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BIOE 109 Summer 2009 Lecture 7- Part II Selection on quantitative characters

BIOE 109 Summer 2009 Lecture 7- Part II Selection on quantitative characters. Selection on quantitative characters What is a quantitative (continuous) character?. Selection on quantitative characters What is a quantitative character?

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BIOE 109 Summer 2009 Lecture 7- Part II Selection on quantitative characters

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  1. BIOE 109 Summer 2009 Lecture 7- Part II Selection on quantitative characters

  2. Selection on quantitative characters What is a quantitative (continuous) character?

  3. Selection on quantitative characters What is a quantitative character? • quantitative characters exhibit continuous variation among individuals.

  4. Selection on quantitative characters • What is a quantitative character? • • quantitative characters exhibit continuous variation among individuals. • • unlike discrete characters, it is not possible to assign phenotypes to discrete groups.

  5. Examples of discrete characters

  6. Example of a continuous character

  7. Two characteristics of quantitative traits: 1. Controlled by many genetic loci

  8. Two characteristics of quantitative traits: 1. Controlled by many genetic loci 2. Exhibit variation due to both genetic and environmental effects

  9. Two characteristics of quantitative traits: 1. Controlled by many genetic loci 2. Exhibit variation due to both genetic and environmental effects • the genes that influence quantitative traits are now called quantitative trait loci or QTLs.

  10. What are QTLs?

  11. What are QTLs? • QTLs possess multiple alleles, exhibit varying degrees of dominance, and experience selection and drift.

  12. What are QTLs? • QTLs possess multiple alleles, exhibit varying degrees of dominance, and experience selection and drift. • some QTLs exhibit stronger effects than others – these are called major effect and minor effect genes, respectively.

  13. What are QTLs? • QTLs possess multiple alleles, exhibit varying degrees of dominance, and experience selection and drift. • some QTLs exhibit stronger effects than others – these are called major effect and minor effect genes, respectively. • the number and relative contributions of major effect and minor effect genes underlies the genetic architecture of the trait.

  14. Mapping QTLs is expensive, labor intensive, and fraught with statistical problems!

  15. Mapping QTLs is expensive, labor intensive, and • fraught with statistical problems! • QTL mapping can reveal: • Number of loci that influence a QT • Magnitude of their effects on phenotype • Their location on genome

  16. Mapping QTLs is expensive, labor intensive, and • fraught with statistical problems! • QTL mapping can reveal: • Number of loci that influence a QT • Magnitude of their effects on phenotype • Their location on genome • QTL mapping CANNOT reveal: • Identity of loci • Proteins they encode

  17. What is heritability?

  18. What is heritability? • heritability is the proportion of the total phenotypic variation controlled by genetic rather than environmental factors.

  19. What is heritability? • heritability is the proportion of the total phenotypic variation controlled by genetic rather than environmental factors.

  20. The total phenotypic variance may be decomposed: VP = total phenotypic variance

  21. The total phenotypic variance may be decomposed: VP = total phenotypic variance VG = total genetic variance

  22. The total phenotypic variance may be decomposed: VP = total phenotypic variance VG = total genetic variance VE = environmental variance

  23. The total phenotypic variance may be decomposed: VP = total phenotypic variance VG = total genetic variance VE = environmental variance VP = VG + VE

  24. The total phenotypic variance may be decomposed: VP = total phenotypic variance VG = total genetic variance VE = environmental variance heritability = VG/VP (broad-sense)

  25. The total genetic variance (VG) may be decomposed:

  26. The total genetic variance (VG) may be decomposed: VA = additive genetic variance

  27. The total genetic variance (VG) may be decomposed: VA = additive genetic variance VD = dominance genetic variance

  28. The total genetic variance (VG) may be decomposed: VA = additive genetic variance VD = dominance genetic variance VI = epistatic (interactive) genetic variance

  29. The total genetic variance (VG) may be decomposed: VA = additive genetic variance VD = dominance genetic variance VI = epistatic (interactive) genetic variance VG = VA + VD + VI

  30. The total genetic variance (VG) may be decomposed: VA = additive genetic variance VD = dominance genetic variance VI = epistatic (interactive) genetic variance heritability = h2 = VA/VP (narrow sense)

  31. Estimating heritability

  32. Estimating heritability • one common approach is to compare phenotypic scores of parents and their offspring:

  33. Estimating heritability • one common approach is to compare phenotypic scores of parents and their offspring: Junco tarsus length (cm) Cross Midparent value Offspring value

  34. Estimating heritability • one common approach is to compare phenotypic scores of parents and their offspring: Junco tarsus length (cm) Cross Midparent value Offspring value F1 x M1 4.34 4.73

  35. Estimating heritability • one common approach is to compare phenotypic scores of parents and their offspring: Junco tarsus length (cm) Cross Midparent value Offspring value F1 x M1 4.34 4.73 F2 x M2 5.56 5.31

  36. Estimating heritability • one common approach is to compare phenotypic scores of parents and their offspring: Junco tarsus length (cm) Cross Midparent value Offspring value F1 x M1 4.34 4.73 F2 x M2 5.56 5.31 F3 x M3 3.88 4.02

  37. Regress offspring value on midparent value  Slope = h2

  38. Heritability estimates from other regression analyses Comparison Slope

  39. Heritability estimates from other regression analyses Comparison Slope Midparent-offspring h2

  40. Heritability estimates from other regression analyses Comparison Slope Midparent-offspring h2 Parent-offspring 1/2h2

  41. Heritability estimates from other regression analyses Comparison Slope Midparent-offspring h2 Parent-offspring 1/2h2 Half-sibs 1/4h2

  42. Heritability estimates from other regression analyses Comparison Slope Midparent-offspring h2 Parent-offspring 1/2h2 Half-sibs 1/4h2 First cousins 1/8h2

  43. Heritability estimates from other regression analyses Comparison Slope Midparent-offspring h2 Parent-offspring 1/2h2 Half-sibs 1/4h2 First cousins 1/8h2 • as the groups become less related, the precision of the h2 estimate is reduced.

  44. Heritabilities vary between 0 and 1

  45. Cross-fostering is a common approach Heritability of beak size in song sparrows

  46. Q: Why is knowing heritability important?

  47. Q: Why is knowing heritability important? A: Because it allows us to predict a trait’s response to selection

  48. Q: Why is knowing heritability important? A: Because it allows us to predict a trait’s response to selection Let S = selection differential

  49. Q: Why is knowing heritability important? A: Because it allows us to predict a trait’s response to selection Let S = selection differential Let h2 = heritability

  50. Q: Why is knowing heritability important? A: Because it allows us to predict a trait’s response to selection Let S = selection differential Let h2 = heritability Let R = response to selection

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