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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 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


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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


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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



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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



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: 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 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: Round x 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



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


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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



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some Mendelian luck

  • Multiple alleles

    • a single gene may have more than two alleles and multiple phenotypes




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some Mendelian luck

  • Incomplete Dominance

    • alters creates new intermediate phenotypes

    • reveals genotypes

  • Co-dominance

    • creates new dominant phenotypes



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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



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some Mendelian luck

  • genes may interact

    • hybrid vigor (heterosis)

      • hybrids are more vigorous than either inbred parent



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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


Some mendelian luck46 l.jpg
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


Slide47 l.jpg

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



More mendelian luck gene linkage l.jpg
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



Recombination frequency depends on distance figure 10 20 l.jpg
recombination frequency depends on distanceFigure 10.20

391/2300=0.17

17 map units


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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


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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



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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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