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Chapter 14 Mendel and the Gene Idea
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Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) Mendel documented a particulate mechanism through his experiments with garden peas Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Beliefs about Heredity Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir de beaux enfants de l’un ou l’autre sexe (Paris, Year VII). Image courtesy of the Bibliothèque Interuniversitaire de Médecine, Paris.
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Homunculus How is “heredity passed on: Spermist vs Ovists Spermist conception of a human sperm
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Homunculus Leeuwenhoek’s black male and white female rabbit experiments: spermist “proof”
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Darwin What he got right.What he got wrong. Acquired characteristics Ex. Blind cave animals Sex. Repro.- gemmules from all over body are packed in sperm and egg Blended inheritance Likes produce likes Change can be permanent There is no limit to cumulative change
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Gregor Mendel Mendelian Genetics
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Fig. 14-5-3 P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP P pp p F 1 Generation Gametes: Genetic makeup: Appearance: Purple flowers Pp P p 1/21/2 1/21/2 F 2 Generation Sperm Eggs P P PPPp p p pp 31
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Mendel’s Three Principles Dominance Segregation Independent Assortment The foundation of “classical” science (1822-1884)
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Dominance Traits of both parents inherited, but one shows over the other Traits are not blended
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Dominance Mechanism Two alleles are carried for each trait In true-breeding individuals, both alleles are the same. Hybrids, on the other hand, have one of each kind of allele. One trait is dominant, the other trait is recessive
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Segregation Half the gametes (egg or sperm) will carry the traits of one parent and half the traits for the other parent Pairs of alleles are separated (=segregated) during meiosis
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Two different parental characteristics will be inherited independently of one another during gamete formation. Independent Assortment Example: flower color and leaf shape
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Genes- genetic material on a chromosome that codes for a specific trait Genotype- the genetic makeup of the organism Phenotype- the expressed trait Allel- an alternative form of a gene
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Dominance Mechanism Two alleles are carried for each trait In true-breeding individuals, both alleles are the same (homozygous). Hybrids, on the other hand, have one of each kind of allele (heterozygous). One trait is dominant, the other trait is recessive
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Genetic Information Genes are traits “Eye color” Ear lobe connectedness Genes produce proteins Enzymes are proteins
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Homologous Chromosomes gene: location allele: specific trait
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Allele Example Gene = “eye color” Alleles brown blue green lavender
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Allele Examples appearance eye color: homozygous
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Allele Examples appearance eye color: heterozygous, brown dominant over blue
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Genotype vs Phenotype homozygous (dominant) heterozygous homozygous (recessive) genotype phenotype appearance Phaner = visible
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Punnett Square If male & female are heterozygous for eye color X brown:3/4 offspring blue:1/4 offspring male female
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PKU Each parent carries one gene for PKU. Pp Pp X P p pP P P P p p P p p Possible genotypes: 1PP 2Pp 1pp Possible phenotypes:no PKUPKU
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Compare this to what would have happened if one parent was homozygous for sickle cell. HbA HbS X HbA HbS HbA HbS HbA HbS HbA all offspring are carriers of sickle cell trait
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Color blind- sex linked XSXs Y X Y XS Xs Y XS Y Xs S= color blind s= normal
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Phenotype HbAHb_ HbAHbB X HbA HbB Hb_HbA Hb_ HbB HbA Hb_ HbB A B AB O Genotype Blood Type AA, A_ BB, B_ AB _ Outcome: AA, A_, AB, B_
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Where Does Genetic Diversity Come From? Mutation Chromosomal Aberrations Genetic Recombination (e.g., from sexual reproduction)
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Mutation Mutations are changes to the sequence of nucleotide bases within a gene. A possible consequence is the change in the amino acid sequence of a protein. Mutations are rare, but occur often enough to be important (each of us will probably pass at least one mutation on to our offspring).
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mutation Sickle Cell Mutation CTG ACT CCT GAG GAG AAG TCT Leu Thr Pro Glu Glu Lys Ser CTG ACT CCT GAG GTG AAG TCT Leu Thr Pro Glu Val Lys Ser NORMAL Hb SICKLE CELL
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Mutation Some mutations may be deleterious, while others are may enhance chances for survival. Probably most mutations are neutral, having little or no effect on the fitness of the organism. Mutations may accumulate generation after generation, eventually leading to substantial differences in the characteristics of a species over time. Some mutations may be deleterious, while others are may enhance chances for survival. Probably most mutations are neutral, having little or no effect on the fitness of the organism. Mutations may accumulate generation after generation, eventually leading to substantial differences in the characteristics of a species over time.
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Autosomes and Sex Chromosomes
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Red-Green Color Blindness Sex-linked trait XCXC Y XCXC XcXc X XCXC XcXc YXCXC XCXC XCXC XCXC Y XcXc XCXC Y XcXc Normal male Normal female recessive gene Possible outcomes:X C X C X C X c X C YX c Y Normal female Normal Female (carrier) Normal male Color-blind male
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Dominance Most traits show complete dominance Blending unexpected
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Eunconnected earlobe econnected earlobe Eunconnected earlobe econnected earlobe allele gene PEE x ee gametes E e F1F1 unconnected connected
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F1F1 Ee x Ee gametes 1/2 E 1/2 e E e Ee EEEe ee F2F2 1 EE 2 Ee 1 ee Punnett Square
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P F1F1 F2F2 EE, ee Ee EE, 2 Ee, ee 50% 100% 75% 1:1 generation genotypes unconnected E:e Basis of the Castle-Hardy-Weinberg Law phenotypes ratio of alleles in the population
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Genotypes Phenotypes Experiment to determine dominant vs. recessive
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Genetic Sleuthing My eye color phenotype is brown. What is my genotype?
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Pedigree phenotypes infer genotypes Alternative: look directly at the DNA
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Complexities Multiple genes for one trait Example: eye color Blended traits (“incomplete dominance”) Influence of the environment
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Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Fig. 14-10-1 Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW
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Fig. 14-10-2 Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2
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Fig. 14-10-3 Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2
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Fig. 14-UN2 Degree of dominance Complete dominance of one allele Incomplete dominance of either allele Codominance Description Heterozygous phenotype same as that of homo- zygous dominant Heterozygous phenotype intermediate between the two homozygous phenotypes Heterozygotes: Both phenotypes expressed Multiple alleles Pleiotropy In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters CRCRCRCR CRCWCRCW CWCWCWCW IAIBIAIB I A, I B, i ABO blood group alleles Sickle-cell disease PP Pp Example
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Disorders Down’s Syndrome (chrom 21) Huntington’s (chrom 4) Alzheimer’s (chrom 1, 10, 14, 19, 21)
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Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Fig. 14-14
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Tongue Roller R = Tongue Roller r = Unable to Roll Tongue
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Widow’s Peak W = Widows Peak w = Lack of Widow’s Peak
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Free Ear Lobe Attached Ear Lobe E = Free Ear Lobe e = Attached Ear Lobe
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Hitchhiker’s Thumb Hi = Straight Thumb hi = Hitchhiker’s Thumb
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Bent Little Finger Bf = Bent Little Finger bf = Straight Little Finger
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Mid-digital Hair M = Mid-Digital Hair m = Absence of Mid-Digital Hair
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Dimples D = Dimples d = Absence of Dimples
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Short Hallux Ha = Short Hallux ha = Long Hallux
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Short Index Finger S s = Short Index Finger S 1 = Long Index Finger *Sex-Influenced Trait
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http://www.youtube.com/w atch?v=gCPuHzbb5hA
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Human Genome Project U.S. govt. project coordinated by the Department of Energy and the National Institutes of Health, launched in 1986 by Charles DeLisi. Definition: GENOME – the whole hereditary information of an organism that is encoded in the DNA. Project Goal: to identify the approximate 100,000 genes in the human DNA. -determine the sequences of the 3 billion bases that make up human DNA. -store this information in databases. -develop tools for data analysis. -address the ethical, legal, and social issues that arise from genome research.
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Bacteria (E. coli, influenza, several others) Yeast (Saccharomyces cerevisiae) Plant (Arabidopsis thaliana) Fruit fly (Drosophila melanogaster) Mouse (Mus musculus) Modeled Organisms
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DNA from 5 humans: 2 males, 3 females 2 caucasians, one each of asian, african, hispanic Cut up DNA with restriction enzymes Ligated into BACs & YACs, then grew them up Sequenced the BACs Let a supercomputer put the pieces together
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Craig Venter Celera Genomics
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Importance of genetics Understanding hereditary diseases and to develop new treatmentsUnderstanding hereditary diseases and to develop new treatments Donor matchesDonor matches PaternityPaternity ForensicsForensics EvolutionEvolution MigrationMigration
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Polynesian Origins Bismarck Archipelago 3.5ka http://www.sciencedaily.com/releases/2011/02/110203124726.htm mtDNA
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Polynesian Origins
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Genetic Testing Would you want to know? Ethical concerns Cost Insurance companies see
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Genetic Testing
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Gel electrophoresis
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PCR is a rapid, inexpensive and simple way of copying specific DNA fragments from minute quantities of source DNA material Three steps are involved in PCR: Denaturation Annealing Extension Polymerase Chain Reaction
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Genetic Testing Paternity Test $99 $99, looks at specific diseases
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Counseling Based on Mendelian Genetics and Probability Rules Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS), a sample of the placenta is removed and tested Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Video: Ultrasound of Human Fetus I Video: Ultrasound of Human Fetus I
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Fig. 14-18 Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several weeks Several weeks (a) Amniocentesis (b) Chorionic villus sampling (CVS) Several hours Several hours Fetal cells Bio- chemical tests Karyotyping Placenta Chorionic villi Fetus Suction tube inserted through cervix
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n About 1-4% of DNA in Modern Europeans and Asians was inherited from Neanderthals Ozzy Osbourne's Genome Reveals Some Neanderthal Lineage http://www.scientificamerican.com/article.cfm?id=ozzy- osbourne-genome
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You should now be able to: 1.Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F 1 generation, F 2 generation 2.Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype 3.Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F 2 generation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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4.Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance 5.Define and give examples of pleiotropy and epistasis 6.Explain why lethal dominant genes are much rarer than lethal recessive genes 7.Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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