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Beyond mendel l.jpg

Beyond Mendel

Not simply “dominant & recessive”

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Multiple allelic traits

  • Can occur in two ways:

    • New alleles arise by mutation

    • Genes may have more than 2 allele types

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New alleles by mutation

  • mutations: rare, stable, & inherited changes in DNA

    • Most common, “expected” allele is defined as the wildtype

    • Other alleles are defined as mutant alleles

      • Ex. Eye color in fruit flys: red eyes  wildtype vs. white eyes  mutant

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New alleles by mutation

  • polymorphic allele: genetic locus with a wild-type that occurs < 99% of the time

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Genes with multiple alleles

  • Occur from random mutation

  • More than 2 allele types exist for the gene, although each individual contains only 2 alleles for the gene (one from mom & one from dad)

  • Increase the number of phenotypes (not just Mendel’s two types)

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Genes with multiple alleles

  • Example:

    coat color in rabbits

    one gene with 4 alleles with a dominant heirarchy:


    C (dark gray) ch (chinchilla)

    h (himalayan) c (albino)

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No dominant over recessive

  • incomplete dominance: one allele is not dominant over another

    • heterozygotes: show interimediates between 2 allele types

  • codominance: one or more alleles are dominant &

    • heterozygotes: both alleles get expressed in the phenotype

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Incomplete dominance

  • Ex. Snapdragons


    white x red

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Incomplete dominance

  • Follows Mendel’s laws because in F2 generation, all 3 phenotypes appear (no blending)

  • Instead of expected 3:1 ratio in F2 generation, get 1:2:1 ratio of offspring

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  • Two alleles at a locus produce two different phenotypes that are both expressed in the heterozygote

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  • Ex. Blood types

    • Four phenotypes possible:

      A, B, AB, and O

    • 3 possible alleles:

      IA, IB, iO

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Pleiotropic alleles

  • single allele can affect more than one phenotype

    Ex. Allele responsible for coloration pattern in Siamese cats

    ( light body, darker extremeties)

    Same allele is responsible for the crossed eyes of the cats

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Pleiotropic alleles

  • Allele results in the same protein, but affects two different characteristics

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Genes effecting genes

  • epistasis: the phenotypic expression of one gene is affected by another gene

    Ex. Coat color in mice

    - wild-type (B): agouti (grayish pattern resulting from bands on the individual hairs)

    bb: no bands  black

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Epistasis in mice

  • A second locus on another chromosome affects early development of hair pigmentation

    A (normal pigmentation)

    aa (no pigmentation)

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Epistasis in mice

  • All mice having aa are albino, regardless of the genotype at the B locus

  • When considering both loci together in a cross, don’t get the expected Mendelian ratio in the F2 generation (9:3:3:1)

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Epistasis in mice

AABB x aabb


AaBb x AaBb

9 agouti: 3 black: 4 albino

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Effect of environment

  • Genotype and environment interact to determine the phenotype of an organism.

    • Light, temp., nutrition

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Effect of environment

  • Consider 2 parameters:

    • Penetrance: proportion of individuals in a group with a given genotype that show the expected phenotype

    • Expressivity: degree to which a genotype is espressed in an individual

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Environment & genotype

  • Result in a population with a continuous variation (instead of two extremes)

    • due to multiple genes, each with multiple alleles, and environmental influences