Mendelian Genetics Chapter 2 Phenotype and Genotype

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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

• 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

• 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

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

All Pp

1/2 P 1/2 p

Gametes

Phenotypic ratio3 purple : 1 whiteGenotypic ratio1 PP : 2 Pp : 1 pp

• 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

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

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 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