© 2006 Jones and Bartlett Publishers Chapter 15Complex Inheritance 15.1quantitative traits 15.2gene/environment interactions 15.3artificial selection

© 2006 Jones and Bartlett Publishers

Chapter 15 Complex Inheritance

15.1 quantitative traits15.2 gene/environment interactions15.3 artificial selection

Up until now…traits have been discrete

either round or wrinkled,either yellow or green,red eyes or white eyes,…

a single gene has different alleles having different phenotypes

very easy to study and understand

But many traits are the result of interactions between multiple genes as well as being affected by the environment

The traits are called:

multifactorial traitsquantitative traits


influenced by:

alternative genotypes of one or more genes

environmental factors

inbred lines


Fig. 15.1. A completely inbred line is homozygous for every gene


influenced by:

alternative genotypes of one or more genes

environmental factors

example height

continuous traitsheight, blood pressure, weightcrop yield, milk production

categorical traitsears of corn/stalkeggs from henridges in fingerprints

threshold traitsfew phenotypesmultiple genes/environement“predisposition to express”

continuous traits

“discrete” traits

like seed color75% yellow, 25% green

like heightdistributions

mean, variance (std. deviation)

Quantification (how do we describe the results)

mean

sum of all heights divided by # of people measured

= average

62” 65” 63” 70”

65”260/4 = = mean


Table 15.1. Distribution of height among British women

54*5+56*33+58*254+…

divided by 4995

=63.1 in. = mean height

€

∑ f ix i

N

€

x =


Fig. 15.2. Graph of distribution of height among 4995 British women

mean=average

mean=

sum of all heightsdivided by

number of people


Fig. 15.3. A living histogram of human height

mean

variance?

standard deviation?

mean

s2 =variance

standarddeviation

€

s2

€

∑ f ix i

N

€

x =μ =

€

∑ f i x i − x ( )2

N −1

σ =


Fig. 15.2. Graph of distribution of height among 4995 British women

mean = 63.1 inches

variance = 7.24 inches2

std dev = 2.69 inches


Fig. 15.5. Features of a normal distribution

annotated bib.

36.3 = mean 2.4 = stdev

67%

95%99.7%

bell curve


Fig. 15.4. Variance of a distribution measures the spread of the distribution around the mean

Variation in a trait

genetic

environmental

•genotypic variation•environmental variation•variation due to genotype-

by-environment interaction

•variation due to genotype-by-environment association


genotypic variation

the distribution of phenotypes, by itself, provides no information about how many genes influence a trait

due to differences in genotype


Fig. 15.6. Segregation of independent genes affecting a quantitative trait

3 genes affect trait

A or a, B or b, C or c

each dominant contributes some to phenotype


Fig. 15.7. Distribution of phenotypes determined by the segregation of 3 and 30 independent genes

3 vs 30 genes?

distribution is the same


genotypic variation

the distribution of phenotypes, by itself, provides no information about how many genes influence a trait

due to differences in genotype


environmental variation

due to differences in environment


Fig. 15.8. Distribution of seed weight in a homozygous line of edible beans

inbred

beans

normalbell curve


Fig. 15.8. Distribution of seed weight in a homozygous line of edible beans


environmental variation

due to differences in environment

the distribution provides no information about the relative importance of genotype or environment. Could be either/or or both


genetic and environmental variation

when both affect phenotype independently, the total variance is the sum of the individual variances


Fig. 15.9. Combined effects of genotypic and environmental variance



when both affect phenotype independently, the total variance is the sum of the individual variances

totalvariance

genotypicvariance

environmentalvariance

= +

€

σ p2 =σ g

2 +σ e2

(eq. 15.3)



REVIEW:

•genotypic (G) variation•environmental (E) variation•variation due to G-E interaction•variation due to G-E association

variation due to G-E interaction

(genotype-by-environment)

corn poor environment

good environment

strain A does better than B

strain B does better than A


Fig. 15.10. Genotype-by-environment interaction in maize. [Data from W. A. Russell. 1974. Annual Corn & Sorghum Research Conference 29: 81]

e.g.,

special varieties of plants developed to suit different growing areas

variation due to G-E (?) interaction

(genotype-by-sex)

sex different phenotype depending on gender of organism

living histogramand height


Fig. 15.3. A living histogram of human height

variation due to G-E association

(genotype-by-environment)

cow example?

A homogeneous population…

…will have no genotypic variance.

€

σ p2 =σ g

2 +σ e2

€

σ g2 = 0

€

σ p2 =σ e

2Therefore:

cave dwelling fish

cross

F1

cross

F2

homogeneouspopulation

heterogeneouspopulation

inbred inbred

see fig 15.6


Fig. 15.6. Segregation of independent genes affecting a quantitative trait

cross

F1

cross

F2

inbred inbred

measureeye sizevariation

homogeneouspopulation

heterogeneouspopulation

cross

F1

cross

F2

inbred inbred

measureeye sizevariation

€

σ 2 = 0.057

€

σ 2 = 0.563

F1

F2

€

σ 2 = 0.057 =σ e2

€

σ 2 = 0.563 =σ p2 =σ g

2 +σ e2

€

0.563−0.057 =σ g2 +σ e

2 −σ e2

€

0.506 =σ g2

Which is more important

genotypeor

environment?

broad-sense heritability H2

shows the importance of genetic variation, relative to environmental variation, in causing variation in phenotype

90% of eye variation in fish is genetic

ratio of genotypic variance to total phenotypic variance

€

H 2 =σ g2

σ p2=

σ g2

σ g2 +σ e

2=0.506

0.563= 0.90


Fig. 15.13. Selection for increased length of corolla tube in tobacco


Fig. 15.13. Selection for increased length of corolla tube in tobacco

M = mean of parental generation

M* = mean of selected parents

M’ = mean of progeny of selected parents

narrow-sense heritability

€

h2 =M ' −M

M* −M€

M ' −M( ) = h2 M* −M( )

narrow-sense heritability

ratio of additive genetic variance to the total phenotypic variance€

h2 =M ' −M

M* −M

broad-sense heritability H2

proportion of phenotypic variancedue to genetic differences

narrow-sense heritability h2

proportion of phenotypic variancedue to differences in additive alleles


that’s all from Chapter 15for now folks

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© 2006 Jones and Bartlett Publishers Chapter 15Complex Inheritance 15.1quantitative traits 15.2gene/environment interactions 15.3artificial selection