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

h2 = 0

VA/VP = 1

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 speed!

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 SA 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 tall.

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 are favored in drought years, what effect will

an El Nino year have on

the population?

h2 = 0.8

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.

h2 = 1

h2 = 0

How do you measure the heritability?

Another way to measure h2

if R = h2 S, then h2 = R/SA 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 population?

h2 = R/S = 1 cm/3 cm = 0.33

Most quantitative traits have substantial random environmental

variance

If h2=0.5, then 50% of the phenotypic variance

is additive genetic. What’s the other 50%?

h2 = VA/VP = VA/(VA+VD+VE)

Hydrangeas

Environmental effects on Quantitative Traits: Monozygotic Twins

Jones’s Farm Smith’s Farmh2 1.0 1.0milk yield 3 qts/day 2 qts/day

Heritability measured in one population does not tell you anything about differences

between populationsEnvironmental variation (VE) is not the same within and between populations

Genetic and Environmental Effects on Quantitative Traits

Can these methods be applied to humans?

Environmental variation cannot be controlled

Twin Studies

• Identical twins share 100% of alleles

• Fraternal twins share 50% of alleles

• Assume both kinds of twins share VE to the same extent

Correlations between identical co-twins

Complete genetic No genetic determination determination

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Trait in Twin 1

Trait in Twin 2

r=1.0

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Trait in Twin 1

Trait in Twin 2

r=0.0

Correlations between co-twins for adult height

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Twin 1 Height

Twin 2 Height

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Twin 1 Height

Twin 2 Height

Monozygotic (identical) Dizygotic (fraternal)

r=0.91 r=0.46

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 0.98Height 0.90 0.57 0.66IQ score 0.83 0.66 0.34Social 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 overestimated

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 membrane.

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.

Conclusions

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

F1

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

sm

sm M

SM

S

MSs

m

Small Intermediate Large

F1:

F2:

MS

MSs

msm M

Ssm

MSs

m

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

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

phenotype

Many Markers

Results

• 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