57 ch13mendel2008
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  • He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants. After the university, Mendel taught at the Brunn Modern School and lived in the local monastery. <br /> The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance. <br />
  • P = parents <br /> F = filial generation <br />
  • In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. <br /> The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. <br /> Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. <br />
  • In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. <br /> The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. <br /> Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. <br />
  • Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry. <br /> Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry. <br />

57 ch13mendel2008 57 ch13mendel2008 Presentation Transcript

  • Genetics & The Work of Mendel AP Biology 2006-2007
  • 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  AP Biology excellent example of scientific method
  • Mendel’s work  Bred pea plants  Pollen transferred from white flower to stigma of purple flower P 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) AP Biology anthers removed all purple flowers result F1 self-pollinate F2
  • Mendel collected data for 7 pea traits AP Biology
  • Looking closer at Mendel’s work P F1 true-breeding true-breeding X white-flower peas purple-flower peas 100% purple-flower peas Where did the white flowers go? 100% generation (hybrids) self-pollinate F2 generation AP Biology 75% purple-flower peas White flowers came back! 25% white-flower peas 3:1
  • 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 AP Biology
  • 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  like having 2 editions of encyclopedia  Encyclopedia Britannica  Encyclopedia Americana AP Biology What are the advantages of being diploid?
  • What did Mendel’s findings mean?  Some traits mask others  purple & white flower colors are separate traits that do not blend I’ll speak for both of us!  purple x white ≠ light purple  purple masked white  dominant allele  functional protein  masks other alleles  mutant allele producing malfunctioning protein recessive allele  allele makes a malfunctioning protein AP Biology wild type allele producing functional protein homologous chromosomes
  • 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 AP Biology 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 x pp X P purple white F1 AP Biology all purple Pp
  • Looking closer at Mendel’s work P true-breeding true-breeding X white-flower peas phenotype purple-flower peas PP pp 100% purple-flower peas F1 genotype 100% generation (hybrids) Pp Pp Pp Pp self-pollinate F2 generation AP Biology 75% purple-flower peas ? ? ? 25% white-flower peas ? 3:1
  • Punnett squares Pp x Pp F1 Aaaaah, phenotype & genotype can have different ratios generation (hybrids) % genotype male / sperm female / eggs P p PP Pp P PP Pp p Pp pp AP Biology % phenotype 25% 75% 50% Pp pp 25% 25% 1:2:1 3:1
  • Genotypes  Homozygous = same alleles = PP, pp  Heterozygous = different alleles = Pp homozygous dominant heterozygous homozygous recessive AP Biology
  • Phenotype vs. genotype  2 organisms can have the same phenotype but have different genotypes purple PP homozygous dominant purple Pp heterozygous How do you determine the genotype of an individual with with a dominant phenotype? AP Biology Can’t tell by lookin’ at ya!
  • 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? x is it PP or Pp? AP Biology pp
  • How does a Test cross work? Am I this? Or am I this? x PP pp x Pp p p P Pp Pp P Pp Pp P Pp Pp p pp pp AP Biology 100% purple p pp p 50% purple:50% white or 1:1
  • Mendel’s 1st law of heredity PP  Law of segregation  during meiosis, alleles segregate P P  homologous chromosomes separate  each allele for a trait is packaged into a separate gamete p pp p P Pp AP Biology p
  • Law of Segregation  Which stage of meiosis creates the law of segregation? Metaphase 1 Whoa! And Mendel didn’t even know DNA or genes existed! AP Biology
  • Monohybrid cross  Some of Mendel’s experiments followed the inheritance of single characters flower color  seed color  monohybrid crosses  AP Biology
  • Dihybrid cross  Other of Mendel’s experiments followed the inheritance of 2 different characters seed color and seed shape  dihybrid crosses  AP Biology Mendel was working out many of the genetic rules!
  • Dihybrid cross P true-breeding yellow, round peas Y = yellow R = round x YYRR yyrr true-breeding green, wrinkled peas y = green r = wrinkled yellow, round peas F1 generation (hybrids) 100% YyRr self-pollinate F2 generation AP Biology 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas
  • What’s going on here?  If genes are on different chromosomes… how do they assort in the gametes?  together or independently?  YyRr Is it this? YR AP Biology yr Or this? YR YyRr Yr Which system explains the data? yR yr
  • YyRr Is this the way it works? YR YyRr x YyRr or yr YyRr YR Yr yR yr 9/16 yellow round YR YR yr AP Biology yr YYRR YyRr YyRr yyrr Well, that’s NOT right! 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled
  • YyRr Dihybrid cross YR YyRr x YyRr YR Yr yR yr or YR Yr yr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr AP Biology YYRr YyRr YYrr Yyrr yyRr yyrr yR yr 9/16 yellow round 3/16 green round YR YYRR YYRr YyRR YyRr Yr YyRr BINGO! 3/16 yellow wrinkled 1/16 green wrinkled
  • Mendel’s 2nd law of heredity Can you think of an exception to this?  Law of independent assortment  yellow different loci (genes) separate into gametes independently green  non-homologous chromosomes align independently  classes of gametes produced in equal amounts round  only true for genes on separate chromosomes or  YR = Yr = yR = yr wrinkled on same chromosome but so far apart that crossing over happens frequently YyRr Yr Yr 1 AP Biology yR : yR 1 YR : YR 1 yr : yr 1
  • Law of Independent Assortment  Which stage of meiosis creates the law of independent assortment? Remember Mendel didn’t even know DNA —or genes— existed! AP Biology Metaphase 1 EXCEPTION  If genes are on same chromosome & close together  will usually be inherited together  rarely crossover separately  “linked”
  • The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring AP Biology
  • Review: Mendel’s laws of heredity  Law of segregation  monohybrid cross  single trait  each allele segregates into separate gametes  established by Metaphase 1  Law of independent assortment  dihybrid (or more) cross  2 or more traits  genes on separate chromosomes assort into gametes independently  established by Metaphase 1 AP Biology EXCEPTION  linked genes metaphase1
  • Mendel chose peas wisely  Pea plants are good for genetic research  available in many varieties with distinct heritable features with different variations  flower color, seed color, seed shape, etc.  Mendel had strict control over which plants mated with which  each pea plant has male & female AP Biology structures  pea plants can self-fertilize  Mendel could also cross-pollinate plants: moving pollen from one plant to another
  • Mendel chose peas luckily  Pea plants are good for genetic research  relatively simple genetically  most characters are controlled by a single gene with each gene having only 2 alleles,  one completely dominant over the other AP Biology
  • Any Questions?? AP Biology 2006-2007