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					  Mendelian Genetics
Simple Probabilities & a Little
           Luck
                Genetics
• the study of heredity & its mechanisms
• Gregor Mendel
   –reported experimental results in 1865/66
   –rediscovered in 1903 by de Vries,
    Correns & von Tschermak
                Genetics
• Before Mendel, heredity was seen as
  –the blending of parental contributions
  –unpredictable
• Mendel demonstrated that heredity
  –involves distinct particles
  –is statistically predictable
Cross pollination
  Figure 10.1
         Mendel’s Experiments
• the model system
   –garden pea varieties
     • easy to grow
     • short generation time
     • many offspring
     • bisexual
        –reciprocal cross-pollination
     • self-compatible
        –self-pollination
        Mendel’s Experiments
• garden pea varieties
  –many variable characters
    • a character is a heritable feature
       –flower color
    • a trait is a character state
       –blue flowers, white flowers, etc.
    • a heritable trait is reliably passed
      down
    • a true-breeding variety produces the
      same trait each generation
    7
characters
    ,

    14
  traits
Table 10.1
one of Mendel’s characters
        Figure 10.2
        Mendel’s Experiments
• Mendel’s experimental design
  –selected 7 characters with distinct traits
  –crossed plants with one trait to plants
   with the alternate trait (P = “parental”
   generation)
  –self-pollinated offspring of P (F1 = first
   filial generation)
  –scored traits in F1 and F2 generations
        Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
    • parents were true-breeding for
      alternate traits of one character
    • parents were reciprocally cross-
      pollinated
    • F1 progeny were self-pollinated
    • traits of F1 & F2 progeny were scored
         Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
  –Results
    • all F1 progeny exhibited the same trait
    • F2 progeny exhibited both parental
      traits in a 3:1 ratio (F1 trait: alternate
      trait)
        Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
  –Analysis
    • F1 trait is dominant
    • alternate trait is recessive
       –disappears from the F1 generation
       –reappears, unchanged, in F2
  –Relevance
    • all seven characters have dominant
      and recessive traits appearing 3:1 in
seven traits were inherited similarly
             Table 10.1
  Mendel’s
interpretatio
       n:
 inheritance
   does not
    involve
   blending
 Figure 10.3
         Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
  –Interpretation
    • inheritance is by discrete units
      (particles)
    • hereditary particles occur in pairs
    • particles segregate at gamete
      formation
    • particles are unaffected by
      combination
       Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
    • symbolic representation

      –P: SS      x ss
      –F1: Ss

    • each parent packages one gene in
      each gamete
    • gametes combine randomly
 recessive
    traits
 disappear
    in the
      F1
generation
Figure 10.4
         Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
    • [terminology
        –different versions of a gene = alleles
        –two copies of an allele =
         homozygous
        –one copy of each allele =
         heterozygous
        –genetic constitution = genotype
        –round or wrinkled seeds =
         Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
    • symbolic representation
       P: SS         x ss
       F1:    Ss gamete formation S or s
        self pollination: S with S
                         s with s
                         S with s or s with S
       F2: SS, ss, Ss, sS
Punnett
 to the
rescue
Figure
  10.4
P: (SS or ss)

p(S)=1          x
p(s)=1

F1: (Ss)   p(Ss) =1 x 1=1


       p(S)=1/2, p(s)=1/2,
so

F2:   p(SS) =1/2 x 1/2=1/4
  F1: Ss
replication
               Punnett
 S-S & s-s
              explained
anaphase I
                 by
S-S or s-s     meiosis
anaphase II    Figure
                10.5
S or S or s
   or s
        Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
    • if you know the genotypes of the
      parental generation you can predict
      the phenotypes of the F1 & F2
      generations

P:      Round x wrinkled
F1:     1/2 Round, 1/2 wrinkled
F2: 3/4 Round, 1/4 wrinkled OR all wrinkled
        Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #1: monohybrid crosses
    • if you know the genotypes of the
      parental generation you can predict
      the phenotypes of the F1 & F2
      generations

P:      Round (Rr) x wrinkled (rr)
F1:     1/2 Round (Rr), 1/2 wrinkled (rr)
F2: 3/4 Round, 1/4 wrinkled OR all wrinkled
       a
  test cross
distinguishe
s between a
homozygou
 s dominant
    and a
heterozygou
   s parent
Figure 10.6
         Mendel’s Experiments
• Mendel’s experimental design
  –Protocol #2: dihybrid crosses
    • P: crossed true breeding plants with
          different traits for two characters
    • F1: scored phenotypes & self-
      pollinated
    • F2: scored phenotypes
        Mendel’s Experiments
• Protocol #2: dihybrid crosses
  –results
    • F1: all shared the traits of one parent
    • F2:
       –traits of both parents occurred in 5/8
        of F2 at a 9:1 ratio
       –non-parental pairs of traits
        appeared in 3/8 of F2 at a 1:1 ratio
 combining
probabilities
   of two
 characters
Figure 10.7
        four
     different
     gametes
         by
or   meiosis
         in
         F1
     dihybrid
     progeny
      Figure
        Mendel’s Experiments
• Protocol #2: dihybrid crosses
  –results
    • F1: all shared traits of one parent
    • F2:
       –traits of both parents occurred in 5/8
        of F2 at a 9:1 ratio
       –nonparental pairs of traits appeared
        in 3/8 of F2 at a 1:1 ratio
       –phenotypic ratios: 9:3:3:1
       Mendel’s Experiments
• Protocol #2: dihybrid crosses
  –phenotypic ratios: 9:3:3:1
    • predictable if alleles assort
      independently
       –character A - 3:1
        dominant:recessive
       –character B - 3:1
        dominant:recessive
       –characters A & B -
         »9 dominant A & dominant B
        Mendel’s Experiments
• Protocol #2: dihybrid crosses
  –a dihybrid test cross (A_B_ x aabb)
    • F1 all with dominant parent
      phenotype, or
    • 1:1:1:1 phenotypes
    Mendel without the experiments:
              pedigrees
• tracking inheritance patterns in human
  populations
   –uncontrolled experimentally
   –small progenies
   –unknown parental genotypes
• Mendelian principles can interpret
  phenotypic inheritance patterns
a pedigree of
Huntington’s
   disease
Figure 10.10
a pedigree of
  albinism
Figure 10.11
        some Mendelian luck
• Multiple alleles
  –a single gene may have more than two
   alleles and multiple phenotypes
One Character, Four Alleles, Five
         Phenotypes
         Figure 10.12
 incomplete
 dominance:
intermediate
 phenotypes
Figure 10.13
        some Mendelian luck
• Incomplete Dominance
   –alters creates new intermediate
    phenotypes
   –reveals genotypes
• Co-dominance
   –creates new dominant phenotypes
co-dominance produces additional
          phenotypes
         Figure 10.14
        some Mendelian luck
• genes may interact
  –epistasis
    • for mouse coat color
       –BB or Bb => agouti, bb => black
       –AA or Aa => colored, aa => white
• AaBb x AaBb => 9 agouti, 3 black, 4
  white
  –9 AA or Aa with BB or Bb
  –3 AA or Aa with bb
white, black & agouti
    Figure 10.15
         some Mendelian luck
• genes may interact
  –hybrid vigor (heterosis)
    • hybrids are more vigorous than either
      inbred parent
hybrid vigor in maize
    Figure 10.16
        some Mendelian luck
• genes may interact
  –quantitative traits
    • some traits are determined by many
      genes, each of which may have many
      alleles
         some Mendelian luck
• environment may alter phenotype
  –some traits are altered by the
    environment of the organism
     • penetrance: proportion of a population
       expressing the phenotype
     • expressivity: degree of expression of
       the phenotype
variation in heterozygotes
    due to differences
       in penetrance
       & expressivity

variation in the population
  due to differences in
penetrance, expressivity &
         genotype


       Figure 10.17
Drosophila melanogaster
     Figure 10.18
More Mendelian luck: gene linkage
• gene linkage was first demonstrated in
  Drosophila melanogaster
  –some genes do not assort
    independently
     • F2 phenotype ratios are not 9:3:3:1
     • F1 test cross ratios are not 1:1:1:1
        –more parental combinations appear
         than are expected
        –fewer recombinant combinations
Mendel’s luck: some genes are linked
            Figure 10.18


                       2300
                        test
                       cross
                      progeny
hypothetical
reproduction
   without
  crossing
   over at
 prophase I
 of meiosis
crossing over can change allele
   combinations of linked loci
         Figure 10.19
recombination frequency depends on
             distance
           Figure 10.20




                           391/2300=0.17
                            17 map units
More Mendelian luck: gene linkage
• if genes were completely linked, only
  parental phenotypes would result
• if genes assort independently phenotypes
  arise in 9:3:3:1 ratio in F2
• when genes are linked, recombinant
  phenotypes are fewer than expected
• recombinant frequencies depend on
  distance
   –distances can be estimated from
         chromosome mapping
             Figure 10.21




  YyMm x yymm      wt    yell.    min.
y/m
   expected/1000        250      250     250
  Mendel’s luck: sex-linked genes
• Sex determination
  –honey bees: diploid female, haploid
   male
  –grasshopper: XX female, XO male
  –mammals: XX female, XY male
    • SRY gene determines maleness
  –Drosophila: XX female, XY male
    • ratio of X:autosomes determines sex
  –birds, moths & butterflies: ZZ male, ZW
   female
  Mendel’s luck: sex-linked genes
• genes carried on X chromosome are
  absent from the Y chromosome
• a recessive sex-linked allele is expressed
  in the phenotype of a male
   –females may be “carriers”
   –males express the single allele
sex-linked genes
  Figure 10.23
  Mendel’s luck: sex-linked genes
• human sex-linked inheritance can be
  deduced from pedigree analysis
inheritance of X-linked
         gene
     Figure 10.24
          Mendel’s Principles
• Principle of segregation
  –two alleles for a character are not
    altered by time spent together in a
    diploid nucleus
• Principle of independent assortment
  –segregation of alleles for one character
    does not affect segregation of alleles for
    another character
     • unless both reside on the same

				
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