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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

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Mendelian Genetics

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Mendelian genetics l.jpg

Mendelian Genetics

Simple Probabilities & a Little Luck


Genetics l.jpg

Genetics

  • the study of heredity & its mechanisms

  • Gregor Mendel

    • reported experimental results in 1865/66

    • rediscovered in 1903 by de Vries, Correns & von Tschermak


Genetics3 l.jpg

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 l.jpg

Cross pollinationFigure 10.1


Mendel s experiments l.jpg

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 experiments6 l.jpg

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 l.jpg

7 characters, 14 traitsTable 10.1


One of mendel s characters figure 10 2 l.jpg

one of Mendel’s charactersFigure 10.2


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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


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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


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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)


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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 F2


Seven traits were inherited similarly table 10 1 l.jpg

seven traits were inherited similarlyTable 10.1


Mendel s interpretation inheritance does not involve blending figure 10 3 l.jpg

Mendel’s interpretation:inheritance does not involve blendingFigure 10.3


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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 particles are genes<=


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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 l.jpg

recessive traits disappear in the F1 generationFigure 10.4


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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 = phenotype

        • the genotype is not always seen in the phenotype]


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Mendel’s Experiments

  • Mendel’s experimental design

    • Protocol#1: monohybrid crosses

      • symbolic representation

        P: SS x ss

        F1:Ssgamete formationS 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 l.jpg

Punnett to the rescueFigure 10.4


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P: (SS or ss) p(S)=1 x p(s)=1F1: (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 p(ss) =1/2 x 1/2=1/4 p(Ss)=[1/2x1/2=1/4] x 2=1/2


Punnett explained by meiosis figure 10 5 l.jpg

Punnett explained by meiosisFigure 10.5

F1: Ss

replication

S-S &s-s

anaphase I

S-S or s-s

anaphase II

S or S or s or s


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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: Roundx wrinkled

        F1:1/2 Round, 1/2 wrinkled

        F2:3/4 Round, 1/4 wrinkled OR all wrinkled


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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

        (RR,Rr,rR,rr) (rr)


A test cross distinguishes between a homozygous dominant and a heterozygous parent figure 10 6 l.jpg

a test cross distinguishes between a homozygous dominantand a heterozygous parentFigure 10.6


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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


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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 l.jpg

combining probabilities of two charactersFigure 10.7


Four different gametes by meiosis in f 1 dihybrid progeny figure 10 8 l.jpg

four different gametes by meiosis in F1dihybrid progenyFigure 10.8

or


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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


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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

          • 3 dominant A & recessive B

          • 3 recessive A & dominant B

          • 1 recessive A & recessive B


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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 l.jpg

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 l.jpg

a pedigree of Huntington’s diseaseFigure 10.10


A pedigree of albinism figure 10 11 l.jpg

a pedigree of albinismFigure 10.11


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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 l.jpg

One Character, Four Alleles, Five PhenotypesFigure 10.12


Incomplete dominance intermediate phenotypes figure 10 13 l.jpg

incomplete dominance: intermediate phenotypesFigure 10.13


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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 l.jpg

co-dominance produces additional phenotypesFigure 10.14


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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

    • 3 aa with BB, Bb; 1 aa with bb = 4 white


White black agouti figure 10 15 l.jpg

white, black & agouti Figure 10.15


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some Mendelian luck

  • genes may interact

    • hybrid vigor (heterosis)

      • hybrids are more vigorous than either inbred parent


Hybrid vigor in maize figure 10 16 l.jpg

hybrid vigor in maizeFigure 10.16


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some Mendelian luck

  • genes may interact

    • quantitative traits

      • some traits are determined by many genes, each of which may have many alleles


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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


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variation in heterozygotes due to differences in penetrance & expressivityvariation in the population due todifferences in penetrance, expressivity & genotypeFigure 10.17


Drosophila melanogaster figure 10 18 l.jpg

Drosophila melanogasterFigure 10.18


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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 appear than are expected


Mendel s luck some genes are linked figure 10 18 l.jpg

Mendel’s luck: some genes are linkedFigure 10.18

2300

test

cross

progeny


Hypothetical reproduction without crossing over at prophase i of meiosis l.jpg

hypotheticalreproduction without crossing over at prophase I of meiosis


Crossing over can change allele combinations of linked loci figure 10 19 l.jpg

crossing over can change allele combinations of linked lociFigure 10.19


Recombination frequency depends on distance figure 10 20 l.jpg

recombination frequency depends on distanceFigure 10.20

391/2300=0.17

17 map units


More mendelian luck gene linkage54 l.jpg

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 recombination rates (1% = 1 map unit)


Chromosome mapping figure 10 21 l.jpg

chromosome mappingFigure 10.21

YyMm x yymm wtyell.min.y/m

expected/1000 250 250 250 250

actual/1000 323 178 177 322


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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 genes57 l.jpg

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 l.jpg

sex-linked genesFigure 10.23


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Mendel’s luck: sex-linked genes

  • human sex-linked inheritance can be deduced from pedigree analysis


Inheritance of x linked gene figure 10 24 l.jpg

inheritance of X-linked geneFigure 10.24


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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 chromosome


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