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Chapter 12: InheritanceChapter 12Patterns of InheritanceWhat is Inheritance?Answer: The passage of genes from parent to offspringModern Genetic Concepts:Locus: Specific location of a gene on achromosomeLocusLocusAllele: Alternate form of a nucleotidesequence for a single geneHeterozygous(“different pair”) Homologous chromosomes havingdifferent alleles for a given geneHomozygous(“same pair”) Homologous chromosomes havingthe same alleles for a given geneChapter 12: InheritanceWho is the Father of Modern Genetics?Chapter 12: InheritancePea Plant Ideal for Genetic Work:1) Normally undergo self-fertilization Same flower provides pollen and eggs True-breeding (homozygous)Answer:Johann Gregor Mendel2) Can undergo cross-fertilization(manually) Different flower provides pollen and eggs Controlled experimentationFigure 12.3 – Audesirk2 & ByersChapter 12: InheritanceInheritance of Single Traits: Mendel raised true-breedersfor different forms of a traitand then cross-fertilized theforms:Chapter 12: InheritanceInheritance of Single Traits – Mendel’s Findings:1) Traits are determined by pairs of discrete physical units Physical units genes One allele of each gene / homologous chromosome2) Differing alleles can mask one another if occupying same cell Dominant Allele: Allele expressed (observed) Recessive Allele: Allele mask (not observed) Mendel then allowed F1 generationto self-fertilize:3) Alleles of a gene segregate from one another during meiosis Law of Segregation Which allele enters which gamete occurs by chance4) True-breeding individuals have two of the same alleles Hybrids have two different alleles for the geneFigure 12.4 / 12.5 – Audesirk2 & Byers1
Figure 12.8 / 12. 9 – Audesirk2 & ByersChapter 12: InheritanceFigure 12.10 – Audesirk2 & ByersChapter 12: InheritanceArrgghh!Inheritance of Single Traits:Inheritance of Single Traits: Phenotype:Outward expression of a trait(e.g., appearance, behavior)Genotype:Actual combination of allelescarried by an individualChapter 12: InheritancePunnett Square Method:Figure 12.11 – Audesirk2 & ByersChapter 12: InheritanceMendel’s hypothesis can be used to predict cross outcomes:(“Genetic Bookkeeping”)Test Cross:Crossing a dominant phenotype individual with a homozygous recessiveindividual to determine the genotype of the dominant individualPPpPpPppPpPpPhenotype:3 purple : 1 whiteGenotype:1 PP : 2 Pp : 1 ppPppPppppPpppPP or PpKeep in Mind:Predicted proportionswill fluctuate in real life100% PurpleppChapter 12: InheritanceInheritance of Multiple Traits: Mendel raised true-breedersfor different forms of two traitsand then cross-fertilized theforms:Figure 12.13 – Audesirk2 & Byers50% Purple / 50% WhiteChapter 12: InheritanceInheritance of Multiple Traits:YYyYYYy3/4 yellow1/4 greenYYSS x yyssyYyyySsSSSsResulting ratio of self-fertilization:9331Yellow, smooth seedsYellow, wrinkled seedsGreen, smooth seedsGreen, wrinkled seeds9:3:3:1Yy Seed ColorYySs Mendel then allowed F1to self-fertilize:SSs Seed Texture3/4 smooth1/4 wrinkledYySs x YySssSsss9/16 Yellow, smooth seeds3/16 Yellow, wrinkled seeds3/16 Green, smooth seeds1/16 Green, wrinkled seeds9:3:3:12
Figure 12.15 – Audesirk2 & ByersChapter 12: InheritanceInheritance of Multiple Traits:Chapter 12: InheritancePunnett Square (Multiple Traits):YySs x YySsLaw of Independent Assortment:Sperm The alleles for one trait may be distributed to the gametes independentlyof the alleles for other traitsYS Occurs via random assortmentof chromosomes duringmeiosis IYsySysYS YYSS Traits located on separatechromosomesEggYsySysFigure 12.14 – Audesirk2 & ByersPunnett Square (Multiple Traits):Figure 12.17-12.20 – Audesirk2 & ByersChapter 12: InheritanceGenes on the Same Chromosome Tend to be Inherited Together:Linked Genes:Genes on same chromosome9331Yellow, smooth seedsYellow, wrinkled seedsGreen, smooth seedsGreen, wrinkled seeds9:3:3:1Linked genes may be separated via crossing over:Crossing Over:Exchange of homologous chromosome segments(occurs during metaphase of meiosis I)Chapter 12: InheritanceSex Chromosomes: Determine sex of individual: Humans X and Y chromosomesChapter 12: InheritanceAutosomes:All other chromosome pairs otherthan sex chromosomesGeneticRecombinationFigure 12.23 – Audesirk2 & ByersExample of Sex-linked Trait: Female XX : Male XY Birds / Reptiles W and Z chromosomes Female WZ : Male ZZ Only one pair per individual in a speciesSex-linked Genes: Genes present on one sex chromosome but not the other Human Y chromosome 20 unique genes Human X chromosome 1500 unique genes Examples: Color vision genes / Blood clotting factors Females Normal dominant / recessive relationships exist foralleles on X chromosomes Males Fully express alleles on X chromosome3
Chapter 12: InheritanceTable 12.1 – Audesirk2 & ByersChapter 12: InheritanceFlower Color in SnapdragonsVariations on Mendelian Theme:Variations on Mendelian Theme:Assumption:One allele is completely dominantover another alleleAssumption:There are only two possible allelesfor each geneFact:Alleles can displayincomplete dominanceFact:Genes can have multiple allelesAA or AOBB or BOABO Most genes have 10 alleles Alleles arise from mutationHeterozygous phenotype isintermediate betweenhomozygous phenotypesRemember:An individual will have only amaximum of two unique allelesfor a gene (diploid organisms)Chapter 12: InheritanceCodominance:Both phenotypes expressedequally in heterozygotesChapter 12: InheritanceVariations on Mendelian Theme:Variations on Mendelian Theme:Assumption:Each trait is completely controlledby a single geneAssumption:Each trait is completely controlledby a single geneFactA:Many traits are influenced byseveral genesFactB:Single genes may have multiplephenotypic effects (Pleiotrophy)Polygenic Inheritance:Interaction of two (2) or more genescontribute to a single phenotypeFertilizationSRY gene:Codes for protein thatactivates otherproteins in male cascadeSex Determining Region of the YHormonesGonad FormationSexual Differentiation of phenotypeFigure 12.25 – Audesirk2 & ByersChapter 12: InheritanceChapter 12: InheritanceHow are Human Genetic Disorders Investigated?Variations on Mendelian Theme:Answer: PedigreesAssumption:Each trait is completely controlledby a single geneFactC:The environment can influencegene expressionDiagrams showing the genetic relationshipamong related individuals:Example:Temperature in Himalayan rabbitsCan reveal whether trait isdominant, recessive, or sex-linked Enzyme deactivated athigh temperatureDominant TraitHumans: Skin color (exposure) Height (nutrition) Intelligence (?)Recessive Trait4
Chapter 12: InheritanceChapter 12: InheritanceCarrier:Heterozygous individualInheritance of Human Disorders (single gene):1) Recessive Alleles: Disease if individual inherits two recessive alleles(not affected)Inheritance of Human Disorders (single gene):2) Dominant Alleles: Potential effects: A) Produce abnormal proteinsB) Perform toxic reactionsC) Produce “overactive” proteins3) Sex-linked Alleles: Genes associated with the sex chromosomes Frequently in males; often skips a generationHuntington Disease:Deterioration of brain(abnormal protein production)Example 1:Color blindness(defective receptors)Albinism (no pigmentation; pink eyes)Sickle-cell Anemia(defective hemoglobin)Chapter 12: InheritanceChapter 12: InheritanceRoyal Families of Europe:Figure 12.32 – Audesirk2 & ByersErrors in Chromosome Number can Occur:1) Sex Chromosomes:XOTurner Syndrome( ; Infertile; lack 2 sexual char.)Example 2:HemophiliaXXXTrisomy X(defective clotting proteins)( ; Fertile; usually tall; potential IQ)XXYKlinefelter Syndrome( ; Fertile; often unnoticed)XYYJacob SyndromeNondisjunction:Homologous pairs do notseparate during meiosis( ; Fertile; excess testosterone (violent?))Chapter 12: InheritanceFigure 12.32 – Audesirk2 & ByersErrors in Chromosome Number can Occur:2) Autosomal Chromosomes:Down Syndrome(Trisomy 21) Physical characteristics Mental retardation Heart malformationsFrequency increase withage at pregnancyNondisjunction:Homologous pairs do notseparate during meiosis5
Chapter 12: Inheritance Inheritance of Single Traits: Genotype: Actual combination of alleles carried by an individual Figure 12.8 / 12. 9 –Audesirk2 & Byers Chapter 12: Inheritance Inheritance of Single Traits: Arrgghh! Figure 12.10 –Audesirk2 & Byers Keep in Mind: Predicted proportion
