Presentation on theme: "It also explains biological variation"— Presentation transcript:
1 It also explains biological variation MENDELIAN GENETICSWhat is genetics?The study of how traits are inherited or how genetic information is passed from one generation to the next.It also explains biological variation
2 Gregor Mendel 1850’s Grew up in a farm wanting to garden Austrian monk (Flunked out of college twice) but became a mathematicianExperimented with garden pea plantsUsing pea plants looked at seven different characters (height of plants, seed color, texture, flower color) and found evidence of how parents transmit genes to offspringMendel’s statistical analysis provided a model for predicting what the next generation would be like
3 What was the prevalent believe about inheritance before Mendel? People believed in “spontaneous generation” and in the “blending of characters”Blending theoryProblem:Would expect variation to disappearVariation in traits persistsEx: Yellow and green parakeets should have all blue babies. This is not what you observe.
4 The gene theoryAn alternative idea is the “gene” idea. Parents pass on discrete individual heritable units: genes
5 Experimental genetics began in an abbey garden Modern geneticsBegan with Gregor Mendel’s quantitative experiments with pea plantsPetalCarpelStamenFigure 9.2 BFigure 9.2 A
6 The Garden Pea PlantMendel chose to work with the pea plant because he could control which plant mated with which. Pea plants areSelf-pollinatingTrue breeding (different alleles not normally introduced)Can be experimentally cross-pollinated
7 Mendel crossed pea plants that differed in certain characteristics And traced traits from generation to generationMendel started his experiments with plants that were “true breeding”.1 Removed stamens from purple flowerWhite2 Transferredpollen from stamens of white flower to carpel of purple flowerStamensCarpelParents (P)Purple3 Pollinated carpel matured into pod4 Planted seeds from podOffspring (F1)Figure 9.2 C
8 Mendel hypothesized that there are alternative forms of genes The units that determine heritable traitsFlower colorFlower positionSeed colorSeed shapePod colorPod shapeStem lengthPurpleWhiteAxialTerminalRoundWrinkledInflatedConstrictedTallDwarfGreenYellowFigure 9.2 D
9 Mendel’s Principles of Genetics Mendel refuted the “blending theory” of heredity and provided an explanation of how inheritance works without knowing anything about chromosomes or genes.He figured that traits must be coded for by some kind of inheritable particle which he called “factors” and now we call “genes”.He said that those genes were transmitted as independent entities from one generation to the next.
10 Mendel’s insight continued… 3 Mendel’s insight continued… 3. He figured that there must be different versions of these “genes” ( we call them now “alleles”)and that every individual has two genes for each trait. (Or we can say that: For each characteristic an organism inherits two alleles, one from each parent) He identified one as dominant, the other as recessive.
11 4. He figured that the two alleles a parent has are separated into different cells when gametes (sex cells) are formed. This actually happens during metaphase of meiosisI ( no one knew about meiosis in those days). This is known as the Law of Segregation What are alleles? Different versions of the same gene
12 Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parentDuring gamete formation, the alleles segregate from each other
13 Mendel’s law of segregation Predicts that allele pairs separate from each other during the production of gametesP plantsGametesGenetic makeup (alleles)F1 plants(hybrids)F2 plantsPPppAll PAll pAll PpSperm12PpPpEggsGenotypic ratio1 PP : 2 Pp: 1 ppPhenotypic ratio3 purple : 1 whiteFigure 9.3 B
14 Mendel’s law of segregation describes the inheritance of a single characteristic From his experimental dataMendel deduced that an organism has two genes (alleles) for each inherited characteristicP generation(true-breeding parents)F1 generationF2 generationPurple flowersWhite flowersAll plants have purple flowersFertilization among F1 plants (F1 F1)of plants have purple flowers34of plants have white flowers1Figure 9.3 A
15 What is a dominant trait What is a dominant trait? The trait that shows, the allele that is fully expressed What is a recessive trait? The alleles that is masked, the gene is there but it doesn’t show What is the phenotype? The observable traits What is the genotype? The genetic make up
16 If the two alleles of an inherited pair differ Then one determines the organism’s appearance and is called the dominant allele ( use capital letters)The other alleleHas no noticeable effect on the organism’s appearance and is called the recessive allele
17 VocabularyWhen you mate two contrasting true breeding plants you get a Hybrid.The true breeding parents are called the “P” (parent) generationThe hybrid offspring of the P generation are called the F1 generationWhen two F1 individuals self pollinate you get the F2 generation
20 Mendel’s Monohybrid Cross Results 5,474 round1,850 wrinkled6,022 yellow2,001 green882 inflated299 wrinkled428 green152 yellowF2 plants showed dominant-to-recessive ratio that averaged 3:1705 purple224 white651 long stem207 at tip787 tall277 dwarf
21 Punnett Square of a Monohybrid Cross Female gametesMalegametesA aAaAaAAaaDominantphenotype canarise 3 ways,recessive onlyone
22 A Test crossIn a pea plant with purple flowers the genotype is not obvious. Could be homozygous or heterozygousWhy do a test cross?It allows us to determine the genotype of an organism with a dominant phenotype but unknown genotype
23 Test CrossYou cross an individual that shows the dominant phenotype with an individual with recessive phenotype ( one who is homozygous recessive for that trait)Examining offspring allows you to determine the genotype of the dominant individual
24 Punnett Squares of Test Crosses Homozygousrecessivea aAaaaAaHomozygousrecessivea aAAaTwo phenotypesAll dominant phenotype
25 Geneticists use the testcross to determine unknown genotypes The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individualCan reveal the unknown’s genotypeTestcross:GenotypesGametesOffspringB_bbTwo possibilities for the black dog:BB or BbBbBbAll black1 black : 1 chocolateFigure 9.6
26 Homologous chromosomes bear the two alleles for each characteristic Alternative forms of a geneReside at the same locus on homologous chromosomesGenotype:PPaaBbHeterozygousPabBGene lociRecessive alleleDominant alleleHomozygous for the dominant alleleHomozygous for the recessive alleleFigure 9.4
28 Mendel’s two Laws 1. Law of segregation The two alleles for a trait segregate during gamete formation and only one allele for a trait is carried in a gamete. The gametes combine at random(In other words:A cell contains two copies of a particular gene, they separate when a gamete is made).2. Law of Independent AssortmentAlleles from one trait behave independently from alleles for another trait. Traits are inherited independently from one another
29 Independent Assortment Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other traitMembers of each pair of homologous chromosomes are sorted into gametes at random during meiosis
30 The law of independent assortment is revealed by tracking two characteristics at once By looking at two characteristics at onceMendel tried to determine how two characteristics were inherited
31 Actual results support hypothesis Mendel’s law of independent assortmentStates that alleles of a pair segregate independently of other allele pairs during gamete formationHypothesis: Dependent assortmentHypothesis: Independent assortmentRRYYrryyGametesRrYyRYrySpermRyRrYYRRYyrrYYrrYyRRyyRryyActual results contradict hypothesisActual results support hypothesisYellow roundGreen roundYellow wrinkledGreen wrinkledEggsP generationF1 generationF2 generation1249163Figure 9.5 A
32 An example of independent assortment Black coat, normal visionB_N_Black coat, blind (PRA)B_nnChocolate coat, normal visionbbN_Chocolate coat, blind (PRA)bbnnBlind9 black coat,normal vision3 black coat,blind (PRA)3 chocolate coat,1 chocolate coat,BbNn BbNnPhenotypesGenotypesMating of heterozygotes(black, normal vision)Phenotypic ratioof offspringFigure 9.5 B
33 A Dihybrid Cross - F1 Results purpleflowers, tallwhiteflowers,dwarfTRUE-BREEDINGPARENTS:AABBxaabbGAMETES:ABABababAaBbF1 HYBRIDOFFSPRING:All purple-flowered, tall
34 16 Allele Combinations in F2 1/41/41/41/4ABAbaBab1/41/161/161/161/16ABAABBAABbAaBBAaBb1/41/161/161/161/16AbAABbAAbbAaBbAabb1/41/161/161/161/16aBAaBBAaBbaaBBaaBb1/41/161/161/161/16abAaBbAabbaaBbaabb
36 Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible1/41/41/41/4ABAbaBab1/41/161/161/161/16ABAABBAABbAaBBAaBb1/41/161/161/161/16AbAABbAAbbAaBbAabb1/41/161/161/161/16aBAaBBAaBbaaBBaaBb1/41/161/161/161/16abAaBbAabbaaBbaabb
37 Mendel’s laws reflect the rules of probability Inheritance follows the rules of probability
38 The rule of multiplication The rule of addition Calculates the probability of two independent eventsThe rule of additionCalculates the probability of an event that can occur in alternate waysF1 genotypesBb femaleFormation of eggsF2 genotypesBb maleFormation of spermBb124Figure 9.7
39 Genetic traits in humans can be tracked through family pedigrees The inheritance of many human traitsFollows Mendel’s lawsDominant TraitsRecessive TraitsFrecklesNo frecklesWidow’s peakStraight hairlineFree earlobeAttached earlobeFigure 9.8 A
40 Family pedigrees Can be used to determine individual genotypes Dd JoshuaLambertAbigailLinnellD ?JohnEddyHepzibahDaggettddJonathanElizabethDd Dd dd Dd Dd Dd ddFemale MaleDeafHearingFigure 9.8 B
41 Recessive Disorders Most human genetic disorders are recessive ParentsOffspringSpermNormalDdD dEggsDdDD(carrier)ddDeafFigure 9.9 A
42 VARIATIONS ON MENDEL’S LAWS The relationship of genotype to phenotype is rarely simpleMendel’s principles are valid for all sexually reproducing speciesBut genotype often does not dictate phenotype in the simple way his laws describe
43 that genes can work together and interact. Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait).We know now that there is a range of dominance andthat genes can work together and interact.Incomplete dominance:When the F1 generation have an appearance in between thephenotypes of the parents.Ex: pink snapdragons offspring of red and white ones.Another way to say it isIn incomplete dominanceHeterozygote phenotype is somewhere between that of twohomozygotes
44 Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plantPink-flowered F1 plants(homozygote)(homozygote)(heterozygotes)
46 Flower Color in Snapdragons: Incomplete Dominance Red flowers - two alleles allow them to make a red pigmentWhite flowers - two mutant alleles; can’t make red pigmentPink flowers have one normal and one mutant allele; make a smaller amount of red pigment
47 Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plantWhite-, pink-, and red-flowered plantsin a 1:2:1 ratio(heterozygote)(heterozygote)
49 Co-Dominance or multiple alleles: Non-identical alleles specify two phenotypes that are both expressed in heterozygotesHaving more than 2 alleles for a given trait and both alleles show in the phenotype. No single one is dominant over the other.Example: ABO blood types
50 Genetics of ABO Blood Types: Three Alleles Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cellsTwo alleles (IA and IB) are codominant when pairedThird allele (i) is recessive to others
52 The ABO blood type in humans Involves three alleles of a single geneThe alleles for A and B blood types are codominantAnd both are expressed in the phenotypeBloodGroup(Phenotype)GenotypesAntibodiesPresent inReaction When Blood from Groups Below Is Mixed withAntibodies from Groups at LeftO A B ABOABABiiIAIAorIAiIBIBIBiIAIBAnti-AAnti-B—Figure 9.13
54 More exceptions to the dominant/recessive rule Pleiotropy:One genes having many effects. Only one gene affects an organism in many ways.Ex: sickle cell anemia and cystic fibrosis
55 PleiotropyAlleles at a single locus may have effects on two or more traitsClassic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia
56 A single gene may affect many phenotypic characteristics In pleiotropyA single gene may affect phenotype in many waysIndividual homozygousfor sickle-cell alleleAbnormal hemoglobin crystallizes,causing red blood cells to become sickle-shapedSickle-cell (abnormal) hemoglobinSickle cellsBreakdown ofred blood cellsClumping of cellsand clogging ofsmall blood vesselsAccumulation ofsickled cells in spleenPhysicalweaknessAnemiaHeartfailurePain andfeverBraindamageDamage toother organsSpleenImpairedmentalfunctionParalysisPneumoniaand otherinfectionsRheumatismKidney5,555Figure 9.14
57 Genetics of Sickle-Cell Anemia Two alleles1) HbAEncodes normal beta hemoglobin chain2) HbSMutant allele encodes defective chainHbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia
58 Pleiotropic effects of the sickle-cell allele in a homozygote
59 Epistasis: Interaction between the products of gene pairs Interaction between two genes in which one of the genes modifies the expression of the other.Ex: fur /hair color in mammals and albinism
60 AlbinismPhenotype results when pathway for melanin production is completely blockedGenotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway
61 Genetics of Coat Color in Labrador Retrievers Two genes involved- One gene influences melanin productionTwo alleles - B (black) is dominant over b (brown)- Other gene influences melanin depositionTwo alleles - E promotes pigment deposition and is dominant over e
62 Allele Combinations and Coat Color Black coat - Must have at least one dominant allele at both lociBBEE, BbEe, BBEe, or BbEEBrown coat - bbEE, bbEeYellow coat - Bbee, BbEE, bbee
64 Human Variation Some human traits occur as a few discrete types Attached or detached earlobesMany genetic disordersOther traits show continuous variationHeightWeightEye color
65 More modifications to Mendel’s rule Polygenic Inheritance:In this case many genes have an additive effect. The characteristic or trait is the result of the combined effect of several genes. Ex: human skin color, height. Controlled by more than one pair of genes
66 Continuous VariationPolygenic inheritance results in a continuous range of small differences in a given trait among individualsThe greater the number of genes that affect a trait, the more continuous the variation in versions of that trait
67 A simplified model for polygenic inheritance of skin color
68 Environmental effects: The degree to which an allele is expressed depends on the environmentEx: Siamese cat fur color ( enzyme for melanin production inhibited by heat), hydrangea flowers ( depends on acidity of soil), height (nutrition)
69 Temperature Effects on Phenotype Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathwayMelanin is produced in cooler areas of body
70 Environmental Effects on Plant Phenotype Hydrangea macrophyllaAction of gene responsible for floral color is influenced by soil acidityFlower color ranges from pink to blue
73 Thomas Hunt Morgan (1910) and Sex Linked Inheritance Morgan’s Experimental Evidence: Scientific InquiryThe first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt MorganMorgan’s experiments with fruit flies (Columbia University, 1910) provided convincing evidence that chromosomes are the location of Mendel’s heritable factors. He provided confirmation of the correctness of the chromosomal theory of inheritance.
74 Morgan’s experimentsDemonstrated the role of crossing over in inheritanceExperimentGray body,long wings(wild type)GgLIFemaleBlack body,vestigialwingsggllMaleOffspringGray long965944206185Black vestigialGray vestigialBlack longParentalphenotypesRecombinantRecombination frequency == 0.17 or 17%391 recombinants2,300 total offspringExplanation(female)(male)GLglEggsSpermFigure 9.20 C
75 Thomas Hunt MorganPerformed some of the early studies of crossing over using the fruit fly Drosophila melanogasterFigure 9.20 B
76 In DrosophilaWhite eye color is a sex-linked traitFigure 9.23 A
78 SEX LINKED INHERITANCE CHROMOSOMESHumans have 22 pairs of AUTOSOMES and one pair of SEX CHROMOSOMES : total=23 prsThomas Morgan discovered SEX LINKED INHERITANCE studying Drosophila (fruit fly)In fruit flies red eyes is the wild type and white eyes is a mutant. He noticed the connection between gender and certain traits. Only the male flies had mutant white eyes.
79 SEX LINKED TRAITS ARE THOSE CARRIED BY THE X CHROMOSOME Red-Green color blindnessInability to see those colors. Red and green look all the same ,like grayHemophilia Blood clotting disorder.The clotting factor VIII is not made, individual can bleed to death.Muscular dystrophyX linked recessive, gradual and progressive destruction of skeletal muscles .Faulty teeth enamelExtremely rare, X linked Dominant
80 Sex-linked genes exhibit a unique pattern of inheritance All genes on the sex chromosomesAre said to be sex-linkedIn many organismsThe X chromosome carries many genes unrelated to sex
81 new technologies can provide insight into one’s genetic legacy Can provide insight for reproductive decisions
82 Identifying Carriers For an increasing number of genetic disorders Tests are available that can distinguish carriers of genetic disorders
83 Newborn Screening Some genetic disorders can be detected at birth By simple tests that are now routinely performed in most hospitals in the United States
84 Fetal Testing Amniocentesis and chorionic villus sampling (CVS) Allow doctors to remove fetal cells that can be tested for genetic abnormalitiesFigure 9.10 AAmniocentesisChorionic villus sampling (CVS)UltrasoundmonitorFetusUterusAmnioticfluidFetalcellsSeveralweeksBiochemicaltestshoursCervixSuction tube insertedthrough cervix to extracttissue from chorionic villiNeedle insertedthrough abdomen toextract amniotic fluidCentrifugationPlacentaChorionicvilliKaryotyping
85 Ethical Considerations New technologies such as fetal imaging and testingRaise new ethical questions
86 Mutations Mutations are permanent changes in DNA Causes? Errors in DNA replication that can be spontaneous. Also caused by high energy radiation (X rays, gamma rays),toxic chemicals in the environment ( pesticides,asbestos, tar) and viruses.
87 MUTATION: A PERMANENT CHANGE IN THE DNA MUTATION: A PERMANENT CHANGE IN THE DNA. When it happens in the gametes it is inheritable. Some mutations are lethal but most are harmless. Mutations are very important because it creates DIVERSITYWHAT CAUSES MUTATIONS?Most mutations are spontaneous, changes in DNA caused by errors in replication ( the DNA is copied incorrectly during cell division). The cell has mechanism to find and correct mistakes but those that get through get passed along.Some mutations can cause genetic disorders.Some environmental factors can cause molecular changes in DNA.X rays, toxic chemicals (insecticides, fertilizers, dry cleaning fluids, tar), some viruses, high energy radiation.
88 Many inherited disorders in humans are controlled by a single gene Some autosomal disorders in humansTable 9.9
89 DISORDERS RESULTING FROM AUTOSOMAL RECESSIVE INHERITANCE These are conditions in which the gene that is defective is recessive.It is only expressed when the child receives both recessive genes for the disorder (one from each parent)If a person is heterozygous, that is it has one dominant regular gene and one recessive abnormal gene for the condition, he will be a CARRIER but not have the disorder. The dominant allele will mask the expression of the abnormal condition.EXAMPLES: ALBINISM:SICKLE CELL ANEMIA:CYSTIC FIBROSIS:TAY- SACHS DISEASE;PHENYLKETONURIA;GALACTOSEMIA:
90 SICKLE CELL ANEMIA: This is also an example of “PLEIOTROPY” DISORDERS RESULTING FROM RECESSIVE INHERITANCE Many not life threatening traits are inherited this way. widows peak, and attached earlobes.ALBINISM: No pigmentation in skin This is also an example of “EPISTASIS”(one pair of genes modifies the expression of another)SICKLE CELL ANEMIA: This is also an example of “PLEIOTROPY”Red blood cells curved shape. Decreased oxygen to brain and muscles (offers resistance to Malaria)
91 DISORDERS RESULTING FROM RECESSIVE INHERITANCE CYSTIC FIBROSIS: Excessive mucus secretions.Impaired lung function, lung infections. Protein channel that transport chloride across cell membrane does not function. Protects against cholera.This is also an example of “PLEIOTROPY”TAY –SACHS DISEASE: Nervous system degeneration in infants. Enzyme fails to breakdown lipids which accumulate in nerve cells and kills the cells. Progressive degeneration starting with the brain cells.
92 DISORDERS RESULTING FROM RECESSIVE INHERITANCE GALACTOSEMIA: Produces brain, liver, eye damage. Enzyme that breaks down lactose is lacking. It accumulates to toxic levels. Death in infancyPHENYLKETONURIA: Results in mental retardation
93 Disorders resulting from Autosomal Dominant Inheritance Dominant genes: Many are harmless for example:freckles, dimples, cleft chin, free earlobe, short big toe, tongue rollers, left thumb on top, curly hair and dark hairDominant traits appear in each generation since the allele shows in the heterozygous individual.
94 Dominant Disorders Some human genetic disorders are dominant Figure 9.9 B
95 Disorders resulting from Dominant Inheritance Acondroplasia or dwarfism:A condition where the bone does not grow properly and can’t make proper cartilage. Person is less than 4 feet with short arms and legs but a regular size trunk.Cholesterolemia:High cholesterol levels in the blood causing arteries to clog and high incidence of early heart attacks.Marfan Syndrome:Abnormal connective tissue
96 Disorders resulting from Autosomal Dominant Inheritance Huntington’s Disorder:Progressive degeneration of nervous system and muscle control. Affects motor and mental abilities and it is irreversible. Late onset, usually late 30’s. Usually the person already had children.Progeria:Premature accelerated aging. Usually dead by 18. Genes that bring about growth and development are abnormal.Polydactily:Extra toes and fingers
97 KaryotypeA karyotype is a visual display of an individual’s chromosomes. A man made picture of a person’s 23 pairs of chromosomes. ( the photo is taken during metaphase when the sister chromatids are lined up together)It is useful in sex determination and diagnosis of certain conditions.
98 INHERITED DISORDERS DUE TO CHROMOSOMES CHANGES Chromosome changes can cause a lot of genetic disorders as well as a lot of varietyWHEN AND HOW CAN A CHROMOSOME CHANGE?Mistakes in replication. During the S phase of the cell cycle segments of a chromosome could be deleted, duplicated, inverted or moved to a new location. Also during Metaphase I (meiosis) there can be improper separation after duplication. This can change the total number of chromosomes in each gamete of the new individual.
99 If during meiosis the paired chromatids fail to separate correctly this is called NON-DISJUNCTION ANEUPLOIDY means an abnormal number of chromosomes.When an individual ends up with the wrong number of chromosomes most of the time it is miscarried ( spontaneous abortion).The wrong number of somatic chromosomes are almost always lethal. Ex: trisomy 21(three chrom. 21): Down SyndromeYou can live with the wrong number of sex pair chromosomes.
100 CHANGES IN THE NUMBER OF SEX CHROMOSOMES X Turner syndrome One X instead of a pair. This happens because of non disjuction of sperm. Most are aborted spontaneously. If they live, she is very short, infertily and with reduced sex characteristics.XXY Klinefelter syndrome One in 500 live male births. Taller than average, infertile, some low intelligence, some normal. Testosterone injections help.XYY “super male” about 1 in taller, mildly retarded but normal phenotype.
101 SEX CHROMOSOMES AND SEX-LINKED GENES Chromosomes determine sex in many speciesIn mammals, a male has one X chromosome and one Y chromosomeAnd a female has two X chromosomes(male)(female)Parents’diploidcellsSpermEggOffspring(diploid)44+XYXX22XYFigure 9.22 A
102 Other systems of sex determination exist in other animals and plants 22+XXXFigure 9.22 B76+ZWZZFigure 9.22 C3216Figure 9.22 D
103 The absence of a Y chromosome The Y chromosomeHas genes for the development of testesThe absence of a Y chromosomeAllows ovaries to develop
104 Alleles at two loci (R and P) interact Comb Shape in PoultryAlleles at two loci (R and P) interactWalnut comb - RRPP, RRPp, RrPP, RrPpRose comb - RRpp, RrppPea comb - rrPP, rrPpSingle comb - rrpp
105 Campodactyly: Unexpected Phenotypes Effect of allele varies:Bent fingers on both handsBent fingers on one handNo effectMany factors affect gene expression