Uses of heritability

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					       Extra credit question
The genes (loci) that influence
 quantitative traits are known as:

         Uses of heritability
• The degree to which offspring resemble
  their parents is determined by the
  narrow-sense heritability h2

• The efficacy of natural and artificial
  selection is also determined by h2
h2 = 1   VA/VP = 1

h2 = 0   VA/VP = 0
Efficacy of artificial selection:
      size of Labradors
            Breeder’s Question
Q: A horse breeder wants to
  win the Kentucky Derby. If
  she breeds her mare to a
  really fast stallion, how
  likely is it that the colt will
  be faster than all the other
  three-year-olds when it runs
  in the Derby?

  A: It depends on the heritability of running
         Breeder’s Equation
• R = h2 S
• S = Selection differential
 difference between selected parents and the population
  as a whole (within a generation)
• R = response to selection
 difference between selected offspring and the unselected
  population (across generations)
        Breeder’s Equation
R = h2 S
A dog breeder chooses his largest dogs to
  breed together. The average height of the
  breed is 60 cm (at the shoulder), and the
  dogs he chooses to breed average 70 cm
He knows from previous work that the
  heritability of height is 0.5.
How big can he expect the offspring to be?

    R = h2 S = 0.5 * 10cm = 5cm
        Breeder’s Equation
    R = h2 S = 0.5 * 10 cm = 5 cm

If the response to selection is 5 cm, he
   can expect his puppies to grow to be

      60 cm + 5 cm = 65 cm tall
Exactly the same equation
can be used to understand
    natural selection!
Efficacy of natural selection:
      Darwin’s finches
               If large bills
h2   = 0.8
             are favored in
             drought years,
             what effect will
                an El Nino
              year have on
                  R = h2 S
Before El Nino mean bill depth = 10 cm. Birds
 that survived the drought had bills that were
 2 mm deeper (on average) than the
 population mean.

Q: What happened to the average bill depth
 in the next generation?

A: R = 0.8 * 2mm = 1.6 mm. Bill depth next
  generation = 10 + 1.6 = 11.6 mm.
How do you measure the heritability?

    h2 = 1

    h2 = 0
    Another way to measure h2
if R = h2 S, then h2 = R/S
A corn breeder chooses to breed from plants that
   have large cobs. The average cob length in his
   crop is 15 cm, but he breeds from plants that
   average 18 cm cobs.
Next year, he measures cob length in the offspring
   of the selected plants and discovers that the mean
   is 16 cm
What is the heritability of cob length in this

     h2 = R/S = 1 cm/3 cm = 0.33
  Most quantitative traits have
substantial random environmental
  h 2   = VA/VP = VA/(VA+VD+VE)

     If  h2=0.5, then 50% of
    the phenotypic variance
        is additive genetic.
     What’s the other 50%?
Environmental effects on
  Quantitative Traits:
  Monozygotic Twins
     Heritability measured in one
     population does not tell you
     anything about differences
        between populations
  Environmental variation (VE) is not the
  same within and between populations
           Jones’s Farm      Smith’s Farm
h2                1.0                1.0
milk yield   3 qts/day     2 qts/day
Genetic and Environmental Effects on
         Quantitative Traits
Can these methods be applied
        to humans?
  Environmental variation cannot be
         Twin Studies

• Identical twins share 100% of
• Fraternal twins share 50% of
• Assume both kinds of twins
  share VE to the same extent
                              Correlations between identical

                   Complete genetic                                                                  No genetic
                    determination                                                                   determination

                  174                                                                  190


                              r=1.0                                                    185

                                                                                       180         r=0.0
Trait in Twin 2

                  168                                                                  175

                                                                     Trait in Twin 2
                  166                                                                  170

                  164                                                                  165

                  162                                                                  160

                  160                                                                  155

                  158                                                                  150

                  156                                                                  145

                  154                                                                  140
                        150     155    160         165   170   175                           150      155     160        165   170   175

                                      Trait in Twin 1                                                       Trait in Twin 1
                        Correlations between co-twins
                               for adult height
                        Monozygotic (identical)                                        Dizygotic (fraternal)
                180                                                            190

                175         r=0.91                                             185

Twin 2 Height

                                                               Twin 2 Height

                165                                                            165



                150                                                            140
                      150    155     160     165   170   175                         150     155     160     165   170   175

                                   Twin 1 Height                                                   Twin 1 Height
  Heritability estimates from
correlations between co-twins
• Identical twins share 100% of alleles
• Fraternal twins share 50% of alleles
                H2 ≈ 2* (ri - rf)

   Height example: ri = 0.91, rf = 0.46

                  H2 = ?
                H2 = 0.90
                      ri            rf
                  Identical   Fraternal     H2
Fingerprint ridges 0.96       0.47        0.98
Height              0.90      0.57        0.66
IQ score            0.83      0.66        0.34
Social maturity     0.97      0.89        0.16
Heritabilty estimates from human
     twin studies are biased
1. H2≈2* (ri - rf) overestimates true H2 by
   0.5* VD/VP
2. H2 includes all genetic effects, not just
   additive genetic
3. GxE interaction increases variability
   among fraternal twins (it lowers rf) but
   does not affect ri, so H2 is
Heritabilty estimates from human
     twin studies are biased
4. Estimates assume identical and
   fraternal twins share environments to
   the same extent.
Do you think this is true?
   a) Identical twins share embryonic
   b) Identical twins share more similar
   social environment.
Heritabilty estimates from human
     twin studies are biased
5. Studies often based on small sample
   sizes, and therefore estimates are not
   very precise (large standard errors)

6. Some studies include male-female
   fraternal twins, whereas identical
   twins are always the same sex.

Heritabiltiy estimates derived
  from human twin studies
 should be considered very
 approximate, and probably
            too high.
Molecular Biology and
Quantitative Genetics
          QTL Mapping
• Use DNA-based markers
• Marker is anything that differs among
  strains (RFLPs, microsatellites,
  single-base pair differnces)
• Markers close to genes will tend to
  be inherited along with genes
  (recombination rare)
  Small                          Large
m         m                     M        M
s         s                     S        S


                 m          M
                 s          S

Markers that are close to the QTL are inherited with it
                (recombination is rare)
F1:                   m          M
                      s          S

 Markers that are close to the QTL are inherited with it
                 (recombination is rare)


 m         m          m          M         M         M
 s         s          s          S         S         S

      Small         Intermediate           Large
 If a marker is far from any gene
affecting fruit weight, there will be
recombination between the fruit-
      weight and the marker
 A marker far from a gene will not show
    a statistical association with the
Many Markers
• 28 QTLs responsible for fruit-weight
  variation in tomato
• One gene has been cloned and
  transferred between plants
• When “small” allele is transferred to
  normally large-fruited variety, the fruit
  weight is reduced by 30%
• Proof that the gene is really a QTL

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