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Fig. 6-1

Chapter 6: from gene to phenotype. Fig. 6-1. Using Neurospora , Beadle & Tatum showed that genes encode enzymes and that most enzymes work in biochemical pathways Wild-type grows on minimal medium ( prototrophic ) (has genes/enzymes to biosynthesize virtually all

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Fig. 6-1

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  1. Chapter 6: from gene to phenotype Fig. 6-1

  2. Using Neurospora, Beadle & Tatum showed • that genes encode enzymes and that most • enzymes work in biochemical pathways • Wild-type grows on minimal medium (prototrophic) • (has genes/enzymes to biosynthesize virtually all • compounds required for life) • Isolated mutants that require specific nutrient in • medium (auxotrophic; defective in a pathway) • Analyzed mutants to identify steps (enzymes) in • the pathway

  3. Fig. 6-4

  4. Fig. 6-4 Gene: arg-1+ arg-2+ arg-3+ X Y Z “One gene – one enzyme” hypothesis

  5. Human metabolism of phenylalanine and known mutations Fig. 6-5

  6. Known mutations in the human phenylalanine hydroxylase gene Fig. 6-6

  7. Consequences of mutations on protein function • Recessive mutations • Partially reduce protein function (“leaky” mutations) • Abolish protein function (“null” mutations) • (will be recessive if one wild-type gene copy if • sufficient to support normal cell function) • Dominant mutations • Haplo-insufficient mutations (one wild-type gene copy is • insufficient) • Gain-of-function mutations (novel function of protein or • mis-expression of gene) • Mutations with no effect on protein function • (“silent” mutations)

  8. Fig. 6-7

  9. Recessive mutant allele of a haplosufficient gene Fig. 6-8

  10. Inter-allelic interactions • Incomplete dominance • heterozygote phenotype is intermediate • F2 phenotypic ratio 1:2:1 • Co-dominance • both alleles produce a phenotype

  11. Example of co-dominance: ABO blood group GroupGenotype AIA / IA or IA / i BIB / IB or IB / i Oi / i ABIA / IB IA , IB , and i are multiple alleles of the I gene

  12. Inter-allelic interactions • Incomplete dominance • heterozygote phenotype is intermediate • F2 phenotypic ratio 1:2:1 • Co-dominance • both alleles produce a phenotype • Lethal alleles

  13. Cross of mice heterozygous for the yellow coat color allele AY/A X AY/A 2 yellow : 1 wild type ratio results from lethality of AY/AY Fig. 6-13

  14. Manx cat (ML/M) Fig. 6-14

  15. pleiotropism: single gene difference can affect • multiple phenotypes • Example: Drosophila white mutation • lack of pigment in eye, testis sheath, Malphighian • tubules • electroretinogram defects • impaired vision, resulting in behavioral deviation • change in primary structure of the white protein

  16. complementation: a test for the allelism of two • recessive genes; if a wild-type phenotype results • from putting both genes in a diploid, we say that • the genes complement each other (i.e., they are • alleles of different genes) • Test: • cross individuals carrying the unknown genes, • and observe the phenotype of the hybrid • “a/a” X “a/a” • normal phenotype recessive phenotype • genes complement -fail to complement • are not alleles -are alleles • a/a+ b/b+ a1/a2

  17. Complementation of flower color mutations in Campanula Fig. 6-16

  18. Complementation tests can be performed heterokaryons in Neurospora Fig. 6-17

  19. w/w; m/m double mutant: is white flower • indistinguishable from w/w; m/+ mutant • gene m mutation is not apparent in the double • mutant (is “masked”)

  20. w/w; m/m double mutant: is white flower • indistinguishable from w/w; m/+ mutant • gene m mutation is not apparent in the double • mutant (is “masked”) • Epistasis: the expression of one gene is not • observed in the presence of another, • non-allelic gene • Gene w mutations are epistatic to gene m • mutations; the product of gene m is apparently • “downstream” in a pathway that includes the • product of gene w.

  21. A molecular example of epistasis Epistasis implies gene interaction Fig. 6-20

  22. Coat color in Labrador dogs is controlled by the B gene (black vs. brown pigment) and the E gene (pigment vs. none) B/-;E/- b/b;E/- B/-;e/e Fig. 6-21

  23. Suppression: a type of epistasis whereby the expression of one gene (the “suppressor” gene) normalizes the phenotype of another gene (the suppressed gene); otherwise, the suppressor gene produces no apparent phenotype.

  24. Suppression of the purpleoid eye color by a non-allelic suppressor (su)

  25. Model for suppression interactions at the protein level Fig. 6-22

  26. Penetrance: frequency with which a phenotype is shown by a particular genotype Expressivity: the degree of phenotype produced by a particular genotype

  27. Fig. 6-25

  28. Variable expressivity of the pie-bald phenotype in beagles Fig. 6-26

  29. Inheritance of a dominant, incompletely penetrant allele Fig. 6-27

  30. Fig. 6-

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