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neo-Mendelian Genetics

neo-Mendelian Genetics. After re-discovery, Mendel’s postulates were applied to many genetic traits, However, some genetic data did not conform to the expected results, Continuous Variation vs. Discontinuous Variation. Alleles. Alleles are alternate forms of the same gene,

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neo-Mendelian Genetics

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  1. neo-Mendelian Genetics • After re-discovery, Mendel’s postulates were applied to many genetic traits, • However, some genetic data did not conform to the expected results, Continuous Variation vs. Discontinuous Variation

  2. Alleles • Alleles are alternate forms of the same gene, • Wild-type allele: the allele that occurs most frequently in the population, • ‘normal’, • usually dominant.

  3. Wild-type vs. ? • wild-type alleles are responsible for the most common phenotype, • mutant alleles: contain modified genetic information, compared to wild-type, • mutant alleles are responsible for mutant • phenotypes.

  4. A1 A1 A2 A2 complete x A1 dom. A2 A2 rec. A1 A2 dom. A1 complete x A1 rec. A2 incomplete x codominant x Dominance(not always complete) phenotypes A1 A2 hybrids A2 A2 A1 A1 to each other to each other

  5. A1 A2 hybrids A2 A2 A1 A1 incomplete x Incomplete Dominance …expression of heterozygote phenotype resulting in offspring that do not resemble either parent, • often, offspring phenotype is intermediate between those of the parents. A1 and A2 are incompletely dominant in regards to each other.

  6. R1: red R2: white Genotypes Phenotypes

  7. Phenotypic Ratio is Equal to Genotypic Ratio Genotypes Phenotypes

  8. Genotype: 1:2:1 Phenotype: 3:1 R2 dom R1 Genotype: 1:2:1 Phenotype: 3:1 R1 dom R2 Genotype: 1:2:1 Phenotype: 1:2:1 incompletely dominant F2 Possible Ratios R1 R1 R1 R2 R1 R2 R2 R2

  9. A1 A2 hybrids A2 A2 A1 A1 codominant x Codominance …expression of heterozygote phenotype resulting in offspring that resemble both parents, • both parental traits show up equally and fully in the offspring. A1 and A2 are codominant to each other.

  10. x CS CS CS CS CD CD CD CD F1 CS CD CS CD CS CD CSspottedCDdotted P F2

  11. F2 Possible Ratios CS CS CS CD CS CD CD CD g: 1:2:1 p: 1:2:1 codominant g: 1:2:1 p: 3:1 CS dom CD g: 1:2:1 p: 1:2:1 incomplete dominance

  12. Fig. 2-20 Think about this. Dominance? Incomplete Dominance? Co-Dominance?

  13. Lethal Alleles • Essential alleles that if not expressed in a sufficient amount, result in lethality, • recessive lethal allele: homozygous recessive individuals die, heterozygous individuals live, • dominant lethal allele: homozygous recessive individuals die, heterozygous individuals die.

  14. A a A AA Aa a Aa aa Recessive Lethal Alleles • recessive lethal allele: homozygous recessive individuals die, heterozygous individuals live, Modifies Ratio: 2:1 Genotypic Ratio all wild-type

  15. Lethality Definitions • Lethal phenotype: doesn’t reproduce*, • *late onset lethal phenotypes, • Lethal allele: expression, or lack of expression causes death.

  16. recessive lethal? t: no tail T: wild-type TT: tailed cat Tt: no tail tt: lethal

  17. what about lethality? TT: tail cat Tt: no tail tt: lethal t: Manx T: wild-type Lack of T results in death, t is recessive… …t does not result in death when T is present.

  18. A a A AA Aa a Aa aa Dominant Lethal Alleles • dominant lethal allele: homozygous recessive individuals die, heterozygous individuals die, The dominant allele is not lethal! The recessive allele is the lethal, it acts as a dominant lethal in the heterozygote.

  19. Huntington’s Disease It is spoken of by those in whose veins the seeds of the disease are know to exist, with a kind of horror, and not at all alluded to except through dire necessity. - George Huntington, 1872

  20. Huntington’s • Neurological and movement disorder with typical onset at about age 40, H: wild-type h: Huntington HH: wild-type Hh: late onset lethal hh: lethal • On March 23, 1993, the Huntington's disease gene was discovered near the tip of chromosome 4. Discovery of the gene has led to direct, genetic testing for the disease. • Gene therapy and other programs are underway and are highly encouraging.

  21. H h H HH Hh h Hh hh Dominant Lethal Alleles • dominant lethal allele: dominant in regards to lethality, H: normal h: Huntington’s lethality is dominant.

  22. Gamete Lethality1:1 ratio • How could this happen, how would you test your hypothesis? Tt x Tt 1/4 Tt 1/4 Tt 1/4 tt 1/4 TT

  23. aha3-1 x wt aha3-1 x wt TT TT T t T T Tt Tt TT Tt T T TT Tt T t pollen lethal? ovule lethal? Reciprocal Crosses YES NO

  24. Aha3-1 wildtype Alexander’s Stain

  25. Aha3 Pollen

  26. More than Two Alleles?

  27. Cell membrane Antigens and Antibodies Antigen: substance that stimulates an immune response, especially the production of antibodies. Antigens are usually proteins and/or polysaccharides. Antibody: protein with the ability to bind to specific molecules...

  28. ABO and Allelic Interactions • ABO blood groups, • A and B antigens are on the surface of red blood cells, • if you add an A-antibody to blood with type A antigens, the blood co-aggulates, • if you add a B-antibody to blood with type B antigens, the blood co-aggulates, • There are four recognizable phenotypes, A B AB O

  29. ...once the antibody binds, the molecules coagulate, or clump together. Antigens, Antibodies and Coagulation

  30. I = Isoagglutinogen ...a gene with three alleles, ...codes for proteins that modify plasma membrane bound, glycosylated proteins in red blood cells.

  31. IB i (ii) no carbohydrate IA codes for a protein that specifically adds a N- acetylgalactosamine IB codes for a protein that adds a galactose A B O IA

  32. A B O AB IA IA IB IB IA IB ii IAi IBi 3 Alleles(4 phenotypes)

  33. A B O AB IA IA IB IB IA IB ii IAi IBi IA and IB are Codominant IA and IB are Dominant to i

  34. IA and IB are Codominant IA and IB are Dominant to i

  35. Know Genotype/Phenotype

  36. Bombay Phenotype How can this be?

  37. H gene product catalyzes this bond. H Substance An entire H substance must be present to permit the I gene product to bind a sugar.

  38. A B O IA H_ ii H_ IA H_

  39. No Antigenicity Genotype hh IA IA IAi IB IB IBi IA IB ii

  40. Bombay Phenotype How can this be?

  41. Epistasis …a gene interaction in which the effects of one gene hides the effects of another gene, • epistatic: gene A genotype hides gene B phenotype, • hypostatic: gene B phenotype is hidden by gene A genoype.

  42. ABO and H Alleles • epistatic: hh genotype hides AB phenotypes, • h gene is epistatic to I gene, • hypostatic: AB phenotypes are hidden by hh genotypes, • I gene is hypostatic to h gene.

  43. Study this chart. Epistasis “Classes” are masked, or “underrepresented”.

  44. Genetics: …in the News

  45. Penetrance …the frequency at which individuals with a given genotype manifest a specific phenotype.

  46. Penetrance • 4 of 6 dogs, or 66% of the population shows the phenotype, at some level, • penetrance is usually referred to as a percentage. all the same genotype

  47. Expressivity …the degree, or range in which a phenotype of a specific genotype is expressed.

  48. Expressivity • range of phenotypes • expressivity may be referred to as a percentage, or another quantifiable measure. all the same genotype

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