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1Mendel and the Gene Idea Chapter 14Mendel and the Gene Idea
2One possible explanation of heredity is a “blending” hypothesis What genetic principles account for the transmission of traits from parents to offspring?One possible explanation of heredity is a “blending” hypothesisThe idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make greenAn alternative to the blending model is the “particulate” hypothesis of inheritance: the gene ideaParents pass on discrete heritable units, genes
3Gregor MendelMendel used the scientific approach to identify two laws of inheritanceMendel discovered the basic principles of heredity by breeding garden peasVocabularyCharacter: a heritable feature, such as flower colorTrait: a variant of a character, such as purple or white flowers
4Mendel also made sure that Mendel chose to trackOnly those characters that varied in an “either-or” mannerEx: Flower color trait is either purple or white, there is no intermediateMendel also made sure thatHe started his experiments with varieties that were “true-breeding”all successive generations display only the desired traitEx: A purple-flowered plant is self-pollinated and all the offspring have purple flowers
5Mendel’s work P F1 F2 Bred pea plants Pollen transferred from white flower to stigma of purple flowerBred pea plantscross-pollinate two true breeding parents (P)hybridizationP = parentalraised seed & then observed traits (F1)Hybrid offspringF = filialallowed offspring to self-pollinate & observed next generation (F2)PanthersremovedP = parentsF = filial generationall purple flowers resultF1self-pollinateF2
6Looking closer at Mendel’s work true-breedingpurple-flower peastrue-breedingwhite-flower peasXPWhere didthe white flowers go?100%F1generation(hybrids)purple-flower peasIn a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.White flowers came back!self-pollinateF2generation3:175%purple-flower peas25%white-flower peas
7Mendel reasoned thatIn the F1 plants, only the purple flower factor was affecting flower color in these hybridsPurple flower color was dominant, and white flower color was recessiveTable 14.1Mendel observed the same patternIn many other pea plant characters
8Mendel’s Experiments and Observations Allowed Mendel to deduce two fundamental laws of heredity:Law of SegregationLaw of Independent Assortment
9Mendel’s Model Mendel developed a hypothesis To explain the 3:1 inheritance pattern that he observed among the F2 offspringFour related concepts make up this modelAlternative versions of genes (alleles)Each Allele is represented twiceIf two alleles differ, the dominant one is expressedTwo alleles segregate during meiosis
10What did Mendel’s findings mean? Traits come in alternative versionspurple vs. white flower colorallelesdifferent alleles vary in the sequence of nucleotides at the specific locus of a genesome difference in sequence of A, T, C, Gpurple-flower allele & white-flower allele are two DNA variations at flower-color locusdifferent versions of gene at same location on homologous chromosomes
11Traits are inherited as discrete units For each characteristic, an organism inherits 2 alleles, 1 from each parentdiploid organisminherits 2 sets of chromosomes, 1 from each parenthomologous chromosomesA genetic locus is actually represented twice, one on each homolog of a pair of chromosomesTwo alleles may be identical or different
12What did Mendel’s findings mean? Some traits mask otherspurple & white flower colors are separate traits that do not blendpurple x white ≠ light purplepurple masked whitedominant allelefunctional proteinmasks other allelesrecessive alleleallele makes a malfunctioning proteinI’ll speak for both of us!wild type allele producing functional proteinmutant allele producing malfunctioning proteinhomologous chromosomes
13Fourth, the law of segregation PPPLaw of segregationduring meiosis, alleles segregatehomologous chromosomes separateeach allele for a trait is packaged into a separate gameteAn egg or sperm only receives one of the two alleles present in the somatic cellpppPpPp
14Law of Segregation Metaphase 1 Which stage of meiosis creates the law of segregation?Metaphase 1Whoa! And Mendel didn’t even know DNA or genes existed!
15Genotype vs. phenotype X Difference between how an organism “looks” & its geneticsphenotypedescription of an organism’s traitthe “physical”genotypedescription of an organism’s genetic makeupF1PXpurplewhiteall purpleExplain Mendel’s results using…dominant & recessive…phenotype & genotype
16PP pp Pp Making crosses x X Can represent alleles as letters flower color alleles P or ptrue-breeding purple-flower peas PPtrue-breeding white-flower peas ppF1PXpurplewhiteall purplePPxppPp
17Mendel’s law of segregation, probability and the Punnett square Try a cross: Pp x PpP GenerationF1 GenerationF2 GenerationPpPpPPppAppearance: Genetic makeup:Purple flowers PPWhite flowers ppPurple flowers PpGametes:F1 spermF1 eggs1/2Each true-breeding plant of theparental generation has identicalalleles, PP or pp.Gametes (circles) each contain onlyone allele for the flower-color gene.In this case, every gamete producedby one parent has the same allele.Union of the parental gametesproduces F1 hybrids having a Ppcombination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.When the hybrid plants producegametes, the two alleles segregate,half the gametes receiving the Pallele and the other half the p allele.3: 1Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation.This box, a Punnett square, showsall possible combinations of allelesin offspring that result from anF1 F1 (Pp Pp) cross. Each squarerepresents an equally probable productof fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.
18Genotypes Homozygous = same alleles = PP, pp True-breeding, all sperm/egg contain PHeterozygous = different alleles = Pp½ sperm/egg contain P other ½ contains phomozygous dominantCan’t tell by lookin’ at ya!heterozygousHow do you determine the genotype of an individual with with a dominant phenotype?homozygous recessive
19Test cross pp x is it PP or Pp? Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allelexHow does that work?is it PP or Pp?pp
20How does a Test cross work? xxAm I this?Or am I this?PPppPpppppppPPPpPpPpPpPpPpPppppp100% purple50% purple:50% white or 1:1
21The Law of Independent Assortment Mendel derived the law of segregationBy following a single traitThe F1 offspring produced in this crossWere monohybrids, heterozygous for one characterCrossing two heterozygotes is a monohybrid crossF1xPpxPp
22The Law of Independent Assortment Mendel identified his second law of inheritanceBy following two characters at the same timeSee color & seed shapeCrossing two, true-breeding parents differing in two charactersProduces dihybrids in the F1 generation, heterozygous for both charactersY = yellowR = roundy = greenr = wrinkledPxyyrrYYRR
23Phenotypic ratio approximately 9:3:3:1 How are two characters transmitted from parents to offspring?As a package?Independently?A dihybrid crossIllustrates the inheritance of two charactersProduces four phenotypes in the F2 generationYYRRP GenerationGametesYRyryyrrYyRrHypothesis ofdependentassortmentindependentF2 Generation(predictedoffspring)1⁄21 ⁄23 ⁄41 ⁄4SpermEggsPhenotypic ratio 3:1YryR9 ⁄163 ⁄161 ⁄16YYRrYyRRYyrrYYrryyRRyyRrPhenotypic ratio 9:3:3:131510810132Phenotypic ratio approximately 9:3:3:1F1 GenerationRESULTSCONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.9:3:3:1Figure 14.8
24What’s going on here? YyRr YyRr YR yr YR Yr yR yr Is it this? Or this? If genes are on different chromosomes…how do they assort in the gametes?together or independently?YyRrIs it this?Or this?YyRrYRyrYRYryRyrWhich system explains the data?
25 Is this the way it works? YyRr x YyRr YR yr YR YYRR YyRr yr YyRr 9/16yellowroundYRyr3/16greenroundWell, that’s NOT right!YRYYRRYyRr3/16yellowwrinkledyrYyRryyrr1/16greenwrinkled
26 Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr YYRR YYRr YyRR orYyRrxYyRr9/16yellowroundYRYryRyrYRYryRyr3/16greenroundYYRRYYRrYyRRYyRrBINGO!YYRrYYrrYyRrYyrr3/16yellowwrinkledYyRRYyRryyRRyyRr1/16greenwrinkledYyRrYyrryyRryyrr
27Mendel’s 2nd law of heredity Using the information from a dihybrid cross, Mendel developed the law of independent assortmentEach pair of alleles segregates independently during gamete formationWorks for alleles on different chromosomes (chromosomes that are not homologous)Or genes far apart from each other on the same chromosome that frequently cross over
28Law of Independent Assortment Which stage of meiosis creates the law of independent assortment?Metaphase 1EXCEPTIONIf genes are on same chromosome & close togetherwill usually be inherited togetherrarely crossover separately“linked”
30Review: Mendel’s laws of heredity Law of segregationmonohybrid crosssingle traiteach allele segregates into separate gametesestablished by Metaphase 1Law of independent assortmentdihybrid (or more) cross2 or more traitsgenes on separate chromosomes assort into gametes independentlyEXCEPTIONlinked genes
31Concept Check 14.1A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a Punnett square for this cross.Pea plants heterozygous for flower position and stem length (AaTt) are allowed to self pollinate, and 400 of the resulting seeds are plants. How many offspring would be predicted to have terminal flowers and be dwarf?
32Concept 14.2: The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortmentReflect the rules of probabilityThe multiplication ruleFinding the probability that two or more independent events will occur together:Multiply the probability of one event by the probability of the other evenEx: Probability of 2 offspring from the same parents are both homozygous recessive?
33What is the likelihood that an offspring is heterozygote? Probability in a monohybrid crossCan be determined using this ruleThe rule of additionStates that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilitiesOne or more possibilities that can occur in the same eventRrSegregation ofalleles into eggsalleles into spermRr1⁄21⁄4SpermEggsFigure 14.9What is the likelihood that an offspring is heterozygote?¼ + ¼ = ½What is the likelihood two offspring from the same parents are both homozygous recessive?¼ x ¼ = 1/16
34Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probabilityTo predict the outcome of crosses involving multiple charactersA dihybrid or other multicharacter crossIs equivalent to two or more independent monohybrid crosses occurring simultaneouslyIn calculating the chances for various genotypes from such crossesEach character first is considered separately and then the individual probabilities are multiplied together
35Concept Check 14.2For any gene with a dominant allele C and recessive allele c, what proportions of the offspring from a CC x Cc cross are expected to be homozygous dominant, homozygous recessive and heterozygous?An organism with the genotype BbDD is mated to one with the genotype BBDd. Assuming independent assortment of these two genes, write the genotypes of all possible offspring from this cross and use the rules of probability to calculate the chance of each type occurring.
36The relationship between genotype and phenotype is rarely simple Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian geneticsThe relationship between genotype and phenotype is rarely simpleThe inheritance of characters by a single geneMay deviate from simple Mendelian patterns
37The Spectrum of Dominance Complete dominanceOccurs when the phenotypes of the heterozygote and dominant homozygote are identicalIn incomplete dominanceThe phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varietiesP GenerationF1 GenerationF2 GenerationRedCRCRGametesCRCWWhiteCWCWPinkCRCWSpermCw1⁄2EggsCR CRCR CWCW CWRRRWWW
38Co-dominance 2 alleles affect the phenotype equally & separately not blended phenotypehuman ABO blood groupsMultiple Alleles: 3 allelesIA, IB, iIA & IB alleles are co-dominantglycoprotein antigens on RBCIAIB = both antigens are producedi allele recessive to both
39The Relation Between Dominance and Phenotype Dominant and recessive allelesDo not really “interact”Dominant alleles do not “subdue” recessive allelesLead to synthesis of different proteins that produce a phenotypeEx: Tay Sachs Disease: autosomal recessive inheritance patternFrequency of Dominant AllelesDominant allelesAre not necessarily more common in populations than recessive allelesEx: Polydactyly: occurs in 1 in 400 births; autosomal dominant
40Pleiotropy In pleiotropy A gene has multiple phenotypic effects Most genes are pleiotrophicEx: A genetic disease caused by a single allele has many symptoms associated with itOne gene can affect many characteristics in an organism
41Extending Mendelian Genetics for Two or More Genes Some traitsMay be determined by two or more genesThis type of expression includes:EpistasisPolygenic Inheritance
42Epistasis One gene completely masks another gene coat color in mice = 2 separate genesC,c: pigment (C) or no pigment (c)B,b: more pigment (black=B) or less (brown=b)cc = albino, no matter B allele9:3:3:1 becomes 9:3:4B_C_B_C_bbC_bbC__ _cc_ _ccHow would you know that difference wasn’t random chance?Chi-square test!
43Epistasis in Labrador retrievers 2 genes: (E,e) & (B,b)pigment (E) or no pigment (e)pigment concentration: black (B) to brown (b)eebbeeB–E–bbE–B–
44Polygenic inheritance Some phenotypes determined by additive effects of 2 or more genes on a single characterphenotypes on a continuumhuman traitsskin colorheightweightintelligencebehaviors
45Skin color: AlbinismHowever albinism can be inherited as a single gene traitaa = albinoenzymetyrosinealbinismmelanin
46Environmental effects Phenotype is controlled by both environment & genesMultifactorial charactersHuman skin color is influenced by both genetics & environmental conditionsThe relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debatea single tree has leaves that vary in size, shape & color, depending on exposure to wind & sunfor humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence testseven identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiencesCoat color in arctic fox influenced by heat sensitive allelesColor of Hydrangea flowers is influenced by soil pH
47Concept Check 14.3If a man with type AB blood marries a woman with type O blood, what blood types would you expect in their children?A rooster with gray feathers is mated with a hen of the same phenotype. Among their offspring, 15 chicks are gray, 6 are black and 8 are white. What is the simplest explanation for the inheritance of these colors in chickens? What phenotypes would you expect in the offspring of a cross between a gray rooster and a black hen?
48Pedigree analysisPedigree analysis reveals Mendelian patterns in human inheritancedata mapped on a family tree= male= female= male w/ trait= female w/ trait
49Simple pedigree analysis What’s the likely inheritance pattern?Simple pedigree analysis123456123456
50Genetic counselingPedigree can help us understand the past & predict the futureThousands of genetic disorders are inherited as simple recessive traitsfrom benign conditions to deadly diseasesalbinismcystic fibrosisTay sachssickle cell anemiaPKU
51Recessive diseases A a AA Aa A a Aa aa The diseases are recessive because the allele codes for either a malfunctioning protein or no protein at allHeterozygotes (Aa)carriershave a normal phenotype because one “normal” allele produces enough of the required proteinAamale / spermAAAaAafemale / eggscarrierAaaacarrierdisease
52Cystic fibrosis (recessive) Primarily whites of European descentstrikes 1 in 2500 births1 in 25 whites is a carrier (Aa)normal allele codes for a membrane protein that transports Cl- across cell membranedefective or absent channels limit transport of Cl- & H2O across cell membranethicker & stickier mucus coats around cellsmucus build-up in the pancreas, lungs, digestive tract & causes bacterial infectionswithout treatment children die before 5; with treatment can live past their late 20snormal lung tissueCystic fibrosis is an inherited disease that is relatively common in the U.S. Cystic fibrosis affects multiple parts of the body including the pancreas, the sweat glands, and the lungs. When someone has cystic fibrosis, they often have lots of lung problems. The cause of their lung problems is directly related to basic problems with diffusion and osmosis in the large airways of the lungs.People without cystic fibrosis have a small layer of salt water in the large airways of their lungs. This layer of salt water is under the mucus layer which lines the airways. The mucus layer in the airways helps to clear dust and other inhaled particles from the lungs.
53Effect on Lungs Chloride channel Cl– Cl– bacteria & mucus build up transports salt through protein channel out of cellOsmosis: H2O follows Cl–Effect on Lungsnormal lungsairwayCl–Cl– channelH2Ocells lining lungsIn people without cystic fibrosis, working cystic fibrosis proteins allow salt (chloride) to enter the air space and water follows by osmosis. The mucus layer is dilute and not very sticky.In people with cystic fibrosis, non-working cystic fibrosis proteins mean no salt (chloride) enters the air space and water doesn't either. The mucus layer is concentrated and very sticky.People with cystic fibrosis have lung problems because:Proteins for diffusion of salt into the airways don't work. (less diffusion)Less salt in the airways means less water in the airways. (less osmosis)Less water in the airways means mucus layer is very sticky (viscous).Sticky mucus cannot be easily moved to clear particles from the lungs.Sticky mucus traps bacteria and causes more lung infections.Therefore, because of less diffusion of salt and less osmosis of water, people with cystic fibrosis have too much sticky mucus in the airways of their lungs and get lots of lung infections. Thus, they are sick a lot.cystic fibrosisCl–H2Obacteria & mucus build upthickened mucus hard to secretemucus secreting glands
54Tay-Sachs (recessive) Primarily Jews of eastern European (Ashkenazi) descent & Cajuns (Louisiana)strikes 1 in 3600 births100 times greater than incidence among non-Jewsnon-functional enzyme fails to breakdown lipids in brain cellsfats collect in cells destroying their functionsymptoms begin few months after birthseizures, blindness & degeneration of muscle & mental performancechild usually dies before 5yo
55Sickle cell anemia (recessive) Primarily Africansstrikes 1 out of 400 African Americanshigh frequencycaused by substitution of a single amino acid in hemoglobinwhen oxygen levels are low, sickle-cell hemoglobin crystallizes into long rodsdeforms red blood cells into sickle shapesickling creates pleiotropic effects = cascade of other symptoms
56Doctors can use regular blood transfusions to prevent brain damage and new drugs to prevent or treat other problems.
57Sickle cell phenotype 2 alleles are codominant both normal & mutant hemoglobins are synthesized in heterozygote (Aa)50% cells sickle; 50% cells normalcarriers usually healthysickle-cell disease triggered under blood oxygen stressexercise
58Dominantly Inherited Disorders Some human disordersAre due to dominant allelesDominant alleles that cause lethal disease are much less commonEx: achondroplasia: a form of dwarfism that is lethal when homozygous for the dominant alleleWhat is the chance that two married dwarves with achondroplasia would have a child who was of normal height?
59Heterozygote advantage Malariasingle-celled eukaryote parasite spends part of its life cycle in red blood cellsIn tropical Africa, where malaria is common:homozygous dominant individuals die of malariahomozygous recessive individuals die of sickle cell anemiaheterozygote carriers are relatively free of bothreproductive advantageHigh frequency of sickle cell allele in African Americans is vestige of African roots
60Inheritance pattern of Achondroplasia Aa x aaAa x Aadominant inheritanceaaAaAaAaAAAaAAdwarfdwarflethalaaaaaaAaaa50% dwarf:50% normal or 1:167% dwarf:33% normal or 2:1
61Huntington’s chorea (dominant) Dominant inheritancerepeated mutation on end of chromosome 4mutation = CAG repeatsglutamine amino acid repeats in proteinone of 1st genes to be identifiedbuild up of “huntingtin” protein in brain causing cell deathmemory lossmuscle tremors, jerky movements“chorea”starts at age 30-50early death10-20 years after start
62Multifactorial Disorders Many human diseasesHave both genetic and environment componentsExamples includeHeart disease, cancer, diabetes, and alcoholismThe hereditary component of these diseases is polygenic
63Genetic Testing and Counseling Based on Mendelian Genetics and Probability Rules Genetic counselorsCan provide information to prospective parents concerned about a family history for a specific diseaseUsing family historiesGenetic counselors help couples determine the odds that their children will have genetic disorders
64Tests for Identifying Carriers For a growing number of diseasesTests are available that identify carriers and help define the odds more accuratelyTests are available for Tay-Sachs, Sickle Cell Anemia, and Cystic Fibrosissequence individual genes
65Fetal & Newborn Testing In amniocentesisThe liquid that bathes the fetus is removed and testedIn chorionic villus sampling (CVS)A sample of the placenta is removed and tested(a) AmniocentesisAmnioticfluidwithdrawnFetusPlacentaUterusCervixCentrifugationA sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.(b) Chorionic villus sampling (CVS)FluidFetalcellsBiochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.SeveralweeksBiochemicaltestshoursChorionic viIIiA sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.Suction tubeInserted throughcervixKaryotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.Karyotyping