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Today: some things Mendel did not tell us… plus Mapping and Epigenetics

Today: some things Mendel did not tell us… plus Mapping and Epigenetics –Exam #3 W 7/30 in class (bonus #2 due)–. Single genes controlling a single trait are unusual. Inheritance of most genes/traits is much more complex…. Dom. Rec. Rec. Dom. Genotype. Phenotype.

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Today: some things Mendel did not tell us… plus Mapping and Epigenetics

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  1. Today: some things Mendel did not tell us… plus Mapping and Epigenetics –Exam #3 W 7/30 in class (bonus #2 due)–

  2. Single genes controlling a single trait are unusual. Inheritance of most genes/traits is much more complex… Dom. Rec. Rec. Dom.

  3. Genotype Phenotype Genes code for proteins (or RNA). These gene products give rise to traits… It is rarely this simple.

  4. Fig 4.4

  5. Sickle-cell anemia is caused by a point mutation Fig4.7

  6. Fig4.7 Sickle and normal red blood cells

  7. Fig4.7 Sickle-Cell Anemia: A dominant or recessive allele? S=sickle-cell H=normal Mom = HS Dad = HS Dad H or S possible offspring 75% Normal 25% Sickle-cell HS HH H or S Mom HS SS

  8. Coincidence of malaria and sickle-cell anemia Fig 24.14

  9. Fig4.7 Sickle-Cell Anemia: A dominant or recessive allele? S=sickle-cell H=normal Mom = HS Dad = HS possible offspring Oxygen transport: 75% Normal 25% Sickle-cell Malaria resistance: 75% resistant 25% susceptible Dad H or S HS HH H or S Mom HS SS

  10. The relationship between genes and traits is often complex Complexities include: • Complex relationships between alleles

  11. Fig 3.18 Sex determination is normally inherited by whole chromosomes or by number of chromosomes.

  12. X/Y chromosomes in humans

  13. The X chromosome has many genes; the Y chromosome only has genes for maleness.

  14. Human sex chromosomes Fig 4.14 (includes Mic2 gene)

  15. Sex-linked traits are genes located on the X chromosome

  16. Color Blind Test

  17. Sex-linked traits: Genes on the X chromosome A= normal; a= colorblind colorblind normal No one affected, female carriers similar to Fig 4.13

  18. Sex-linked traits: Genes on the X chromosome A= normal; a= colorblind normal normal 50% of males affected, 0 % females affected similar to Fig 4.13

  19. Sex-linked traits: Genes on the X chromosome A= normal; a= colorblind colorblind normal 50% males affected, 50% females affected similar to Fig 4.13

  20. Sex-linked traits: Genes on the X chromosome A= normal ; a= colorblind No one affected, female carriers 50% of males affected, 0 % female affected 50% males affected, 50% females affected similar to Fig 4.13

  21. Fig 3.18 males and females may have different numbers of chromosomes

  22. Tbl 7.1 dosage compensation

  23. Fig 7.4 The epithelial cells derived from this embryonic cell will produce a patch of white fur At an early stage of embryonic development While those from this will produce a patch of black fur

  24. Mammalian X-inactivation involves the interaction of 2 overlapping genes. Promotes compaction Prevents compaction

  25. The Barr body is replicated and both copies remain compacted Barr body compaction is heritable within an individual

  26. A few genes on the inactivated X chromosome are expressed in the somatic cells of adult female mammals • Pseudoautosomal genes(Dosage compensation in this case is unnecessary because these genes are located both on the X and Y) • Up to a 25% of X genes in humans may escape full inactivation • The mechanism is not understood

  27. Lamarck was right? Sort of… Epigenetics: http://www.pbs.org/wgbh/nova/sciencenow/3411/02.html Image from: http://www.sparknotes.com/biology/evolution/lamarck/section2.rhtml

  28. Genomic Imprinting • Genomic imprinting is a phenomenon in which expression of a gene depends on whether it is inherited from the male or the female parent • Imprinted genes follow a non-Mendelian pattern of inheritance • Depending on how the genes are “marked”, the offspring expresses either the maternally-inherited or the paternally-inherited allele **Not both

  29. Genomic Imprinting: Methylation of genes during gamete production.

  30. A hypothetical example of imprinting a B* a B* A* b A=curly hair a=straight hair B=beady eyes b=normal *=methylation A* in males B* in females A* b

  31. A hypothetical example of imprinting a B* a B* A* b A=curly hair a=straight hair B=beady eyes b=normal *=methylation A* in males B* in females A* b A*a bB* A*a bB*

  32. A hypothetical example of imprinting a B* a B* A* b A=curly hair a=straight hair B=beady eyes b=normal *=methylation A* in males B* in females A* b A*a bB* A*a bB* A*a bB Aa bB*

  33. A hypothetical example of imprinting similar to Fig 7.10 a B* a B* A* b A=curly hair a=straight hair B=beady eyes b=normal *=methylation A* in males B* in females A* b A*a bB* A*a bB* A*a bB Aa bB* Ab, AB*, ab, aB* A*b, A*B, ab, aB

  34. Thus genomic imprinting is permanent in the somatic cells of an animal • However, the marking of alleles can be altered from generation to generation

  35. Imprinting and DNA Methylation • Genomic imprinting must involve a marking process • At the molecular level, the imprinting is known to involve differentially methylated regions • They are methylated either in the oocyte or sperm • Not both

  36. For most genes, methylation results in inhibition of gene expression • However, this is not always the case

  37. Fig 7.11 Changes in methylation during gamete development alter the imprint Haploid female gametes transmit an unmethylated gene Haploid male gametes transmit a methylated gene

  38. Tbl 7.2 To date, imprinting has been identified in dozens of mammalian genes

  39. Tbl 7.2

  40. Imprinting plays a role in the inheritance of some human diseases: Prader-Willi syndrome (PWS) and Angelman syndrome (AS) • PWS is characterized by: reduced motor function, obesity, mental deficiencies • AS is characterized by: hyperactivity, unusual seizures, repetitive muscle movements, mental deficiencies • Usually, PWS and AS involve a small deletion in chromosome 15 • If it is inherited from the mother, it leads to AS • If it is inherited from the father, it leads to PWS

  41. AS results from the lack of expression of UBE3A (encodes a protein called EA-6P that transfers small ubiquitin molecules to certain proteins to target their degradation) • The gene is paternally imprinted (silenced) • PWS results (most likely) from the lack of expression of SNRNP (encodes a small nuclear ribonucleoprotein that controls gene splicing necessary for the synthesis of critical proteins in the brain) • The gene is maternally imprinted (silenced)

  42. Fig 7.12 The deletion is the same in males and females, but the expression is different depending on who you received the normal version from.

  43. The relationship between genes and traits is often complex Complexities include: • Multiple genes controlling one trait

  44. Two genes control coat color in mice Fig 4.21

  45. The interaction of these two proteins explains their affect on a single trait (in fruit flies).

  46. Variation in Peas Fig 3.2

  47. Fig 2.8 Inheritance of 2 independent genes

  48. Approximate position of seed color and shape genes in peas Gene for seed color y Y r R Gene for seed shape Chrom. 1/7 Chrom. 7/7

  49. Fig 2.9 There must be a better way…

  50. Inheritance can be predicted by probability Section 2.2, pg 30-32

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