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Mendelian GeneticsMendelian GeneticsMendelian GeneticsMendelian Genetics
Chapter 2Chapter 2
Phenotype and GenotypePhenotype and Genotype
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
Mendel’s Experimental DesignMendel’s Experimental Design
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
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
Pea TraitsPea TraitsPea TraitsPea Traits
Monohybrid Crosses and Mendel’s Principle of Segregation
Monohybrid Crosses and Mendel’s Principle of Segregation
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
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
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
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
Monohybrid CrossMonohybrid CrossMonohybrid CrossMonohybrid Cross
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
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
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
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
Test Test CrossCrossTest Test CrossCross
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
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
Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent
Assortment
Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent
Assortment
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
Branch Diagram of Dihybrid CrossBranch Diagram of Dihybrid CrossBranch Diagram of Dihybrid CrossBranch Diagram of Dihybrid CrossPhenotype
Genotype vs. Genotype vs. PhenotypePhenotypeGenotype vs. Genotype vs. PhenotypePhenotype
Test Cross With DihybridTest Cross With DihybridTest Cross With DihybridTest Cross With Dihybrid
Trihybrid CrossTrihybrid CrossTrihybrid CrossTrihybrid Cross
Tribble Traits ActivityTribble Traits ActivityTribble Traits ActivityTribble Traits Activity
Statistical Analysis of Genetic Data: The Chi-Square Test
Statistical Analysis of Genetic Data: The Chi-Square Test
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
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
Mendelian Genetics in HumansMendelian Genetics in Humans
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
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
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
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
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