Quantitative genetics of natural variation some questions
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Quantitative Genetics of Natural Variation: some questions. Do most adaptations involve the fixation of major genes?. micromutationist view : adaptations arise by allelic substitution of slight effect at many (innumerable) loci, and no single substitution constitutes a major

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Quantitative genetics of natural variation some questions

Quantitative Genetics of Natural Variation: some questions

Do most adaptations involve the fixation of major genes?

micromutationist view: adaptations arise by allelic substitution of slight effectat many (innumerable) loci, and no single substitution constitutes a major

portion of an adaptation (Darwin, Fisher)

macromutationist views:

1. single “systemic” mutations produce complex adaptations in essentially perfect form (Goldschmidt)

2. adaptation often involves one or a few alleles having large effects

• Of 8 studies, only 3 consistent with changes involving > 5 loci (Orr and Coyne 1992)


Quantitative genetics of natural variation some questions1

Quantitative Genetics of Natural Variation: some questions

• How many loci contribute to naturally occurring phenotypic

variation, and what are the magnitudes of their effects?

• What sorts of genes —and changes in these genes—are responsible for trait variation within populations (e.g., transcription factors, structural genes, metabolic genes)

• Do the same genes that contribute to variation within species also contribute to variation between species?

• What genes underlie evolutionary novelties?

• What are the genetic bases for evolutionary novelties?

• How do pleiotropic effects of genes evolve?

Answers require a mechanistic approach towards identifying the relevant loci and how genetic differences are translated into phenotypic differences


Quantitative traits depend on multiple underlying loci

Quantitative traits depend on multiple underlying loci

one locus +

environment

two loci +

environment

one locus

four loci +

environment

many loci +

environment


Phenotypic value and population means falconer and mackay ch 7

A2A2

A1A2

A1A1

genotype

– a

0

d

+ a

genotypic value

Phenotypic Value and Population Means (Falconer and Mackay Ch. 7)

P = G + E

Phenotypic value = Genotypic value + Environmental Deviation

GenotypeFreq Value Freq x Val

A1A1p2 +a p2a

A1A22pq d 2pqd

A2A2 q2 -a -q2a

Sum = Pop Mean = a(p-q) + 2dpq


Quantitative genetics of natural variation some questions

Timing of Metamorphosis

The majority of organisms on planet earth have complex life cycles

Predictable

Larval Habitat

Hatching

Metamorphosis

Predictable

Ephemeral Pond

Time


Quantitative genetics of natural variation some questions

Thyroid Hormone Receptors as Candidate Genes for

Variation in Metamorphic Timing

Hypothalamus

TRH

Pituitary

TSH

Thyroid

TH

Target cells

T4

deiodionation

T3

TRs

transcription

An extreme difference in

metamorphic timing


Quantitative genetics of natural variation some questions

Thyroid Hormone Receptors : A Hypothetical Example

Thyroid Hormone Receptor

Alpha Genotype

A1A1

A1A2

A2A2

Timing of

Metamorphosis

(Days)

200

160

150

d

-15

-a

a

-25

25

0

Homozygote

Midpoint

(175)


Quantitative genetics of natural variation some questions

GenotypeFreq Value Freq x Val

A1A1p2 25 p2(25)

A1A22pq -15 2pq(-15)

A2A2 q2 -25 -q2(25)

Sum = Pop Mean = 25(p-q) + 2(-15)pq

(adds time)

(reduces time)

p = f(A1)

q = f(A2)

A1A1A1A2A2A2

Mean

00-25

2.25-6.3-12.25

6.25-7.5-6.25

12.25-6.3-2.25

2500

0.0

0.3

0.5

0.7

1.0

1.0

0.7

0.5

0.3

0.0

-25 (150)

-16.3 (158.7)

-7.5 (167.5)

3.7 (178.7)

25 (200)


Quantitative genetics of natural variation some questions

Let’s Consider a Second Locus

Thyroid Hormone Receptor

Alpha Genotype

A1A1

A1A2

A2A2

Timing of

Metamorphosis

(Days)

200

160

150

0

Thyroid Hormone Receptor

Beta Genotype

A1A1

A1A2

A2A2

Timing of

Metamorphosis

(Days)

200

140

0

-a

a

-30

30

Homozygote

Midpoint

(170)


Quantitative genetics of natural variation some questions

GenotypeFreq Value Freq x Val

A1A1p2 30 p2(30)

A1A22pq 0 2pq(0)

A2A2 q2 -30 -q2(30)

Sum = Pop Mean = 30(p-q) + 2(0)pq

(adds time)

(reduces time)

P = f(A1)

Q = f(A2)

A1A1A1A2A2A2

Mean

-30 (140)

-12 (158)

0 (170)

12 (182)

30 (200)

00-30

2.70-14.7

00 0

14.70-2.7

3000

0.0

0.3

0.5

0.7

1.0

1.0

0.7

0.5

0.3

0.0


Quantitative genetics of natural variation some questions

Consider the joint effect of both TH Loci

Total Range = 2Sa=110

Tha A1A1

Thb A1A1

Tha A2A2

Thb A2A2

Timing of

Metamorphosis

(Days)

227.5

117.5

0

-a

a

55

55

Average

Homozygote

Midpoint

(172.5)

Overall

Mean

=

Sa(p-q) + S2dpq


Quantitative genetics of natural variation some questions

Genotypic value is not transferred from parent to

offspring; genes are.

Need a value that reflects the genes that an

individual carries and passes on to it’s offspring

Breeding Value

Empirically: An individual’s value based on the mean deviation of its progeny

from the population mean.

Theoretically: An individual’s value based on the sum of the average effects

of the alleles/genes it carries.


Quantitative genetics of natural variation some questions

.

a1 = pa + qd - [ a (p – q) + 2dpq ]

population mean

f (A1)

f (A2)

Average Effect of an Allele

Type of Values and FreqMean valuePopulation Average

gamete of gametesof genotypesmean effect of

gene

A1A1 A1A2 A2A2

a d -a

A1 p q pa + qd -a(p-q) + 2dpq q[a+d(q-p)]

A2 p q -qa + pd -a(p-q) + 2dpq -p[a+d(q-p)]

average effect of An:

an = mean deviation from the population mean of individuals that received An from one parent, if the other parent’s allele chosen randomly

a1 = q [ a + d (q – p)]

a2 = –p [ a + d (q – p)]


Quantitative genetics of natural variation some questions

When there are only two alleles at a locus

Average effect of a gene substitution

A1A1

A1A2

A2A2

+a

d

-a

(a - d)

(d + a)

p(a - d) +q(d + a)

a = a + d(q - p)

a

pa


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