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Chapter 14~ Mendel & The Gene Idea

Chapter 14~ Mendel & The Gene Idea. Gregor Mendel. Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used experimental method used quantitative analysis collected data & counted them

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Chapter 14~ Mendel & The Gene Idea

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  1. Chapter 14~ Mendel & The Gene Idea

  2. Gregor Mendel • Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas • used experimental method • usedquantitative analysis • collected data & counted them • excellent example of scientific method

  3. Mendel’s work Pollen transferred from white flower to stigma of purple flower P • Bred pea plants • cross-pollinate true breeding parents (P) • P = parental • raised seed & then observed traits (F1) • F = filial • allowed offspring to self-pollinate& observed next generation (F2) anthers removed all purple flowers result F1 self-pollinate F2

  4. true-breeding purple-flower peas true-breeding white-flower peas 100% purple-flower peas F1 generation (hybrids) 100% 75% purple-flower peas 25% white-flower peas 3:1 F2 generation Looking closer at Mendel’s work X P Where did the whiteflowers go? Whiteflowers cameback! self-pollinate

  5. What did Mendel’s findings mean? • Traits come in alternative versions • purple vs. white flower color • alleles • different alleles vary in the sequence of nucleotides at the specific locus of a gene • some difference in sequence of A, T, C, G purple-flower allele & white-flower allele are two DNA variations at flower-color locus different versions of gene at same location on homologous chromosomes

  6. Traits are inherited as discrete units • For each characteristic, an organism inherits 2 alleles, 1 from each parent • diploid organism • inherits 2 sets of chromosomes, 1 from each parent • homologous chromosomes

  7. What did Mendel’s findings mean? • Some traits mask others • purple & white flower colors are separate traits that do not blend • purple x white ≠ light purple • purplemaskedwhite • dominant allele • functional protein • masks other alleles • recessive allele • allele makes a malfunctioning protein I’ll speak for both of us! mutantallele producingmalfunctioningprotein wild typeallele producingfunctional protein homologouschromosomes

  8. X P purple white F1 all purple Genotype vs. phenotype • Difference between how an organism “looks” & its genetics • phenotype • description of an organism’s trait • the “physical” • genotype • description of an organism’s genetic makeup Explain Mendel’s results using …dominant&recessive …phenotype&genotype

  9. PP pp x X P purple white F1 all purple Making crosses • Can represent alleles as letters • flower color alleles  P or p • true-breeding purple-flower peas  PP • true-breeding white-flower peas  pp Pp

  10. PP pp x X P purple white F1 all purple Making crosses • Can represent alleles as letters • flower color alleles  P or p • true-breeding purple-flower peas  PP • true-breeding white-flower peas  pp Pp

  11. true-breeding purple-flower peas true-breeding white-flower peas 100% purple-flower peas F1 generation (hybrids) 100% 75% purple-flower peas 25% white-flower peas Looking closer at Mendel’s work phenotype X P PP pp genotype Pp Pp Pp Pp self-pollinate 3:1 F2 generation ? ? ? ?

  12. PP 25% male / sperm P p Pp 50% 75% P Pp female / eggs pp p 25% 25% Punnett squares Aaaaah, phenotype & genotypecan have different ratios F1 generation (hybrids) Pp x Pp % genotype % phenotype PP Pp Pp pp 1:2:1 3:1

  13. Genotypes • Homozygous = same alleles = PP, pp • Heterozygous = different alleles = Pp homozygousdominant heterozygous homozygousrecessive

  14. purple PP homozygous dominant purple Pp heterozygous How do you determine the genotype of an individual withwith a dominant phenotype? Phenotype vs. genotype • 2 organisms can have the same phenotype but have different genotypes Can’t tellby lookin’at ya!

  15. x Test cross • Breed the dominant phenotype —the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele How does that work? is itPP or Pp? pp

  16. x x How does a Test cross work? Am I this? Or am I this? PP pp Pp pp p p p p P P Pp Pp Pp Pp P p Pp Pp pp pp 100% purple 50% purple:50% whiteor 1:1

  17. P P P p PP Pp pp p p Mendel’s 1st law of heredity • Law of segregation • during meiosis, alleles segregate • homologous chromosomes separate • each allele for a trait is packaged into a separate gamete

  18. Metaphase 1 Law of Segregation • Which stage of meiosis creates the law of segregation? Whoa!And Mendeldidn’t even knowDNA or genesexisted!

  19. Monohybrid cross • Some of Mendel’s experiments followed the inheritance of single characters • flower color • seed color • monohybrid crosses

  20. Dihybrid cross • Other of Mendel’s experiments followed the inheritance of 2 different characters • seed color andseed shape • dihybrid crosses Mendelwas working outmany of the genetic rules!

  21. F1 generation (hybrids) yellow, round peas 100% F2 generation Dihybrid cross P true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas

  22. YR YR yR Yr yr yr YyRr What’s going on here? • If genes are on different chromosomes… • how do they assort in the gametes? • together or independently? Is it this? Or this? YyRr Which systemexplains the data?

  23. 9/16 yellow round YyRr YyRr 3/16 green round YR yr YR yR Yr yr 3/16 yellow wrinkled 1/16 green wrinkled Is this the way it works? or YyRr x YyRr YR yr  Well, that’sNOT right! YR YYRR YyRr yr YyRr yyrr

  24. 9/16 yellow round YyRr YyRr 3/16 green round YR YR yr YR yR Yr yr Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross or YyRr x YyRr YR Yr yR yr  YYRR YYRr YyRR YyRr BINGO! YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

  25. yellow green round wrinkled Can you thinkof an exceptionto this? Mendel’s 2nd law of heredity • Law of independent assortment • different loci (genes) separate into gametes independently • non-homologous chromosomes align independently • classes of gametes produced in equal amounts • YR = Yr = yR = yr • only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1

  26. Metaphase 1 Law of Independent Assortment • Which stage of meiosis creates the law of independent assortment? RememberMendel didn’teven know DNA—or genes—existed! EXCEPTION • If genes are on same chromosome & close together • will usually be inherited together • rarely crossover separately • “linked”

  27. Mendelian inheritance reflects rule of probability • Mendel’s laws of segregation and independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice. • We can use the rule of multiplication to determine the chance that two or more independent events will occur together in some specific combination • The rule of addition also applies to genetic problems. • Under the rule of addition, the probability of an event that can occur two or more different ways is the sum of the separate probabilities of those ways • We can combine the rules of multiplication and addition to solve complex problems in Mendelian genetics.

  28. Extending Mendelian genetics • Mendel worked with a simple system • peas are genetically simple • most traits are controlled by a single gene • each gene has only 2 alleles, 1 of which is completely dominant to the other • The relationship between genotype & phenotype is rarely that simple

  29. Incomplete dominance • appearance between the phenotypes of the 2 parents. Ex: carnations • Heterozygote shows an intermediate, blended phenotype • example: • RR = red flowers • rr = white flowers • Rr = pink flowers • make 50% less color • Incomplete dominance in carnations: red, pink, white

  30. Co-dominance • 2 alleles affect the phenotype equally & separately • not blended phenotype • human ABO blood groups • 3 alleles • IA, IB, i • IA & IB alleles are co-dominant • glycoprotein antigens on RBC • IAIB = both antigens are produced • i allele recessive to both

  31. Multiple alleles: • more than 2 possible alleles for a gene. Ex: human blood types

  32. Pleiotropy: • genes with multiple phenotypic effect. • one gene affects more than one phenotypic character • Ex: sickle-cell anemia • Normal and sickle red blood cells

  33. Pleiotropy • Most genes are pleiotropic • one gene affects more than one phenotypic character • 1 gene affects more than 1 trait • dwarfism (achondroplasia) • gigantism (acromegaly)

  34. Inheritance pattern of Achondroplasia Aa x aa Aa x Aa dominantinheritance a a A a  Aa Aa AA Aa A A dwarf dwarf lethal a a aa aa Aa aa 50% dwarf:50% normal or1:1 67% dwarf:33%normalor2:1

  35. Epistasis • One gene completely masks another gene • coat color in mice = 2 separate genes • C,c:pigment (C) or no pigment (c) • B,b:more pigment (black=B) or less (brown=b) • cc = albino, no matter B allele • 9:3:3:1 becomes 9:3:4 B_C_ B_C_ bbC_ bbC_ _ _cc _ _cc

  36. Epistasis in Labrador retrievers • 2 genes: (E,e) & (B,b) • pigment (E) or no pigment (e) • pigment concentration: black (B) to brown(b) eebb eeB– E–bb E–B–

  37. Polygenic inheritance • Some phenotypes determined by additive effects of 2 or more genes on a single character • phenotypes on a continuum • human traits • skin color • height • weight • intelligence • behaviors

  38. Human disorders • The family pedigree • Recessive disorders: • •Cystic fibrosis •Tay-Sachs •Sickle-cell • Dominant disorders: •Huntington’s achondroplasia

  39. Recessive diseases • The diseases are recessive because the allele codes for either a malfunctioning protein or no protein at all • Heterozygotes (Aa) • carriers • have a normal phenotype because one “normal” allele produces enough of the required protein

  40. male / sperm A a AA Aa A A a A female / eggs Aa Aa a Aa aa a Heterozygote crosses • Heterozygotes as carriers of recessive alleles Aa x Aa AA Aa carrier Aa aa carrier disease

  41. = male = female = male w/ trait = female w/ trait Pedigree analysis • Pedigree analysis reveals Mendelian patterns in human inheritance • data mapped on a family tree

  42. Human disorders • Testing: •amniocentesis •chorionic villus sampling (CVS) • Examination of the fetus with ultrasound is another helpful technique

  43. Genetic counseling • Pedigree can help us understand the past & predict the future • Thousands of genetic disorders are inherited as simple recessive traits • from benign conditions to deadly diseases • albinism • cystic fibrosis • Tay sachs • sickle cell anemia • PKU

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