach’sSach’s Disease (TSD)Disease (TSD)
•• AutosomalAutosomal recessiverecessive
•• Affects the enzyme Affects the enzyme hexosaminidasehexosaminidase A A
•• LysosomalLysosomal enzymeenzyme
•• Fatty substance builds up in brainFatty substance builds up in brain
•• Mental, physical deterioration; death by age 4Mental, physical deterioration; death by age 4
PunnettPunnett SquaresSquares
•• Scenario: 2 healthy individuals have a child with Scenario: 2 healthy individuals have a child with
TayTay--Sach’sSach’s
•• AutosomalAutosomal recessive disease so child must be recessive disease so child must be
homozygoushomozygous
•• One allele inherited from each parent, yet each One allele inherited from each parent, yet each
parent is healthyparent is healthy
•• Both parents must be Both parents must be heterozygousheterozygous
•• We call these individuals We call these individuals carrierscarriers
•• Have the disease gene, but do not have the Have the disease gene, but do not have the
diseasedisease
Maternal allelesMaternal alleles
T T ttPa
tern
al a
llel
esPa
tern
al a
llel
es
T
T
tt
•• Possible offspring genotypes? Phenotypes?Possible offspring genotypes? Phenotypes?
TayTay--Sach’sSach’s DiseaseDisease
DeafnessDeafness
•• Let’s do a pedigree for an Let’s do a pedigree for an autosomalautosomal recessive recessive condition: hereditary deafness (condition: hereditary deafness (dddd))
•• Trait may skip a generationTrait may skip a generation
•• Assign a genotype to each individualAssign a genotype to each individual
DeafnessDeafness
•• Step 1: Assign a genotype to anyone we know is Step 1: Assign a genotype to anyone we know is
homozygoushomozygous
•• Step 2: Give all unaffected individuals one DStep 2: Give all unaffected individuals one D
•• Step 3: Give all offspring of affected Step 3: Give all offspring of affected
individuals one dindividuals one d
•• Step 4: Work backwards Step 4: Work backwards –– look at affected look at affected
individuals; d must be present in both parentsindividuals; d must be present in both parents
•• Step 5: Double check, but some will remain a Step 5: Double check, but some will remain a
mysterymystery
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DeafnessDeafness
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InheritanceInheritance
•• In reality, inheritance is much more complicatedIn reality, inheritance is much more complicated
•• Many factors at play that can alter expected Many factors at play that can alter expected
inheritance patternsinheritance patterns
•• More than two alleles for one geneMore than two alleles for one gene
•• More than one gene affects a traitMore than one gene affects a trait
•• One gene modifies expression of another One gene modifies expression of another
gene (gene (epistasisepistasis))
•• We will look at 2 factors here:We will look at 2 factors here:
•• Incomplete dominanceIncomplete dominance
•• CodominanceCodominance
Incomplete DominanceIncomplete Dominance
•• Sometimes there is not one clear dominant alleleSometimes there is not one clear dominant allele
•• In a heterozygous individual, both alleles are In a heterozygous individual, both alleles are
expressedexpressed
•• Phenotype is a blend of both traitsPhenotype is a blend of both traits
Incomplete DominanceIncomplete Dominance
•• Example: snapdragon colorExample: snapdragon color
•• Both red (RR) and white (Both red (RR) and white (rrrr) are dominant) are dominant