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1Mendel and the Gene Idea Chapter 14Mendel and the Gene Idea
2What genetic principles account for the transmission of traits from parents to offspring?
3One possible explanation of heredity is the “blending” hypothesis The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green
4An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea Parents pass on discrete heritable units, genes
5Gregor MendelDocumented a particulate mechanism of inheritance through his experiments with garden peasFigure 14.1
6He discovered the basic principles of heredity Concept 14.1: Mendel used the scientific approach to identify two laws of inheritanceHe discovered the basic principles of heredityBy breeding garden peas in carefully planned experimentsWhy Garden peas?
7Mendel’s Experimental, Quantitative Approach Mendel chose to work with peas for two main reasons;Because they are available in many varietiesBecause he could strictly control which plants mated with which
8Crossing pea plants Figure 14.2 Removed stamens from purple flower APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns ofinheritance. In this example, Mendel crossed pea plantsthat varied in flower color.2Transferred sperm-bearing pollen fromstamens of whiteflower to egg-bearing carpel ofpurple flowerParentalgeneration(P)TECHNIQUEStamens(male)Carpel(female)Pollinated carpelmatured into pod34Planted seedsfrom podWhen pollen from a white flower fertilizeseggs of a purple flower, the first-generation hybrids all havePurple flowers. The result is the same for the reciprocal cross,the transfer of pollen from purple flowers to white flowers.TECHNIQUERESULTSExaminedoffspring:all purpleflowers5Firstgenerationoffspring(F1)Figure 14.2
9Some genetic vocabulary Character: a detectable inheritable feature of an organism, such as flower colorTrait: a variant of a character, such as purple or white flowers
10Mendel also made sure that Mendel chose to trackOnly those characters that varied in an “either-or” mannerMendel also made sure thatHe started his experiments with varieties that were “true-breeding”
11In a typical breeding experiment Mendel mated two contrasting, true-breeding varieties, a process called hybridizationThe true-breeding parentsAre called the P generation; always producing offspring with the same trait as the parents.
12The hybrid offspring of the P generation Are called the F1 generationWhen F1 individuals are self-pollinatedThe F2 generation is produced.Because Mendel followed three generations ( P, F1 and F2) he discovered two lows of heredity:Law of SegregationLaw of independent assortment
13When Mendel crossed the F1 plants The Law of SegregationWhen Mendel crossed contrasting, true-breeding white and purple flowered pea plantsAll of the offspring were purple.Hypothesis; if the inheritable factor for white had been lost, then a cross between F1 plants should produce only purple flowers.When Mendel crossed the F1 plantsMany of the plants had purple flowers, but some had white flowers in a ratio of 3:1
14Mendel discoveredA ratio of about 3:1, purple to white flowers, in the F2 generationP Generation(true-breedingparents)PurpleflowersWhiteF1 Generation(hybrids)All plants hadpurple flowersF2 GenerationEXPERIMENT True-breeding purple-flowered peaplants and white-flowered pea plants were crossed(symbolized by ). The resulting F1 hybrids wereallowed to self-pollinate or were cross-pollinatedwith other F1 hybrids. Flower color was then observedin the F2 generation.RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. InMendel’s experiment, 705 plants had purple flowers,and 224 had white flowers, a ratio of about 3 purple: 1 white.Figure 14.3
15Mendel reasoned thatIn the F1 plants, only the purple flower factor was affecting flower color in these hybrids and it masks the white color.Mendel concluded that Purple flower color was dominant, and white flower color was recessive.
16Mendel observed the same pattern In many other pea plant characters Table 14.1
17Mendel developed a hypothesis Mendel’s ModelMendel developed a hypothesisTo explain the 3:1 inheritance pattern that he observed among the F2 offspringFour related concepts make up this model
18First, alternative versions of genes Account for variations in inherited characters, which are now called alleles (variations in genes nucleotide sequence).Figure 14.4Allele for purple flowersLocus for flower-color geneHomologouspair ofchromosomesAllele for white flowers
19Second, for each character An organism inherits two alleles, one from each parentA genetic locus is actually represented twice
20Third, if the two alleles at a locus differ Then one, the dominant allele, determines the organism’s appearanceThe other allele, the recessive allele, has no noticeable effect on the organism’s appearance
21Fourth, the law of segregation The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
22Useful Genetic Vocabulary Homozygous: having two identical alleles for a given trait (e.g. PP or pp).all gametes carry that allele.Homozygotes are true breeding.Heterozygous: having two different alleles for a trait (e.g. Pp).half of the gametes carries one allele (P) and the remaining half carries the other (p).heterozygotes are not true breeding.
23Useful Genetic Vocabulary Phenotype: An organisms appearance (e.g. purple or white flowers).in Mendel’s experiment, F2 generation had a 3:1 phenotypic ratio.Genotype: An organism genetic make up e.g. PP or Pp or pp)the genotypic ratio of the F2 generation was 1:2:1 (1 PP:2Pp:1pp).
24QuestionDoes Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?We can answer this question using a Punnett square
25Mendel’s law of segregation, probability and the Punnett square Each true-breeding plant of the parental generation has identical alleles, PP or pp.Gametes (circles) each contain only one allele for the flower-color gene.In this case, every gamete produced by one parent has the same allele.P GenerationF1 GenerationF2 GenerationAppearance: Genetic makeup:Purple flowers PPWhite flowers ppGametes:PpUnion of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, all these hybrids have purple flowers.When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the Pallele and the other half the p allele.Appearance: Genetic makeup:Purple flowers PpGametes:1/21/2PpThis box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized bya P sperm.F1 spermPpPPPPpF1 eggspPpppRandom combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation.3: 1Figure 14.5
26Phenotype versus genotype Figure 14.6312PhenotypePurpleWhiteGenotypePP(homozygous)Pp(heterozygous)ppRatio 3:1Ratio 1:2:1
27In pea plants with purple flowers The TestcrossIn pea plants with purple flowersThe genotype is not immediately obviousHow can we determine the genotype?By the Testcross
28A testcrossAllows us to determine the genotype of an organism with the dominant phenotype, but Unknown genotype. How?Crossing an individual with the dominant phenotype with an individual that is homozygous recessive for a trait.The result of a testcross can indicate whether an individual who has the dominant phenotype is heterozygous or homozygous dominant.If any of the offspring of a testcross have the recessive phenotype, the parent with the dominant phenotype must be heterozygous
29then 1⁄2 offspring purple The testcrossAPPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.Dominant phenotype,unknown genotype:PP or Pp?Recessive phenotype,known genotype:ppIf PP,then all offspringpurple:If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:pPPpTECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.RESULTSFigure 14.7
30The Law of Independent Assortment Mendel derived the law of segregationBy following a single traitThe F1 offspring produced in this crossWere monohybrids, heterozygous for only one character
31Mendel identified his second law of inheritance By following two characters at the same timeCrossing two, true-breeding parents differing in two characters (Eg. Stem length & pod shape; seed color & seed shape)Produces dihybrids in the F1 generation, heterozygous for both characters
32How are two characters transmitted from parents to offspring? Are these 2 traits transmitted from parents to offspring as:1- a package or2- are they inherited independentlyHypothesis:If the two characters segregate together, the F1 hybrids can only produce the same two classes of gametes (RY and ry) that they receive from parents.If the two characters segregate independently, the F1 hybrids will produce four classes of gametes (RY, Ry, rY and ry).
33Phenotypic ratio approximately 9:3:3:1 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.Figure 14.8
34Using the information from a dihybrid cross, Mendel developed the law of independent assortment Each pair of alleles segregates independently during gamete formation
35Concept 14.2: The laws of probability govern Mendelian inheritance. Mendel’s laws of segregation and independent assortment Reflects theRules of probability; We can apply this rule To predict the outcome of crosses involving multiple characters
36The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication ruleStates that the probability that two or more independent events will occur together is the product of their individual probabilities
37Probability in a monohybrid cross Can be determined using this ruleRrSegregation ofalleles into eggsalleles into spermRr1⁄21⁄4SpermEggsFigure 14.9
38Probability that an egg from F1 (Pp) will receive a (p) allele = ½ MonohybridsProbability that an egg from F1 (Pp) will receive a (p) allele = ½Probability that a sperm from F1 (Pp) will receive a (p) allele = ½The probability that two recessive alleles will unite at fertilization = ½ x ½ = ¼DihybridsFor a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR.Probability that an egg from a YyRr parent will receive the Y and the R alleles = ½ x ½ = 1/4Probability that a sperm from a YyRr parent will receive the Y and the R alleles = ½ x ½ = ¼Tthe overall probability of an F2 plant having the genotype YYRR= ¼ x ¼ = 1/16.
39Solving Complex Genetics Problems with the Rules of Probability Question; For the following cross: AaBbCc x AaBbCc, what is the probability of producing an offspring with the genotype aabbcc.Answer:Aa x Aa: probability for an aa offspring = ¼Bb x Bb: probability for an bb offspring = ¼Cc x Cc: probability for an cc offspring = ¼The probability that the offspring will be aabbcc is:¼ aa x ¼ bb x ¼ cc = 1/64.
40The rule of additionStates that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities.Example;The probability for one possible way of obtaining F2 heterozygote ( Rr) is ¼.Probability for the other possible way (rR) is also ¼.Using the rule of addition we can calculate the probability of an F2 heterozygote as ¼ + ¼ = ½
41Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian geneticsThe relationship between genotype and phenotype is rarely simple
42Extending Mendelian Genetics for a Single Gene The inheritance of characters by a single geneMay deviate from simple Mendelian patterns
43The Spectrum of Dominance Complete dominanceOccurs when the phenotypes of the heterozygote and dominant homozygote are identicalOther patterns of inheritance were not described by Mendel, but his laws can be extended to more complex cases as:
44I. Codominance (Full expression of both alleles in the heterozygote). Two dominant alleles affect the phenotype in separate, distinguishable waysThe human blood group MN Is an example of codominance (The MN blood group locus codes for the production of surface glycoproteins on RBC).In this system there are 3 blood types: M, N & MN. The result is a full phenotypic expression of both alleles in the heterozygote.That is, both molecules (M & N) are produced on the cell surface.
45II. Incomplete dominance (One allele is NOT dominant over the other, so the heterozygote has a phenotype that is intermediate between the parents).The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties. F2 generation have a phenotype ratio of 1 red: 2 pink: 1 white and not 3:1.P GenerationF1 GenerationF2 GenerationRedCRCRGametesCRCWWhiteCWCWPinkCRCWSpermCw1⁄2EggsCR CRCR CWCW CWFigure 14.10
46The Relation Between Dominance and Phenotype Dominant and recessive allelesDo not really “interact”Lead to synthesis of different proteins that produce a phenotype
47Frequency of Dominant Alleles Are not necessarily more common in populations than recessive alleles
48The ABO blood group in humans Multiple AllelesIII. Multiple alleles: Most genes exist in populations In more than two allelic formsE.g. ABO Blood groups in humans.The ABO blood group in humansIs determined by multiple alleles [3 alleles (IA , IB and i) producing 4 blood groups: A, B, AB and O].
49i allele codes for no Ag (neither A nor B). The IA , IB alleles codes for production of A and B antigens, respectively.i allele codes for no Ag (neither A nor B).Table 14.2
50IV. Pleiotropy ( from pleion which is Greek word means more): is the ability of a single gene to have multiple phenotypic effects (one gene is responsible for many traits).E.g. Sickle-cell anemia, a disease caused by a single defective gene but causes complex set of symptoms (phenotypes).
51Extending Mendelian Genetics for Two or More Genes VI. Epistasis( from the Greek word for stopping)Some traits may be determined by two or more genesE.g; Interaction between two different genes in which a gene at one locus alters (modifies) the phenotypic expression of a gene at a second locus.
52Epistasis…. Cont.If epistasis occurs between non-allelic genes, the phenotypic ratio resulting from a dihybrid cross will NOT be 9:3:3:1Example: coat color in mice is determined by two allels B and b.The coat is either black (BB or Bb) or white (bb), but the deposition of pigments (color) depends on another gene, C.CC, Cc = pigment deposited, cc = pigment NOT deposited (white).BB, Bb = black coat, bb = brown coat. This happens only if a mouse gets CC or Cc. However, if a mouse gets a cc, it will be white.
53An example of epistasis (The homozygous recessive bb & cc prevents the expression of the dominant B & C alleles and produces white mice all the time).BCbCBcbc1⁄4BBCcBbCcBBccBbccbbccbbCcBbCCbbCCBBCC9⁄163⁄164⁄16SpermEggs9/16 black, 3/16 brown, 4/16 whiteFigure 14.11
54Polygenic Inheritance VII. Polygenic Inheritance (converse of pleiotropy)Many human characters vary in the population along a continuum and are called quantitative charactersMany genes determine one phenotype.Each allele has the same degree of influence.The additive effect of two or more genes determines a single phenotypic character.Example: skin color in humans is determined by three separately inherited genes.· Three genes will be dark skin alleles (A, B, C) contribute one unit of darkness to the phenotype. These alleles are incompletely dominant over the (a, b, c) alleles.· AABBCC person would be very dark.· AaBbCc person would have intermediate skin color.
55The population shows a continuous variation (bell shape) AaBbCcaabbccAabbccAaBbccAABbCcAABBCcAABBCC20⁄6415⁄646⁄641⁄64Fraction of progenyFigure 14.12
60Multifactorial characters Are those that are influenced by both genetic and environmental factors
61Integrating a Mendelian View of Heredity and Variation An organism’s phenotypeIncludes its physical appearance, internal anatomy, physiology, and behaviorReflects its overall genotype and unique environmental history
62Even in more complex inheritance patterns Mendel’s fundamental laws of segregation and independent assortment still apply
63Humans are not convenient subjects for genetic research Concept 14.4: Many human traits follow Mendelian patterns of inheritanceHumans are not convenient subjects for genetic researchHowever, the study of human genetics continues to advance
64Pedigree Analysis A pedigree Is a family tree that describes the interrelationships of parents and children across generations
65Inheritance patterns of particular traits Can be traced and described using pedigreesWwwwWWorFirst generation(grandparents)Second generation(parents plus auntsand uncles)Thirdgeneration(two sisters)FfffFF or FfFFWidow’s peakNo Widow’s peakAttached earlobeFree earlobe(a) Dominant trait (widow’s peak)(b) Recessive trait (attached earlobe)Figure A, B
66PedigreesCan also be used to make predictions about future offspring
67Recessively Inherited Disorders Many genetic disordersAre inherited in a recessive manner
68Recessively inherited disorders Show up only in individuals homozygous for the alleleCarriersAre heterozygous individuals who carry the recessive allele but are phenotypically normal
69Symptoms of cystic fibrosis include Mucus buildup in the some internal organsAbnormal absorption of nutrients in the small intestine
70Sickle-Cell Disease Sickle-cell disease Symptoms include Affects one out of 400 African-AmericansIs caused by the substitution of a single amino acid in the hemoglobin protein in red blood cellsSymptoms includePhysical weakness, pain, organ damage, and even paralysis
71Mating of Close Relatives Matings between relativesCan increase the probability of the appearance of a genetic diseaseAre called consanguineous matings
72Dominantly Inherited Disorders Some human disordersAre due to dominant alleles
73One example is achondroplasia A form of dwarfism that is lethal when homozygous for the dominant alleleFigure 14.15
74Huntington’s disease Is a degenerative disease of the nervous system Has no obvious phenotypic effects until about 35 to 40 years of ageFigure 14.16
75Multifactorial Disorders Many human diseasesHave both genetic and environment componentsExamples includeHeart disease and cancer
76Genetic Testing and Counseling Genetic counselorsCan provide information to prospective parents concerned about a family history for a specific disease
77Counseling Based on Mendelian Genetics and Probability Rules Using family historiesGenetic counselors help couples determine the odds that their children will have genetic disorders
78Tests for Identifying Carriers For a growing number of diseasesTests are available that identify carriers and help define the odds more accurately
79In chorionic villus sampling (CVS) Fetal TestingIn amniocentesisThe liquid that bathes the fetus is removed and testedIn chorionic villus sampling (CVS)A sample of the placenta is removed and tested
80(b) Chorionic villus sampling (CVS) Fetal testing(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.KaryotypingFigure A, B
81Some genetic disorders can be detected at birth Newborn ScreeningSome genetic disorders can be detected at birthBy simple tests that are now routinely performed in most hospitals in the United States