Presentation on theme: "Overview: Locating Genes Along Chromosomes"— Presentation transcript:
1Overview: Locating Genes Along Chromosomes Mendel’s “hereditary factors” were genesToday we can show that genes are located on chromosomesThe location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene
2Figure 15.1Figure 15.1 Where are Mendel’s hereditary factors located in the cell?
3CONCEPT 15.1: MENDELIAN INHERITANCE HAS ITS PHYSICAL BASIS IN THE BEHAVIOR OF CHROMOSOMES
4Mendel and Meiosis Figure 15.2 The chromosomal basis of Mendel’s laws. P GenerationYellow-round seeds (YYRR)Green-wrinkled seeds (yyrr)YryRYRryMeiosisFertilizationRYyrGametesAll F1 plants produce yellow-round seeds (YyRr).F1 GenerationRRyyrrYYMeiosisLAW OF SEGREGATION The two alleles for each gene separate during gamete formation.LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.RrrRMetaphase IYyYy11RrrRAnaphase IYyYyRrrRFigure 15.2 The chromosomal basis of Mendel’s laws.22YyMetaphase IIYyyYYyYYyyGametesRRrrrrRR1/4YR1/4yr1/4Yr1/4yRF2 GenerationAn F1 F1 cross-fertilization3Fertilization recombines the R and r alleles at random.3Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.9: 3: 3: 1
5Mendel and Meiosis P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr)YyrRYRryMeiosisFertilizationRYyrGametesFigure 15.2 The chromosomal basis of Mendel’s laws.
6F1 Generation All F1 plants produce yellow-round seeds (YyRr). Figure 15.2bAll F1 plants produce yellow-round seeds (YyRr).F1 GenerationRRyyrrYYLAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.MeiosisRrrRMetaphase IYyYy11RrrRAnaphase IYyYyFigure 15.2 The chromosomal basis of Mendel’s laws.RrrRMetaphase II22YyYyYyYYyYyyGametesRRrrrrRR1/4YR1/4yr1/4Yr1/4yR
7Mendel and Meiosis LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT F2 GenerationAn F1 F1 cross-fertilization3Fertilization recombines the R and r alleles at random.3Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.9: 3: 3: 1Figure 15.2 The chromosomal basis of Mendel’s laws.
8Morgan’s Experimental Evidence for Mendel The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologistMorgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
9Morgan’s Choice of Experimental Organism Several characteristics make fruit flies a convenient organism for genetic studiesThey produce many offspringA generation can be bred every two weeksThey have only four pairs of chromosomes
10Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes
11Figure 15.4EXPERIMENTP GenerationThe F1 generation all had red eyesThe F2 generation showed the 3:1 red:white eye ratio, but only males had white eyesF1 GenerationAll offspring had red eyes.RESULTSF2 GenerationCONCLUSIONP GenerationwwXXXYwwSpermEggsF1 GenerationwwFigure 15.4 Inquiry: In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?wMorgan determined that the white-eyed mutant allele must be located on the X chromosomewwSpermEggswwF2 Generationwwwwww
12P Generation F1 Generation F2 Generation EXPERIMENTP GenerationF1 GenerationAll offspring had red eyes.RESULTSFigure 15.4 Inquiry: In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?F2 Generation
13P Generation F1 Generation F2 Generation CONCLUSIONP GenerationwwXXXYwwSpermEggsF1 GenerationwwwwwSpermFigure 15.4 Inquiry: In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?EggswwF2 Generationwwwwww
14CONCEPT 15.2: SEX-LINKED GENES EXHIBIT UNIQUE PATTERNS OF INHERITANCE In humans and some other animals, there is a chromosomal basis of sex determinationCONCEPT 15.2: SEX-LINKED GENES EXHIBIT UNIQUE PATTERNS OF INHERITANCE
15The Chromosomal Basis of Sex Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosomeThe SRY gene on the Y codes for a protein that directs the development of male anatomical featuresXY
16Sex Determination Systems 44 XYParents44 XXSex Determination Systems22 X22 Y22 XorSpermEgg44 XXor44 XYHuman males are XYHuman females are XXOther organisms use different systems!Zygotes (offspring)(a) The X-Y system22 XX22 X(b) The X-0 system76 ZW76 ZZ(c) The Z-W system32 (Diploid)16 (Haploid)(d) The haplo-diploid system
1744 XY 44 XX Parents 22 X 22 X 22 Y or Sperm Egg 44 XX Figure 15.6a44 XY44 XXParents22 X22 X22 YorSpermEgg44 XX44 XYorZygotes (offspring)(a) The X-Y systemFigure 15.6 Some chromosomal systems of sex determination.22 XX22 X(b) The X-0 system
1876 ZW 76 ZZ 32 (Diploid) 16 (Haploid) Figure 15.6b76 ZW76 ZZ(c) The Z-W system32 (Diploid)16 (Haploid)Figure 15.6 Some chromosomal systems of sex determination.(d) The haplo-diploid system
19Sex-LinkageA gene that is located on either sex chromosome is called a sex-linked geneGenes on the Y = Y-linked genes; there are few of theseGenes on the X = X-linked genes
20X-linked InheritanceX chromosomes code for many genes not related to sexSome disorders caused by recessive alleles on the X chromosome in humansColor blindness (mostly X-linked)Duchenne muscular dystrophyHemophilia
21Transmitting X-linked genes: Colorblindness XNXNXnYXNXnXNYXNXnXnYSpermXnYSpermXNYSpermXnYEggsXNXNXnXNYEggsXNXNXNXNYEggsXNXNXnXNYXNXNXnXNYXnXNXnXnYXnXnXnXnY(a)(b)(c)Figure 15.7 The transmission of X-linked recessive traits.For a recessive X-linked trait to be expressedA female needs two copies of the allele (homozygous)A male needs only one copy of the allele (hemizygous)X-linked recessive disorders are much more common in males than in females
22X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic developmentThe inactive X condenses into a Barr bodyIf a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character
23Female Mosaics: X Inactivation X chromosomesAllele for orange furEarly embryo:Allele for black furCell division and X chromosome inactivationTwo cell populations in adult cat:Active XInactive XActive XBlack furOrange furFigure 15.8 X inactivation and the tortoiseshell cat.
24Each chromosome has hundreds or thousands of genes (except the Y chromosome) Genes located on the same chromosome that tend to be inherited together are called linked genesConcept 15.3: Linked genes tend to be inherited together Because of Location
26How Linkage Affects Inheritance Morgan did other experiments with fruit flies to see how linkage affects inheritance of two charactersMorgan crossed flies that differed in traits of body color and wing sizeDouble mutant (black body, vestigial wings)Wild type (gray body, normal wings)
27P Generation (homozygous) Double mutant (black body, vestigial wings) FigureEXPERIMENTP Generation (homozygous)Double mutant (black body, vestigial wings)Wild type (gray body, normal wings)b b vg vgb b vg vgFigure 15.9 Inquiry: How does linkage between two genes affect inheritance of characters?
28P Generation (homozygous) Double mutant (black body, vestigial wings) FigureEXPERIMENTP Generation (homozygous)Double mutant (black body, vestigial wings)Wild type (gray body, normal wings)b b vg vgb b vg vgF1 dihybrid (wild type)Double mutantTESTCROSSb b vg vgb b vg vgFigure 15.9 Inquiry: How does linkage between two genes affect inheritance of characters?
29Wild type (gray-normal) FigureEXPERIMENTP Generation (homozygous)Double mutant (black body, vestigial wings)Wild type (gray body, normal wings)b b vg vgb b vg vgF1 dihybrid (wild type)Double mutantTESTCROSSb b vg vgb b vg vgTestcross offspringEggsb vgb vgb vgb vgWild type (gray-normal)Black- vestigialGray- vestigialBlack- normalb vgFigure 15.9 Inquiry: How does linkage between two genes affect inheritance of characters?Spermb b vg vgb b vg vgb b vg vgb b vg vg
30Wild type (gray-normal) FigureEXPERIMENTP Generation (homozygous)Double mutant (black body, vestigial wings)Wild type (gray body, normal wings)b b vg vgb b vg vgF1 dihybrid (wild type)Double mutantTESTCROSSb b vg vgb b vg vgTestcross offspringEggsb vgb vgb vgb vgWild type (gray-normal)Black- vestigialGray- vestigialBlack- normalb vgFigure 15.9 Inquiry: How does linkage between two genes affect inheritance of characters?Spermb b vg vgb b vg vgb b vg vgb b vg vgPREDICTED RATIOSIf genes are located on different chromosomes:1:1:1:1If genes are located on the same chromosome and parental alleles are always inherited together:1:1::RESULTS965:944:206:185
31b+ vg+b vgF1 dihybrid female and homozygous recessive male in testcrossb vgb vgb+ vg+b vgMost offspringorb vgb vgFigure 15.UN01 In-text figure, p. 294Morgan found body color and wing size are usually inherited together in specific combinations (parental phenotypes)
32However, nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent
33Genetic Recombination and Linkage Offspring with a phenotype matching one of the parental phenotypes are called parental typesOffspring with nonparental phenotypes are called recombinant types, or recombinantsA 50% frequency of recombination is observed for any two genes on different chromosomes
34Parental vs. Recombinant Phenotypes Gametes from yellow-round dihybrid parent (YyRr)YRyrYryRGametes from green- wrinkled homozygous recessive parent (yyrr)yrYyRryyrrYyrryyRrFigure 15.UN02 In-text figure, p. 294Parental- type offspringRecombinant offspring
35Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observedSome process must occasionally break the physical connection between genes on the same chromosome
36Crossing Over and Recombination Testcross parentsGray body, normal wings (F1 dihybrid)Black body, vestigial wings (double mutant)Crossing Over and Recombinationb vgb vgb vgb vgReplication of chromosomesReplication of chromosomesb vgb vgb vgb vgb vgb vgb vgb vgMeiosis Ib vgMeiosis I and IIRecombinant chromosomes bring alleles together in new combinations in gametesGenetic variation drives evolution!b vgb vgb vgMeiosis IIRecombinant chromosomesbvgb vgb vgb vgEggsFigure Chromosomal basis for recombination of linked genes.Testcross offspring965 Wild type (gray-normal)944 Black- vestigial206 Gray- vestigial185 Black- normalb vgb vgb vgb vgb vgb vgb vgb vgb vgSpermParental-type offspringRecombinant offspringRecombination frequency391 recombinants 2,300 total offspring 17%
37Recombination frequencies Mapping Distance Between Genes Using Recombination DataRecombination frequencies1 map unit = 1% recombination frequency9%9.5%Chromosome17%bcnvgA linkage map is a genetic map of a chromosome based on recombination frequenciesThe farther apart two genes are, the higher the probability crossover will occur and therefore the higher the recombination frequencyDistances between genes can be expressed as map units.Figure Research Method: Constructing a Linkage Map
38Genes that are far apart on the same chromosome can have a recombination frequency near 50% Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes
39Genetic Linkage Map: Cytogenic Mapping Mutant phenotypesShort aristaeBlack bodyCinnabar eyesVestigial wingsBrown eyes48.557.567.0104.5Figure A partial genetic (linkage) map of a Drosophila chromosome.Long aristae (appendages on head)Gray bodyRed eyesNormal wingsRed eyesWild-type phenotypes
40Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disordersPlants tolerate such genetic changes better than animals doConcept 15.4: Alterations of chromosome number or structure cause some genetic disorders
41Chromosomal Changes and Disorders Abnormal Chromosome NumberAlteration of Chromosome Structure
42Abnormal Chromosome Number Meiosis INondisjunctionIn nondisjunction, pairs of homologous chromosomes do not separate normally during meiosisFigure Meiotic nondisjunction.
43Abnormal Chromosome Number Meiosis INondisjunctionMeiosis IINon- disjunctionAs a result, one gamete receives two of the same type of chromosome, and another gamete receives no copyFigure Meiotic nondisjunction.
44Abnormal Chromosome Number Meiosis INondisjunctionMeiosis IINon- disjunctionGametesFigure Meiotic nondisjunction.n 1n 1n 1n 1n 1n 1nnNumber of chromosomesNondisjunction of homo- logous chromosomes in meiosis I(a)Nondisjunction of sister chromatids in meiosis II(b)
45AneuploidyThe fertilization of gametes in which nondisjunction occurredOffspring with this condition have an abnormal number of a particular chromosome
46A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome
47Polyploidy: More than 2 Sets of Chromosomes Triploidy (3n) is three sets of chromosomesTetraploidy (4n) is four sets of chromosomesPolyploidy is common in plants, but not animalsPolyploids are more normal in appearance than aneuploidsHexaploidy (6n)Triploidy (3n)Octoploidy (8n)
48Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structureDeletion removes a chromosomal segmentDuplication repeats a segmentInversion reverses orientation of a segment within a chromosomeTranslocation moves a segment from one chromosome to another
49Alterations of Chromosome Structure (a) DeletionACDEFGHBA deletion removes a chromosomal segment.ABCEFGH(b) DuplicationABCDEFGHFigure Alterations of chromosome structure.A duplication repeats a segment.ABCDEFGH
50Alterations of Chromosome Structure (c) InversionABCDEFGHAn inversion reverses a segment within a chromosome.ADCBEFGH(d) TranslocationABCDEFGHMNOPQRFigure Alterations of chromosome structure.A translocation moves a segment from one chromosome to a nonhomologous chromosome.GMNOCHFEDABPQR
51Human Disorders Due to Chromosomal Alterations Most mammalian aneuploidy are lethal but some survive to birth and beyondThese surviving individuals have a set of symptoms, or syndrome.
52Down Syndrome (Trisomy 21) Down syndrome is an aneuploid condition that results from three copies of chromosome 21It affects about 1in 700 children born in the USThe frequency of Down syndrome increases with the age of the mother
53Aneuploidy of Sex Chromosomes SyndromeGenotypeOccurrenceSymptomsKlinefelter SyndromeXXY1/500 ~ 1/1000Underdeveloped testesMale is sterileSome secondary female characteristicsXYY SyndromeXYY1/1000Normal sexual developmentTaller than averageIncreased aggressionTurner Syndrome (Monosomy X)XO1/2500Only viable monosomy in humansSex organs do not develop; sterileTrisomy XXXXLearning disabledFertileSlightly taller
55Disorders Caused by Structurally Altered Chromosomes The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood
56CML: Chronic Myelogenous Leukemia Normal chromosome 9CML caused by translocations of chromosomesNormal chromosome 22ReciprocaltranslocationFigure Translocation associated with chronic myelogenous leukemia (CML).Translocated chromosome 9Translocated chromosome 22 (Philadelphia chromosome)
57Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance There are two normal exceptions to Mendelian geneticsOne exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleusIn both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance
58Genomic ImprintingFor a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traitsSuch variation in phenotype is called genomic imprintingGenomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production
59Normal Igf2 allele is expressed. Figure 15.17Normal Igf2 allele is expressed.Paternal chromosomeMaternal chromosomeNormal-sized mouse (wild type)Normal Igf2 allele is not expressed.(a) HomozygoteMutant Igf2 allele inherited from motherMutant Igf2 allele inherited from fatherNormal-sized mouse (wild type)Dwarf mouse (mutant)Figure Genomic imprinting of the mouse Igf2 gene.Normal Igf2 allele is expressed.Mutant Igf2 allele is expressed.Mutant Igf2 allele is not expressed.Normal Igf2 allele is not expressed.(b) Heterozygotes
60It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotidesGenomic imprinting is thought to affect only a small fraction of mammalian genesMost imprinted genes are critical for embryonic development
61Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasmMitochondria, chloroplasts, and other plant plastids carry small circular DNA moleculesExtranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the eggThe first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant
62Figure 15.18Figure Variegated leaves from English holly (Ilex aquifolium).
63Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systemsFor example, mitochondrial myopathy and Leber’s hereditary optic neuropathy
64Sperm Egg C c B b D A d E a P generation gametes e F f Figure 15.UN03SpermEggCcBbDAdEaP generation gameteseFfThe alleles of unlinked genes are either on separate chromosomes (such as d and e) or so far apart on the same chromosome (c and f) that they assort independently.This F1 cell has 2n 6 chromo- somes and is heterozygous for all six genes shown (AaBbCcDdEeFf). Red maternal; blue paternal.DeCBdFigure 15.UN03 In-text figure, p. 294AGenes on the same chromo- some whose alleles are so close together that they do not assort independently (such as a, b, and c) are said to be genetically linked.Each chromosome has hundreds or thousands of genes. Four (A, B, C, F) are shown on this one.EFfacb
65Figure 15.UN04Figure 15.UN04 In-text figure, p. 294
66Figure 15.UN05Figure 15.UN05 In-text figure, p. 294
67Figure 15.UN06Figure 15.UN06 In-text figure, p. 294