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Genes, Alleles, Loci, and Chromosomes
- 2. Chapter 12 2 InheritanceInheritance Inheritance is the process by which theInheritance is the process by which the characteristics of individuals arecharacteristics of individuals are passed to their offspringpassed to their offspring GenesGenes encode these characteristicsencode these characteristics AA genegene is a unit of heredity that encodesis a unit of heredity that encodes information for the form of a particularinformation for the form of a particular characteristiccharacteristic The location of a gene on a chromosomeThe location of a gene on a chromosome is called itsis called its locuslocus
- 3. Chapter 12 3 AllelesAlleles Homologous chromosomes carry theHomologous chromosomes carry the same kinds of genes for the samesame kinds of genes for the same characteristicscharacteristics Genes for the same characteristic areGenes for the same characteristic are found at the same loci on bothfound at the same loci on both homologous chromosomeshomologous chromosomes
- 4. Chapter 12 4 AllelesAlleles Genes for a characteristic found onGenes for a characteristic found on homologous chromosomes may nothomologous chromosomes may not be identicalbe identical Alternate versions or forms of genesAlternate versions or forms of genes found at the same gene locus arefound at the same gene locus are calledcalled allelesalleles
- 5. Chapter 12 5 AllelesAlleles Each cell carries two alleles perEach cell carries two alleles per characteristic, one on each of the twocharacteristic, one on each of the two homologous chromosomeshomologous chromosomes If both homologous chromosomes carry theIf both homologous chromosomes carry the samesame allele (gene form) at a given geneallele (gene form) at a given gene locus, the organism islocus, the organism is homozygoushomozygous at thatat that locuslocus If two homologous chromosomes carryIf two homologous chromosomes carry differentdifferent alleles at a given locus, thealleles at a given locus, the organism isorganism is heterozygousheterozygous at that locus (aat that locus (a hybridhybrid))
- 6. Chapter 12 6 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626Loci:Loci: Genes, Alleles,Genes, Alleles, Loci, and ChromosomesLoci, and Chromosomes Chromosome from One ParentChromosome from One Parent Homologous Chromosome from Other ParentHomologous Chromosome from Other Parent 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626Loci:Loci: M locus has gene that controls leaf color. Plant homozygous for this gene D locus has gene that controls plant height. Plant homozygous for this gene Bk locus has gene that controls fruit shape. Plant heterozygous for this gene
- 7. Chapter 12 7 Definitions 1Definitions 1 Must know these!!!Must know these!!! TraitTrait—A variable characteristic of organism—A variable characteristic of organism GeneGene—A segment of chromosomal DNA—A segment of chromosomal DNA controlling a specific traitcontrolling a specific trait LocusLocus—Chromosomal position where DNA—Chromosomal position where DNA for a specific gene livesfor a specific gene lives GenomeGenome—Refers to all standard loci for a—Refers to all standard loci for a speciesspecies
- 8. Chapter 12 8 Definitions 2Definitions 2 Must know these!!!Must know these!!! AllelesAlleles—Different forms of a—Different forms of a genegene • ““Flower color” is a gene;Flower color” is a gene; • ““Purple” is one flower-color allelePurple” is one flower-color allele • ““White” is another flower-color alleleWhite” is another flower-color allele GenotypeGenotype—List of alleles for an individual at—List of alleles for an individual at specific genesspecific genes • Familiar organisms are diploidFamiliar organisms are diploid • One or two alleles per individualOne or two alleles per individual
- 9. Chapter 12 9 Definitions 3Definitions 3 HomozygousHomozygous—Maternal & paternal alleles—Maternal & paternal alleles samesame • Father donates purple-flower alleleFather donates purple-flower allele • Mother donates purple-flower alleleMother donates purple-flower allele HeterozygousHeterozygous—Maternal & paternal alleles—Maternal & paternal alleles differdiffer • Father donates purple-flower alleleFather donates purple-flower allele • Mom donates white-flower alleleMom donates white-flower allele
- 10. Chapter 12 10 Definitions 4Definitions 4 PhenotypePhenotype:: • List of traits exhibited by individualList of traits exhibited by individual • Doesn’t always represent genotypeDoesn’t always represent genotype DominantDominant—Allele that is expressed 100% in—Allele that is expressed 100% in heterozygoteheterozygote RecessiveRecessive—Allele is not expressed in—Allele is not expressed in heterozygoteheterozygote Incomplete dominanceIncomplete dominance—heterozygote—heterozygote displays intermediate traitdisplays intermediate trait
- 11. Chapter 12 11 Genetic SymbolismGenetic Symbolism Often use initial letter of dominant alleleOften use initial letter of dominant allele • CapitalCapital letter represents dominantletter represents dominant • Lower caseLower case ofof same lettersame letter representsrepresents recessiverecessive If purple flower dominant to white…If purple flower dominant to white… • ““P” represents allele for purpleP” represents allele for purple • ““p” represents allele for whitep” represents allele for white
- 12. Chapter 12 12 Cross Fertilization of ParentsCross Fertilization of Parents True-breedingTrue-breeding Purple-floweredPurple-flowered ParentParent True-breedingTrue-breeding White-floweredWhite-flowered ParentParent Cross-FertilizeCross-Fertilize All Purple-floweredAll Purple-flowered OffspringOffspring Pollen Pollen P P F1
- 13. Chapter 12 13 Self-fertilization of FSelf-fertilization of F22 F1 Self-FertilizeSelf-Fertilize F2 F2 F2 F2 75% Purple75% Purple 25% White25% White
- 14. Chapter 12 14 Genotype vs PhenotypeGenotype vs Phenotype Phenotype is how we look/behavePhenotype is how we look/behave • PurplePurple flowersflowers • WhiteWhite flowersflowers Genotype is what our genes sayGenotype is what our genes say • WhiteWhiteFlowers /Flowers / WhiteWhiteFlowersFlowers • WhiteWhiteFlowers /Flowers / PurplePurpleFlowersFlowers • PurplePurpleFlowers /Flowers / PurplePurpleFlowersFlowers
- 15. Chapter 12 15 Genotype vs Phenotype 2Genotype vs Phenotype 2 GenotypesGenotypes • PP = homozygous forPP = homozygous for purplepurple flowerflower • pp = homozygous forpp = homozygous for whitewhite flowerflower • Pp = heterozygous for flower colorPp = heterozygous for flower color Phenotype from genotype:Phenotype from genotype: • PP =PP = purplepurple flowerflower • Pp =Pp = purplepurple flowerflower • pP =pP = purplepurple flowerflower • pp =pp = WhiteWhite flowerflower
- 16. Chapter 12 16 How Meiosis Separates GenesHow Meiosis Separates Genes The two alleles for a characteristic separateThe two alleles for a characteristic separate during gamete formation (meiosis)during gamete formation (meiosis) • Homologous chromosomes separate inHomologous chromosomes separate in meiosis anaphase Imeiosis anaphase I • Each gamete receives one of each pair ofEach gamete receives one of each pair of homologous chromosomes and thus one ofhomologous chromosomes and thus one of the two alleles per characteristicthe two alleles per characteristic The separation of alleles in meiosis isThe separation of alleles in meiosis is known as Mendel’s Law of Segregationknown as Mendel’s Law of Segregation
- 17. Chapter 12 17 Gametes of HomozygotesGametes of Homozygotes A A A A Homozygous ParentHomozygous Parent GametesGametes All gametes identicalAll gametes identical regarding this generegarding this gene
- 18. Chapter 12 18 Gametes of HeterozygotesGametes of Heterozygotes A a A a Heterozygous ParentHeterozygous Parent GametesGametes Gametes 50/50Gametes 50/50 regarding this generegarding this gene
- 19. Chapter 12 19 pp homozygous recessive Homozygous DominantHomozygous Dominant X Homozygous RecessiveX Homozygous Recessive P p P p PurpleParentPurpleParent PP homozygous dominant WhiteParentWhiteParent spermsperm nucleinuclei eggegg nucleinuclei spermsperm nucleinuclei eggegg nucleinuclei
- 20. Chapter 12 20 Pp pP P Sperm + p EggsP Sperm + p Eggs same as p Sperm + P Eggssame as p Sperm + P Eggs PurpleFPurpleF11PurpleFPurpleF11 P p spermsperm nucleusnucleus eggegg nucleusnucleus ++ p P eggegg nucleusnucleus spermsperm nucleusnucleus ++
- 21. Chapter 12 21 PurplePurple homozygoushomozygous dominant (PP)dominant (PP) PurplePurple heterozygousheterozygous (Pp)(Pp) PurplePurple heterozygousheterozygous (pP)(pP) WhiteWhite homozygoushomozygous recessive (pp)recessive (pp) Pp X Pp CrossPp X Pp Cross P p p P p P P p ++ ++ ++ ++ FF11 SpermSperm FF11 EggsEggs FF22 OffspringOffspring
- 22. Chapter 12 22Using Punnett SquaresUsing Punnett Squares in Genetic Crossesin Genetic Crosses Named after geneticist ReginaldNamed after geneticist Reginald PunnettPunnett Figured usingFigured using Punnett squaresPunnett squares • Considers only genes of interestConsiders only genes of interest • List sperm genotypes across topList sperm genotypes across top • List egg genotypes down sideList egg genotypes down side • Fill in boxes with zygote genotypesFill in boxes with zygote genotypes
- 23. Chapter 12 23 Consider Flower ColorConsider Flower Color Pretend flower color affected by only onePretend flower color affected by only one gene (gene (monohybrid crossmonohybrid cross)) Assume all alleles are purple or whiteAssume all alleles are purple or white Purple (P) is dominant to white (p)Purple (P) is dominant to white (p) HeterozygotesHeterozygotes will have flowers as purplewill have flowers as purple as homozygous dominantsas homozygous dominants
- 24. Chapter 12 24 P p 1(25%) White 3 (75%)3 (75%) PurplePurple FrequenciesFrequencies PhenotypesPhenotypes GenotypesGenotypes FrequenciesFrequencies Making a Punnett Square:Making a Punnett Square: Heterozygous X HeterozygousHeterozygous X Heterozygous Eggs of Heterozygous PlantEggs of Heterozygous Plant Pollen ofPollen of Heterozygous PlantHeterozygous Plant 1111 22 P p pP PpPP pp PP pppP Pp
- 25. Chapter 12 25 Practical Application: The Test CrossPractical Application: The Test Cross AA test crosstest cross is used to deduce the actualis used to deduce the actual genotype of an organism with agenotype of an organism with a dominant phenotype (i.e., is thedominant phenotype (i.e., is the organismorganism PPPP oror PpPp?)?) 1.1. Cross the unknown dominant-phenotypeCross the unknown dominant-phenotype organism (organism (PP_) with a homozygous_) with a homozygous recessive organism (recessive organism (pppp)…)…
- 26. Chapter 12 26 Practical Application: The Test CrossPractical Application: The Test Cross 2. If the dominant-phenotype organism is2. If the dominant-phenotype organism is homozygous dominant (homozygous dominant (PPPP), only), only dominant-phenotype offspring will bedominant-phenotype offspring will be produced (produced (PpPp)) 3.3. If the dominant-phenotype organism isIf the dominant-phenotype organism is heterozygous (heterozygous (PpPp), approximately half of), approximately half of the offspring will be of recessivethe offspring will be of recessive phenotype (phenotype (pppp))
- 27. Chapter 12 27 p p (50%) White (50%)(50%) PurplePurple FrequenciesFrequencies PhenotypesPhenotypes GenotypesGenotypes FrequenciesFrequencies Test Cross:Test Cross: Heterozygous X Homozygous RecessiveHeterozygous X Homozygous Recessive Eggs of Homozygous RecessiveEggs of Homozygous Recessive Pollen of unknownPollen of unknown plant with dominantplant with dominant phenotypephenotype (Heterozygous)(Heterozygous) 22 P p pp PpPP pp Pp pppP pp 22
- 28. Chapter 12 28 p p (100%)(100%) PurplePurple FrequenciesFrequencies PhenotypesPhenotypes GenotypesGenotypes FrequenciesFrequencies Test Cross:Test Cross: Homozygous X Homozygous RecessiveHomozygous X Homozygous Recessive Eggs of Homozygous RecessiveEggs of Homozygous Recessive Pollen of unknownPollen of unknown plant with dominantplant with dominant phenotypephenotype (Homozygous)(Homozygous) P Pp PpPp Pp Pp PpPp Pp P 44
- 29. Chapter 12 29Traits of PeasTraits of Peas Studied by MendelStudied by Mendel Plant size Flower location Flower color Pod color Pod shape Seed shape Seed color
- 30. Chapter 12 30 Traits Are Inherited IndependentlyTraits Are Inherited Independently Seed color (yellow vs. green peas) and seedSeed color (yellow vs. green peas) and seed shape (smooth vs. wrinkled peas) wereshape (smooth vs. wrinkled peas) were the characteristics studiedthe characteristics studied The allele symbols were assigned:The allele symbols were assigned: • YY = yellow (dominant),= yellow (dominant), yy = green (recessive)= green (recessive) • SS = smooth (dominant),= smooth (dominant), ss = wrinkled (recessive)= wrinkled (recessive) Two trait cross was between two trueTwo trait cross was between two true breeding varieties for each characteristicbreeding varieties for each characteristic • P:P: SSYYSSYY xx ssyyssyy
- 31. Chapter 12 31 RecombinationRecombination Genes on the same chromosome do notGenes on the same chromosome do not alwaysalways sort togethersort together Crossing overCrossing over in Prophase I of meiosisin Prophase I of meiosis creates new gene combinationscreates new gene combinations Crossing over involves the exchange ofCrossing over involves the exchange of DNA between chromatids of pairedDNA between chromatids of paired homologous chromosomes inhomologous chromosomes in synapsissynapsis
- 32. Chapter 12 32 red red Purple Purple round round Long Long PP PP pp pp LL LL ll ll PP pp pp LL LL ll ll PP LL pp LL ll ll PP pp PP LL pp LL ll ll PP pp LL LL ll ll PP PP pp pp PP PP pp pp LL LL ll ll PP pp pp LL LL ll ll Crossing OverCrossing Over SisterSister ChromatidsChromatids DuplicatedDuplicated ChromosomeChromosome DuplicatedDuplicated ChromosomeChromosome LL LL ll ll PP PP pp pp SisterSister ChromatidsChromatids HomologousHomologous ChromosomesChromosomes PP PP pp pp LL LL ll ll PP pp pp LL LL ll ll pp LL PP ll LLPP llpp old combinationold combination new combinationnew combination new combinationnew combination old combinationold combination Flower Color Pollen Shape
- 33. Chapter 12 33
- 34. Chapter 12 34
- 35. Chapter 12 35 Sex Chromosomes and AutosomesSex Chromosomes and Autosomes Mammals and many insect species have aMammals and many insect species have a set ofset of sex chromosomessex chromosomes that dictatethat dictate gendergender • Females have twoFemales have two X chromosomesX chromosomes • Males have anMales have an X chromosomeX chromosome and aand a YY chromosomechromosome • Sex chromosomesSex chromosomes segregate duringsegregate during meiosismeiosis • [The rest of the (non-sex) chromosomes[The rest of the (non-sex) chromosomes are calledare called autosomes]autosomes]
- 36. Chapter 12 36
- 37. Chapter 12 37 XX11 XX22 Sex DeterminationSex Determination in Mammalsin Mammals EGGSEGGS Male ParentMale Parent YYXXmm SS PP EE RR MM Female OffspringFemale Offspring Male OffspringMale Offspring YY XXmm XXmmXX11 XX22XXmm YY YYXX11 XX22 XX11 XX22 Female ParentFemale Parent
- 38. Chapter 12 38 Sex-Linked Genes Are on the X or the YSex-Linked Genes Are on the X or the Y Genes carried on one sex chromosome areGenes carried on one sex chromosome are sex-sex- linkedlinked • X chromosome is much larger than the Y andX chromosome is much larger than the Y and carries over 1000 genescarries over 1000 genes • Y chromosome is smaller and carries only 78Y chromosome is smaller and carries only 78 genesgenes The X and the Y have very few genes in commonThe X and the Y have very few genes in common • Females (XX) can be homozygous orFemales (XX) can be homozygous or heterozygous for a characteristicheterozygous for a characteristic • Males (XY) have onlyMales (XY) have only one copyone copy of the genes onof the genes on the X or the Ythe X or the Y
- 39. Chapter 12 39How Sex-Linkage AffectsHow Sex-Linkage Affects InheritanceInheritance Patterns of sex-linked inheritance were firstPatterns of sex-linked inheritance were first discovered in fruit flies (discovered in fruit flies (DrosophilaDrosophila) in) in early 1900searly 1900s Eye color genes were found to be carried byEye color genes were found to be carried by the X chromosomethe X chromosome • RR = red eyes (dominant)= red eyes (dominant) • rr = white eyes (recessive)= white eyes (recessive)
- 40. Chapter 12 40How Sex-Linkage AffectsHow Sex-Linkage Affects InheritanceInheritance Sex-linked (specificallySex-linked (specifically X-linkedX-linked) recessive) recessive alleles displayed their phenotype morealleles displayed their phenotype more often in malesoften in males • Males showed recessive white-eyedMales showed recessive white-eyed phenotype more often than females in anphenotype more often than females in an XXRRXXrr xx XXrrY crossY cross Males do not have a second X-linked geneMales do not have a second X-linked gene (as do females) which can mask a(as do females) which can mask a recessive gene if dominantrecessive gene if dominant
- 41. Chapter 12 41 25%25% Normal fNormal f Carrier fCarrier f Normal mNormal m 25%25% 25%25% 25% White-e m FrequenciesFrequencies PhenotypesPhenotypes GenotypesGenotypes FrequenciesFrequencies Sex Linkage:Sex Linkage: Eye Color in Fruit FliesEye Color in Fruit Flies Eggs ofEggs of XR Xr FemaleFemale Sperm ofSperm of XXRRY MaleY Male 1111 YXR XRXrXRXR YXr XRXR XrYXRXr XRY R r R Female Female Male Male 11 11
- 42. Chapter 12 42 RR RR (100%)(100%) Pink (intermediate)Pink (intermediate) FrequenciesFrequencies PhenotypesPhenotypes GenotypesGenotypes FrequenciesFrequencies Incomplete Dominance:Incomplete Dominance: Homozygous-X Homo RecessiveHomozygous-X Homo Recessive Eggs of HomozygousEggs of Homozygous RR Red ParentRed Parent Pollen ofPollen of HomozygousHomozygous R'R' White ParentWhite Parent R' R' R'R R'RR'R R'R R'R R'RR'R R'R Pink Pink Pink Pink 11
- 43. Chapter 12 43 CodominanceCodominance Some alleles are always expressed evenSome alleles are always expressed even in combination with other allelesin combination with other alleles Heterozygotes display phenotypes ofHeterozygotes display phenotypes of both the homozygote phenotypes inboth the homozygote phenotypes in codominancecodominance
- 44. Chapter 12 44 CodominanceCodominance Example: Human blood group allelesExample: Human blood group alleles • Alleles A and B are codominantAlleles A and B are codominant • Type AB blood is seen where individualType AB blood is seen where individual has the genotype ABhas the genotype AB
- 45. Chapter 12 45 10%10% 40%40% 46%46% 4%4% B or ABB or AB A or ABA or AB O,AB,O,AB, A,BA,B (universal)(universal) ABAB (universal)(universal) B or OB or O A or OA or O OO AB, A,AB, A, B, OB, O (universal)(universal) AA BB BothBoth NeitherNeither BB or BOBB or BO AA or AOAA or AO OOOO ABAB OO ABAB BB AA FreqFreqDonatesDonatesRe-Re- ceivesceives Anti-Anti- bodiesbodiesRBCsRBCsGenotypeGenotypeTypeType Human ABO Blood GroupHuman ABO Blood Group
- 46. Chapter 12 46 Polygenic InheritancePolygenic Inheritance Phenotypes produced byPhenotypes produced by polygenicpolygenic inheritanceinheritance are governed by theare governed by the interaction of more than two genes atinteraction of more than two genes at multiple locimultiple loci Human skin color is controlled by at least 3Human skin color is controlled by at least 3 genes, each with pairs of incompletelygenes, each with pairs of incompletely dominant allelesdominant alleles
- 47. Chapter 12 47
- 48. Chapter 12 48 Pedigree AnalysisPedigree Analysis Records of gene expression over severalRecords of gene expression over several generations of a family can begenerations of a family can be diagrammeddiagrammed Careful analysis of this diagram (aCareful analysis of this diagram (a pedigreepedigree) can reveal inheritance) can reveal inheritance pattern of a traitpattern of a trait Pedigree analysis is often combined withPedigree analysis is often combined with molecular genetics technology tomolecular genetics technology to elucidate gene action and expressionelucidate gene action and expression
- 49. Chapter 12 49 How to Read PedigreesHow to Read Pedigrees = male= male = female= female = parents= parents oror = individual who shows the trait= individual who shows the trait oror = heterozygous carrier of= heterozygous carrier of autosomal traitautosomal trait = offspring= offspring 11 22 33 I, II, III, IV, or VI, II, III, IV, or V = generation= generation
- 50. Chapter 12 50 A Recessive PedigreeA Recessive Pedigree
- 51. Chapter 12 51Pedigrees:Pedigrees: Legacy of Queen VictoriaLegacy of Queen Victoria
- 52. Chapter 12 52 Sickle-Cell AnemiaSickle-Cell Anemia Hemoglobin is an oxygen-transporting proteinHemoglobin is an oxygen-transporting protein found in red blood cellsfound in red blood cells A mutant hemoglobin gene causesA mutant hemoglobin gene causes hemoglobin molecules in blood cells tohemoglobin molecules in blood cells to clump togetherclump together • Red blood cells take on a sickle (crescent)Red blood cells take on a sickle (crescent) shape and easily breakshape and easily break • Blood clots can form, leading to oxygenBlood clots can form, leading to oxygen starvation of tissues and paralysisstarvation of tissues and paralysis • Condition is known asCondition is known as sickle-cell anemiasickle-cell anemia
- 53. Chapter 12 53 Normal Red Blood CellsNormal Red Blood Cells
- 54. Chapter 12 54 Sickled CellsSickled Cells
- 55. Chapter 12 55 Sex-Linked Genetic DisordersSex-Linked Genetic Disorders Several defective alleles forSeveral defective alleles for characteristics encoded on the Xcharacteristics encoded on the X chromosome are knownchromosome are known Sex-linked disorders appear moreSex-linked disorders appear more frequently in males and often skipfrequently in males and often skip generationsgenerations Examples of sex-linked (X-linked)Examples of sex-linked (X-linked) disordersdisorders • Red-green color blindnessRed-green color blindness
- 56. Chapter 12 56
- 57. Chapter 12 57
- 58. Chapter 12 58 Non-DisjunctionNon-Disjunction Incorrect separation of chromosomes orIncorrect separation of chromosomes or chromatids in meiosis known aschromatids in meiosis known as non-non- disjunctiondisjunction Most embryos arising from gametes withMost embryos arising from gametes with abnormal chromosome numbers abortabnormal chromosome numbers abort spontaneously (are miscarried)spontaneously (are miscarried) Some combinations of abnormalSome combinations of abnormal chromosome number survive to birthchromosome number survive to birth or beyondor beyond
- 59. Chapter 12 59
- 60. Chapter 12 60
- 61. Chapter 12 61 Incidence of Down SyndromeIncidence of Down Syndrome 1010 2020 3030 4040 5050 00 100100 200200 300300 400400 Age of Mother (years)Age of Mother (years) Numberper1000BirthsNumberper1000Births
- 62. Chapter 12 The endThe end
Hinweis der Redaktion
- Each homologous chromosome carries the same set of genes. Each gene is located at the same relative position, or locus, on its chromosome. Differences in nucleotide sequences at the same gene locus produce different alleles of the gene. Diploid organisms have two alleles of each gene.
- Must know these!!! Trait—A variable characteristic of organism. It’s something about the organism’s appearance, behavior, etc., that you’re interested in. Gene—A segment of chromosomal DNA controlling a specific trait. This refers to the genetic material that produces a product that determines the trait. Locus—The chromosomal position where a specific gene lives. This is the gene’s address, in terms of which chromosome does it live on and where on that chromosome does it live? Genome—Refers to all standard loci for a species. We can speak of the “human genome.” It is the list of the genes that humans have.
- Must know these!!! Alleles—Different forms of a gene “Eye color” is a gene; “Blue eyes” is one allele (version) of the eye color gene. “Brown eyes” is another allele (version) of the eye color gene.There are others, but, for the purpose of simplicity, we will pretend there are only two alleles for eye color. Genotype—List of alleles an individual has at specific genes Familiar organisms are diploid. Each individual has an allele of each gene from Mom, and another allele of the same gene from Dad. These two copies of a gene may have identical alleles (the individual is homozygous) or different (the individual is heterozygous).
- 1. Alleles are various molecular forms of a gene for the same trait. 2. If homozygous, both alleles are the same. 3. If heterozygous, the alleles differ. 4. When heterozygous, one allele is dominant (A), and the other is recessive (a). 5. Thus, homozygous dominant = AA, homozygous recessive = aa, and heterozygous = Aa. 6. Genotype is the sum of the genes, and phenotype is how the genes are expressed (what you observe). Example: Homozygous—Maternal & paternal alleles same Dad donates blue-eyed allele Mom donates blue-eyed allele Heterozygous—Maternal & paternal alleles differ Dad donates blue-eyed allele Mom donates brown-eyed allele
- Phenotype—List of traits exhibited by individual Doesn’t always reveal genotype. Sometimes the presence of a dominant allele on the maternal chromosome will mask the presence of a recessive allele on the other chromosome. Dominant—Allele that is expressed 100% in heterozygote Recessive—Allele is not expressed at all in heterozygote but only in homozygote. Incomplete dominance—heterozygote displays intermediate version of the trait about half way between the full two homozygous phenotypes.
- Often use initial letter of dominant allele Capital letter represents dominant Lower case of same letter represents recessive If black fur dominant to white… B represents allele for black b represents allele for white
- Mendel pea experiments, flower color: cross fertilization of parental generation.
- Mendel pea experiments, flower color: self-fertilization of F2.
- Phenotype is how we look/behave Brown eyes Blue eyes Genotype is what our genes say BlueEyes/BlueEyes BlueEyes/BrownEyes BrownEyes/BrownEyes
- BB = homozygous for black fur bb = homozygous for white fur Bb = heterozygous for fur color Phenotypes: BB = Black Bb = Black bB = Black bb = White
- Mendel pea experiments, flower color: gametes of a homozygous parent
- Mendel pea experiments, flower color: F1 generation from homozygous parents
- Mendel pea experiments, flower color: F2 from heterozygous F1
- Named after geneticist Reginald Punnett Figured using Punnett squares Considers only genes of interest List all possible sperm genotypes across top List all possible egg genotypes down side Fill in boxes with zygote genotypes
- Other genes also affect eye color, but we will pretend there is only one gene and that it has only two alles Eye color affected mainly by one gene (monohybrid cross) Most common alleles are brown and blue Blue is recessive to brown Heterozygotes have eyes as brown as homozygous dominants
- Note: You should be very familiar with how to work these. In a cross between two heterozygotes involving dominant and recessive alleles: 1/4 of the offspring will typically show the recessive phenotype because they are homozygous for the recessive allele. 3/4 will have the dominant phenotype, even though 2/3 of these (1/2 total) are heterozygous. The Punnett square method allows you to predict both genotypes and phenotypes of specific crosses; here we use it for a cross between plants that are heterozygous for a single trait, flower color. (1) Assign letters to the different alleles; use uppercase for dominant and lowercase for recessive. (2) Determine all the types of genetically different gametes that can be produced by the male and female parents. (3) Draw the Punnett square, with each row and column labeled with one of the possible genotypes of sperm and eggs, respectively. (We have included the fractions of these genotypes with each label.) (4) Fill in the genotype of the offspring in each box by combining the genotype of sperm in its row with the genotype of the egg in its column. (We have placed the fractions in each box.) (5) Count the number of offspring with each genotype. (Note that Pp is the same as pP.) (6) Convert the number of offspring of each genotype to a fraction of the total number of offspring. In this example, out of four fertilizations, only one is predicted to produce the pp genotype, so 1/4 of the total number of offspring produced by this cross is predicted to be white. To determine phenotypic fractions, add the fractions of genotypes that would produce a given phenotype. For example, purple flowers are produced by 1/4 PP + 1/4 Pp + 1/4 pP, for a total of 3/4 of the offspring.
- Traits of pea plants that Mendel studied
- FIGURE 12-17 Replicated homologous chromosomes of the sweet pea
- FIGURE 12-18 Crossing over between homologous chromosomes of the sweet pea
- FIGURE 12-21 Photomicrograph of human sex chromosomes Notice the small size of the Y chromosome, which carries relatively few genes.
- Figure: FIGURE 12.9 Title: Sex determination in mammals Caption: Male offspring receive their Y chromosome from the father; female offspring receive the father’s X chromosome (labeled Xm). Both male and female offspring receive an X chromosome (either X1 or X2) from the mother.
- Figure: 19-2 part a Title: Viral structure and replication part a Caption: (a) A cross section of the virus that causes AIDS. Inside, genetic material is surrounded by a protein coat and molecules of reverse transcriptase, an enzyme that catalyzes the transcription of DNA from the viral RNA template after the virus enters the host cell. This virus is among those that also have an outer envelope that is formed from the host cell's plasma membrane. Spikes made of glycoprotein (protein and carbohydrate) project from the envelope and help the virus attach to its host cell.
- Note: You should be very familiar with how to work these. In a cross between two heterozygotes involving dominant and recessive alleles: 1/4 of the offspring will typically show the recessive phenotype because they are homozygous for the recessive allele. 3/4 will have the dominant phenotype, even though 2/3 of these (1/2 total) are heterozygous. The Punnett square method allows you to predict both genotypes and phenotypes of specific crosses; here we use it for a cross between plants that are heterozygous for a single trait, flower color. (1) Assign letters to the different alleles; use uppercase for dominant and lowercase for recessive. (2) Determine all the types of genetically different gametes that can be produced by the male and female parents. (3) Draw the Punnett square, with each row and column labeled with one of the possible genotypes of sperm and eggs, respectively. (We have included the fractions of these genotypes with each label.) (4) Fill in the genotype of the offspring in each box by combining the genotype of sperm in its row with the genotype of the egg in its column. (We have placed the fractions in each box.) (5) Count the number of offspring with each genotype. (Note that Pp is the same as pP.) (6) Convert the number of offspring of each genotype to a fraction of the total number of offspring. In this example, out of four fertilizations, only one is predicted to produce the pp genotype, so 1/4 of the total number of offspring produced by this cross is predicted to be white. To determine phenotypic fractions, add the fractions of genotypes that would produce a given phenotype. For example, purple flowers are produced by 1/4 PP + 1/4 Pp + 1/4 pP, for a total of 3/4 of the offspring.
- The inheritance of flower color in snapdragons is an example of incomplete dominance. (In such cases, we will use capital letters for both alleles, here R and R’.) Hybrids (RR’) have pink flowers, whereas the homozygotes are red (RR) or white (R’R’).
- Figure: TABLE 12.1 Title: Human blood group characteristics Caption: Human blood group characteristics
- Figure 12-25 Polygenic inheritance of skin color in humans (a) At least three separate genes, each with two incompletely dominant alleles, determine human skin color (the inheritance is actually much more complex than this). The backgrounds of each box indicate the depth of skin color expected from each genotype. (b) The combination of complex polygenic inheritance and environmental effects (especially exposure to sunlight) produces an almost infinite gradation of human skin colors.
- Figure: FIGURE 12.14 Title: A family pedigree Caption: This pedigree is for a recessive trait, such as albinism. Both of the original parents are carriers. Because the allele for albinism is rare, pairing between carriers is an unlikely event. However, the chance that each of two related people will carry a rare recessive allele (inherited from a common ancestor) is much higher than normal. As a result, pairings between cousins or even closer relations are the cause of a disproportionate number of recessive diseases. In this family, pairings between cousins occurred three times—between III 3 and III 5, III 4 and IV 3, and IV 1 and IV 2.
- Figure: FIGURE 12.18 Title: Hemophilia among the royal families of Europe Caption: A famous genetic pedigree involves the transmission of sex-linked hemophilia from Queen Victoria of England (seated center front, with cane, 1885) to her offspring and eventually to virtually every royal house in Europe. Because Victoria’s ancestors were free of hemophilia, the hemophilia allele must have arisen as a mutation either in Victoria herself or in one of her parents (or as a result of marital infidelity). Extensive intermarriage among royalty spread Victoria’s hemophilia allele throughout Europe. Her most famous hemophiliac descendant was great-grandson Alexis, tsarevitch (crown prince) of Russia. The Tsarina Alexandra (Victoria’s granddaughter) believed that the monk Rasputin, and no one else, could control Alexis’s bleeding. Rasputin may actually have used hypnosis to cause Alexis to cut off circulation to bleeding areas by muscular contraction. The influence that Rasputin had over the imperial family may have contributed to the downfall of the tsar during the Russian Revolution. In any event, hemophilia was not the cause of Alexis’s death; he was killed with the rest of this family by the Bolsheviks (Communists) in 1918.
- Normal red blood cells are disc-shaped with indented centers.
- Figure: FIGURE 12.16b Title: Sickle-cell anemia Caption: Sickled red blood cells in a person with sickle-cell anemia occur when blood oxygen is low. In this shape they are fragile and tend to clump together, clogging capillaries.
- Figure 12-30a Color blindness, a sex-linked recessive trait (a) This figure, called an Ishihara chart after its inventor, distinguishes color-vision defects. People with red-deficient vision see a 6, and those with green-deficient vision see a 9. People with normal color vision see 96.
- Figure 12-30b Color blindness, a sex-linked recessive trait (b) Pedigree of one of the authors (G. Audesirk, who sees only a 6 in the Ishihara chart), showing sex-linked inheritance of red-green color blindness. Both the author and his maternal grandfather are color deficient; his mother and her four sisters carry the trait but have normal color vision. This pattern of more-common phenotypic expression in males and transmission from affected male to carrier female to affected male is typical of sex-linked recessive traits.
- Figure 12-32 Nondisjunction during meiosis Nondisjunction may occur either during meiosis I (left) or meiosis II (right), resulting in gametes with too many (n + 1) or too few (n - 1) chromosomes.
- Figure 12-33a Trisomy 21, or Down syndrome (a) This karyotype of a Down syndrome child reveals three copies of chromosome 21 (arrow).
- Figure: FIGURE 12.20 Title: Down syndrome frequency increases with maternal age Caption: The increase in frequency of Down syndrome after maternal age 35 is quite dramatic.