mendelian genetics
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Mendelian GeneticsMendelian GeneticsMendelian GeneticsMendelian Genetics
Chapter 2Chapter 2
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Phenotype and GenotypePhenotype and Genotype
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Genotype and PhenotypeGenotype and PhenotypeGenotype and PhenotypeGenotype and Phenotype
• Genotype – genetic constitution of an organism
• Phenotype – observable characteristic• Genotype and environment
• Contribution of environment varies between genes
• Can be controlled by many genes
• Random developmental events
• Genotype – genetic constitution of an organism
• Phenotype – observable characteristic• Genotype and environment
• Contribution of environment varies between genes
• Can be controlled by many genes
• Random developmental events
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Mendel’s Experimental DesignMendel’s Experimental Design
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Mendelian GeneticsMendelian GeneticsMendelian GeneticsMendelian Genetics
• Modern genetics began with Gregor Mendel’s quantitative genetic experiments
• Austrian monk • Mathematician• Numerical and observational
data• Several generations
• Modern genetics began with Gregor Mendel’s quantitative genetic experiments
• Austrian monk • Mathematician• Numerical and observational
data• Several generations Stamen
Carpel
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Mendelian GeneticsMendelian GeneticsMendelian GeneticsMendelian Genetics
• Heritable, obvious traits• Simple crosses at first• Used peas because:
• Easy to grow and available• Many distinguishable
characteristics• Self-fertilization
• True breeding peas
• Heritable, obvious traits• Simple crosses at first• Used peas because:
• Easy to grow and available• Many distinguishable
characteristics• Self-fertilization
• True breeding peas
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Pea TraitsPea TraitsPea TraitsPea Traits
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Monohybrid Crosses and Mendel’s Principle of Segregation
Monohybrid Crosses and Mendel’s Principle of Segregation
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Breeding CrossesBreeding CrossesBreeding CrossesBreeding Crosses
• Initial cross is the P generation• Parents
• Progeny of parents is first filial generation• F1 generation
• Inbreeding of first generation creates second filial generation• F2 generation
• Initial cross is the P generation• Parents
• Progeny of parents is first filial generation• F1 generation
• Inbreeding of first generation creates second filial generation• F2 generation
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Monohybrid CrossesMonohybrid CrossesMonohybrid CrossesMonohybrid Crosses
• Cross between true-breeding individuals with one different trait
• Mendel’s first crosses • Resembled only one of
the parents• Planted progeny and
allowed self-fertilization• Revealed both phenotypes
• Cross between true-breeding individuals with one different trait
• Mendel’s first crosses • Resembled only one of
the parents• Planted progeny and
allowed self-fertilization• Revealed both phenotypes
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Monohybrid CrossMonohybrid CrossMonohybrid CrossMonohybrid Cross
• Mendel determined that• Particulate factors for
genes, each contains a set of two
• Transmitted by both parents• Alternate forms called
alleles• True breeding forms
contains identical set
• Mendel determined that• Particulate factors for
genes, each contains a set of two
• Transmitted by both parents• Alternate forms called
alleles• True breeding forms
contains identical set
GENETIC MAKEUP (ALLELES)
P PLANTS
F1 PLANTS(hybrids)
F2 PLANTS
PP pp
All P
All p
All Pp
1/2 P 1/2 p
EggsP
p
P
PPp
Sperm
Pp
Pp
pp
Gametes
Gametes
Phenotypic ratio3 purple : 1 whiteGenotypic ratio1 PP : 2 Pp : 1 pp
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Monohybrid CrossMonohybrid CrossMonohybrid CrossMonohybrid Cross
• F1 generation had both alleles
• Only one expresses• One allele masks
• Dominant
• Recessive
• Identical alleles – homozygous• Different alleles - heterozygous
• F1 generation had both alleles
• Only one expresses• One allele masks
• Dominant
• Recessive
• Identical alleles – homozygous• Different alleles - heterozygous
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Monohybrid CrossMonohybrid CrossMonohybrid CrossMonohybrid Cross
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Principle of SegregationPrinciple of SegregationPrinciple of SegregationPrinciple of Segregation
• Recessive characteristics are masked• Reappear in F2
• Members of a gene pair (alleles) segregated during gamete formation
• Recessive characteristics are masked• Reappear in F2
• Members of a gene pair (alleles) segregated during gamete formation
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How cells carry characteristicsHow cells carry characteristicsHow cells carry characteristicsHow cells carry characteristics
• Genes on chromosomes• At a specific loci
• Homologous pairs carry the same genes at the same locus• Different versions
• Separation of homologous chromosomes yields separation of alleles
• Genes on chromosomes• At a specific loci
• Homologous pairs carry the same genes at the same locus• Different versions
• Separation of homologous chromosomes yields separation of alleles
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Branch DiagramsBranch DiagramsBranch DiagramsBranch Diagrams
• Punnett squares can become messy with more than one gene
• Use branch diagram to figure out genotype and phenotype expected frequency
• Punnett squares can become messy with more than one gene
• Use branch diagram to figure out genotype and phenotype expected frequency
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Test CrossTest CrossTest CrossTest Cross
• Mendel did several crosses• Followed over several
generations• Selfing also very
important• Allowed plants to
reveal their genotype and not just their phenotye
• Mendel did several crosses• Followed over several
generations• Selfing also very
important• Allowed plants to
reveal their genotype and not just their phenotye
TESTCROSS:
B_GENOTYPES bb
BB Bbor
Two possibilities for the black dog:
GAMETES
OFFSPRING All black 1 black : 1 chocolate
B
b
B
b
b
Bb Bb bb
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Test Test CrossCrossTest Test CrossCross
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Recessive AllelesRecessive AllelesRecessive AllelesRecessive Alleles
• Wild-type allele – functional allele • Predominates in population• Dominant allele
• Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional• Recessive• Function of other in heterozygote is sufficient
• Wild-type allele – functional allele • Predominates in population• Dominant allele
• Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional• Recessive• Function of other in heterozygote is sufficient
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Wrinkled PeasWrinkled PeasWrinkled PeasWrinkled Peas
• SS type contains more starch and lower sucrose• Also more water
• SBEI - starch-branching enzyme
• Extra 800 bp piece in mutation
• SS type contains more starch and lower sucrose• Also more water
• SBEI - starch-branching enzyme
• Extra 800 bp piece in mutation
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Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent
Assortment
Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent
Assortment
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The Principle of Independent The Principle of Independent AssortmentAssortmentThe Principle of Independent The Principle of Independent AssortmentAssortment
• Factors for different traits assort independently of one another• Genes are inherited
independently of each other
• Segregate randomly in gametes
• Dihybrid Cross
• Factors for different traits assort independently of one another• Genes are inherited
independently of each other
• Segregate randomly in gametes
• Dihybrid Cross
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Branch Diagram of Dihybrid CrossBranch Diagram of Dihybrid CrossBranch Diagram of Dihybrid CrossBranch Diagram of Dihybrid CrossPhenotype
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Genotype vs. Genotype vs. PhenotypePhenotypeGenotype vs. Genotype vs. PhenotypePhenotype
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Test Cross With DihybridTest Cross With DihybridTest Cross With DihybridTest Cross With Dihybrid
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Trihybrid CrossTrihybrid CrossTrihybrid CrossTrihybrid Cross
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Tribble Traits ActivityTribble Traits ActivityTribble Traits ActivityTribble Traits Activity
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Statistical Analysis of Genetic Data: The Chi-Square Test
Statistical Analysis of Genetic Data: The Chi-Square Test
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Statistical AnalysisStatistical AnalysisStatistical AnalysisStatistical Analysis
• Data from genetics is quantitative• Use statistics to show deviation of
observed results from predicted results• Chance factors cause deviations
• Null-hypothesis – no difference between the predicted and observed• If not accepted then have to come up with a
new hypothesis for deviation
• Data from genetics is quantitative• Use statistics to show deviation of
observed results from predicted results• Chance factors cause deviations
• Null-hypothesis – no difference between the predicted and observed• If not accepted then have to come up with a
new hypothesis for deviation
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Chi-Square TestChi-Square TestChi-Square TestChi-Square Test
• Goodness of fit test• How much observed number deviates from
the expected number
• Goodness of fit test• How much observed number deviates from
the expected number
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Mendelian Genetics in HumansMendelian Genetics in Humans
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Pedigree AnalysisPedigree AnalysisPedigree AnalysisPedigree Analysis
• Inheritance patterns are studied using family trees• Pedigree analysis
• Phenotypic records• Proband is where gene
was discovered
• Inheritance patterns are studied using family trees• Pedigree analysis
• Phenotypic records• Proband is where gene
was discovered
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Examples of Human Genetic TraitsExamples of Human Genetic TraitsExamples of Human Genetic TraitsExamples of Human Genetic Traits
• Most genetic disorders are recessive
• Due to lack of function
• Homozygous recessive expression
• Dominant usually selected out
• Albinism
• Most genetic disorders are recessive
• Due to lack of function
• Homozygous recessive expression
• Dominant usually selected out
• Albinism
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Characteristics of Recessive Characteristics of Recessive Inheritance TraitsInheritance TraitsCharacteristics of Recessive Characteristics of Recessive Inheritance TraitsInheritance Traits
• Most have normal heterozygous parents
• Heterozygotes have 3:1 ratio• When both parents have the
trait then all progeny have the trait
• Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs
• Most have normal heterozygous parents
• Heterozygotes have 3:1 ratio• When both parents have the
trait then all progeny have the trait
• Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs
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Characteristics of Dominant Characteristics of Dominant Inheritance TraitsInheritance TraitsCharacteristics of Dominant Characteristics of Dominant Inheritance TraitsInheritance Traits
• Gain of function mutations• New property of the mutant
gene• No loss of function
• Must have one parent with disease
• Does not skip generations• Will transmit to half its
progeny• Huntingtons disease, Marfan
syndrome, achondroplasia
• Gain of function mutations• New property of the mutant
gene• No loss of function
• Must have one parent with disease
• Does not skip generations• Will transmit to half its
progeny• Huntingtons disease, Marfan
syndrome, achondroplasia