1 / 30

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

anitra
Télécharger la présentation

Fig. 6-1

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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-

More Related