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Genes and ChromosomesThe Chromosome Theory of HeredityMutationsRegulation of Gene Expression

ObjectivesState the chromosome theory of heredityExplain how gene linkage affects inherited traitsDescribe the process of crossing-over and explain how it increases genetic varietyDescribe gene mappingDescribe the process of sex determination and the patterns of inheritance for sex-linked traitsChromosome Theory of HeredityMendels work was incomplete because he never asked an important question: Where in the cell are the factors that control heredity? Where are the genes?

What do you see in this picture?What do you recall about chromosomes?

Chromosome Theory and HeredityKey termsChromosome theory of heredityLinked geneLinkage groupRecombinantSex chromosomeAutosomeX chromosomeY chromosomeSex-linked

Genes and ChromosomesNucleuscontains chromsomeschromosome=threadlike structure in a cell that contains the genetic information that is passed on from one generation of cells to the next

Genes and ChromosomesWalter Sutton1902, discovered gene locationChromosome theory of heredity genes are located on the chromosomes and each gene occupies a specific place on a chromosomeEach gene may exist in several forms or allelesEach chromosome contains just one of the alleles for each of its genesSuttons development of the chromosome theory is an example of how the work of one scientist builds on the work of another scientist

Gene LinkageGenes on a chromosome are linked togetherThey are inherited togetherLinked genes do not undergo independent assortment Linked genes=genes that are inherited in a groupGene LinkageThomas Hunt Morgan studied drosphiliaEffects of gene linkageMorgan crossed purebred gray bodies and normal wings with purebred black bodies and small wingsGray (G) black (g) Normal wings (W) small wings (w)F1 should have been gray with normal wings (GgWw)When F1 crossed with black small-winged drosphilia (ggww) Morgan did not observe the expected resultsMost gray-bodied drosphilia had normal wings and most black-bodied flies had small wingsGene for body color and gene for wing size were somehow connected, or linkedThey could not assort independently

Gene LinkageLinkage groupsMorgan studied more and more genesDiscovered genes fell into distinct linkage groups of genes that always tended to be inherited togetherThe linkage groups (chromosomes) assorted independently, but all genes on one group were inherited togetherBecause homologous chromosomes contain the same genes, there is one linkage group for every homologous pair of chromosomes(drosphilia has four linkage groups, four pairs of chromsomes)A cobra has 38 chromosomes. How many linkage groups would this make? Tomorrow!Crossing over in linked genesGene mappingSex linkageCrossing-overDuring prophase I of meiosis, homologous chromosomes may exchange sections of their chromatids in a process called crossing-overIncreases genetic variety

Crossing-OverLinkage groups explains some of the results of the drosphilia crosses but does not provide a complete explanation83% have gene combinations like their parents17% have new gene combinationsRecombinants=individuals with new combinations of genesCrossing-OverIf the genes for body color and wing size are linked, why arent they linked all the time?Morgan proposed that linkages could be broken some of the timeIf two homologous chromosomes were positioned side by side, sections of the two chromosomes might cross, break, and reattach.This process would rearrange the genes on the chromosome and produce new linkage groupsGene MappingFurther reasoned that crossing-over occurs at random along the linkage groups, and the distance between two genes determines how often crossing-over occurs between themClose together crossing-over is rareFar apart crossing-over more common

Gene MappingKnowing the frequency with which crossing-over between two genes occurs makes it possible to map the positions of genes on a chromosomeToday we have detailed maps of Drosphilia that pinpoint the locations of more than 1500 different genesSex Linkage1905 American biologist Nettie Stevens discovered that not every chromosome has a corresponding homologous chromosomeDiscovered female mealworm contain 20 large chromsomes and male contain19 large and one small One of male chromosome pairs is not homologousThe pair has very different shapesSame thing was found in drosphiliaSex LinkageThese mismatched chromosmes are the sex chromosomesfemale sex chromosomes=two matching sex chromosomes (XX)Male sex chromosomes=two dissimilar sex chromosomes(XY)Y chromosome=small and hook shapedThe other chromosomes, which are the same in both males and females, are called autosomes

Sex LinkageSex determinationThe sex chromosomes in the males gametes determine the sex of the offspringXYXXXXYXXXXYSex LinkageSex determinationWhen female gametes are produced, meiosis separates one of the X chromosomes into each egg cellIn the male, meiosis separates the X and Y chromosomes so that 50% carry X chromosome and 50% carry Y chromosomeWhen an X sperm fertilizes an egg a female is formedWhen a Y sperm fertilizes an egg a male is formedSex LinkageGenes on sex chromosomesSex chromosomes also carry genes that affect other traitsSex-Linked a gene located on one of the sex chromosomesNon-DisjunctionOccurs during meiosisSome gametes contain extra chromosomesSome gametes are missing chromosomesKlinefelters SyndromeMale XXY sex chromosomesSterileShow female characteristicsUnderdeveloped testesBreast developmentPoor beard growth

Turners SyndromeFemaleX_Mental retardationSterileShort in statureUnderdeveloped ovariesIncreased chance of thyroid problemsTomorrow!Gene Mapping Lab!Board work 21How are genes related to chromosomes?How does crossing-over make genetic mapping possible?What are sex chromosomes? Autosomes?Why are the effects of recessive sex-linked alleles seen more often in males than in females?Mutations

MutationsMistakes in duplicating genetic info and transmitting it to the next generation are rare, but they do occurMutations=change in the genetic material of the cellNot all are harmfulNo effectSlight effectHarmlessbeneficial

MutationsMutations in reproductive cells (germ cells)Germ mutationsinheritableMutations that affect other cells of the bodySomatic mutationsCancer2 levelsChromosomal mutationsGene mutations

Chromosomal mutationsInvolveSegments of chromosomesWhole chromosomesEntire sets of chromosomesResults in change in number or structure4 typesdeletionsDuplicationsInversionstranslocationsChromosomal mutationsDeletionsThe loss of part of a chromosome

Chromosomal mutationsDuplicationsOpposite of deletion, segment of chromosome is repeated

Chromosomal mutationsInversionsPart of a chromosome becomes reversed

Chromosomal mutationsTranslocationsPart of one chromosome breaks off and attaches to another, nonhomologous chromosome

Chromosomal mutationsNondisjunctionsInvolve whole chromosomes or complete sets of chromosomesFailure of homologous chromosomes to separate normally during meiosisNot coming apart1 chromosome involveextra copy in one cell and loss from anotherMore than 1dramatic increase in number, producing triploid (3N) or tetraploid (4N) organismsExtra sets of chromosomespolyploidyAlmost always fatal in animalsPlants are often larger and hardierGene mutationsInvolveIndividual genesCauseChemical change that affects DNAGene mutationsPoint mutationsaffect no more than a single nucleotide

Gene mutationsInsertion or deletion of nucleotideFrameshift mutationscompletely change the polypeptide product produced by a gene

Board Work 22Compare a chromosomal mutation and a gene mutation.What is a somatic mutation? How does it differ from a germ mutation?How does nondisjunction result in chromosomal mutations?Regulation of Gene ExpressionGene interactionsIncomplete dominanceCodominancePolygenic inheritanceGene expression in prokaryotesOperonRepressorGene activationGene expression in eukaryotes

Regulation of Gene ExpressionAs biologists have intensified their studies of gene activity, it has become clear that interactions between different genes and between genes and their environment are critically important

Gene InteractionsDominanceHow genes interact with each otherRemember.A gene is a section of DNAcodes for a polypeptideDominant allele codes codes for a specific polypeptide that works, recessive for one that does not workGene InteractionsIncomplete dominanceInheritance in which an active allele does not entirely compensate for an inactive allele

rrRRrRrRRrRrRrRRRRrrRrrrR=redr=whiteF1 generationAll pinkRed carnationWhite carnationPink carnationPink carnationF2 generation1 red2 pink1 whiteGene InteractionsCodominanceCondition in which both alleles of a gene are expressedWritten as capital letters with subscripts or superscriptsEx: B1 and B2 or R and RSeen in many organismscattle=red hair is codominant with white hair (HRHW)Look roan or pinkish whiteChickens=black feather are codominant with white feathers (FBFW)Erminant chickens (speckled black and white)

Gene InteractionsPolygenic InheritanceA trait that is controlled by two or more genesMany traits are produced by the interaction of many genespolygenicShape of your noseColor and markings on an animals coat

Gene Expression in ProkaryotesThe genes of a single organism cannot be activated ate the same timeMake many molecules it did not needWaste energyMust be able to produce the product of a gene quickly and in adequate amounts

Gene Expression in ProkaryotesWhen the product of a gene (a specific protein) is being actively produced by a cell, we say that the gene is being expressedWithin a single organism, some gene are rarely expressed, some are constantly expressed, and some are expressed for a time and then turned offHow does a cell know when to make a protein and when not toTurn off and turn on?Gene Expression in ProkaryotesThe OperonGenes and regions of DNA that operate together; consists of a gene cluster and regions involved in the regulation and expression of that clusterConsists ofOperator=region of chromosome near the cluster of genes in an operon to which the repressor binds when the operon is turned offPromoter=region of chromosome next to the operator in an operon to which RNA polymerase binds at the beginning of transcriptionGene Expression in ProkaryotesThe OperonThe gene cluster in the operon studied by Jacob and Monod produces enzymes that break down lactoseBacteria does not produce enzymes in large amounts unless lactose is presentLactose induces production of enzymes to break down lactose for use as foodThis operon systeminducer because it induces the production of enzymesEnzymes not produced in the absence of lactoseGene Expression in ProkaryotesThe RepressorWhen repressor nears the operator it attaches itself to the operator so that it sits between the promoter and the genesPosition blocks the access of RNA polymeraseGene Expression in EukaryotesInducers induce the activation of genesBind directly to DNA and either start or increase transcription of particular genesmRNA produced during transcription may be altered before it is used to make protein during translationThe presence of DNA sequences that do not code for proteinExons=sequences that are complementary code for protein expressedIntrons=segments that are not complementary and do not code for protein interveningBoard work 23 How do gene interactions affect gene expression?Compare incomplete dominance and codominance.What is a polygenic trait?Mutations in introns are less likely to affect phenotype than mutations in exons.? Defend or refute this statement.