L22Biol261W2013Q Genetics Copy

8/9/2019 L22Biol261W2013Q Genetics Copy

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Lecture 22:

Quantitativegenetics

Estimating broad

(H2)and narrow sense

(h2)heritabilities

(1)Problems for

next week are on the

Moodle site


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A

B

C

D

D

F

G

1

2

3

4

5

6

7

8

Additive effectsat

some or all loci

coding for

enzymes A-G

where there are:

Partial loss of function

mutations

or, enzyme over expression,

or some net expression

of a pathway - a

character variant

in a population

andEnvironmental effects

(temperature,

nutrients, ph) on

the rate of pigment

production

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Variance Components

V = variance: variancesare squared, so they maybe added (s2or 2).

VG- genetic variance

VE- environmentalvariance

VP- phenotypic variance

H2 = VG

(VG+V

E)= VG

(VP)

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Broad sense heritability(H2) indicates the influence (%) ofinherited variation,

variation transmitted from parents to offspring,

variation segregating in a populationsharing a defined environment.

Broad-sense heritability(H2) measures what proportion of phenotype ( a

character e.g. height) variationis due to differences in genotype (all sources of

transmitted genetic variance: (1) ancestry; (2)number and kind of interacting genes

and ; (3) for each of these, the effects of different alleles, of substituting alleles one for

another )


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Generation Mean

height (in)

Phenotypic

variance

(in height)

P1Ramona

Strain

13.0 11.0

P2Baart

Strain

27.6 10.0

F1 18.5 5.0

F2 21.2 40.0

There are 2 pure-breeding varieties or strains of

wheat that a farmer wants to cross, trying to breed a

mid-sized strain of grain with low size variation.

Broad sense heritability H2?

VP= VG+ VE

VE=

VP

=

VG= (VP- VE)=

H2=5

(1) Variance Components


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Expected and observed H2, based on familial relationships:

There is an expected positivegenetic correlationamong individuals in

a family (relatives) due to their (high) proportion of shared alleles

(100%, 50%, 25% etc.).

TRC is one of several fingerprint characters.

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Method 2


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Genetic Contribution to Variation in IQ tests

IQ tests , score word use, number problems, memory processes, spatial configuration

matrices and reaction times and are thought to measure both cognitive abilities and a

general inherited intelligence.

IQ heritability is estimated using sib data, preferred estimates use monozygotic

estimates, assuming they represent minimize environmental differences:


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Different kinds of twin and other familial studies find that IQ heritability estimates

center on 0.75 (Nessier et al. 1996.American Psychologist 51: 77 - 101), although the

range is from 0.4 to 0.9. This suggests that variation due to environmental differences

(education) has less influence on IQ than inheritance.

The problem with this argument is the progressive gains in IQ scores, about 6 IQ points

per decade(Flynn, J.R. 1999 Am. Psychologist 54 5-20). How would you explain this ?

Note: the differences

between countries indicate

how the tests were

standardized, gain can be

compared among nations

but IQ score cannot.

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There are other influences some which are genetic but are not directly

inherited: dominance (VD);genotype/ environment interaction (VGE);

epistatic variation or inter-locus interaction (VEp).

All of these variance components are in the numerator of broad

sense heritability (H2) of monozygotic twins, clones or inbred strains,

or they inflate the size of H2compared to h2.

h2 =VA

VE + (VA +VEp +VGxE +VD )

H2=

VG

(VG+V

E)

=

VG

(VP)

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Broad sense heritability (H2) isestimated from:1) Partitioning components of variancein breeding designs

2) expected genetic correlationsamong relatives,3) In particular, monozygotic and dizygotic twin studies.

Broad-sense twin heritability estimates make assumptions :

(a)

Ignoring treatments that decrease the environmentalvariance of a group - maternal effects, socially treating twinsthe same.

(b) Ignoring differences between monozygotic and dizygotictwins (dizygotic differences in sex, monozygotic same).

(c) dominance effects are unimportant.

(d) genotype - environment interaction is insignificant.11


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h2 =VA

VE + (VA +VEp +VGxE +VD )

H2=

VG

(VG+V

E)=

VG

(VP)

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Dominance varianceis the variance that is due to interactions betweenalleles: effects due to two alleles interacting to make the trait greater (or lesser)than the sum of the two alleles acting alone (non-additive), and it is nottransmitted, because gametes are haploid.

Interactions between different genes, is called epistasis variance, by

definition it is also non additive and it depends on genotype which is partlyrecombined each generation

GE interactions depend on genotype and environment which are nottransmitted directly.

VD Dominance varianceEpistatic varianceVEp


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H2indicates the proportion of phenotype variation attributable to

genotype differences in a population, but it does not tell us how

estimate or predict the fraction of inherited resemblance

transmitted between generations, unless VG= VA.

Narrow sense heritability (VA/VP= h2) is a measurement of the extent

to which variation in phenotypesis determined by the sum of allelic

differences of genes with additive effects, segregating in the parental

generation, transmitted to offspring in a population.

In other words, h2represents the reliability of the phenotypic value of

the parents as an index of the phenotypic value of the offspring

(breeding value).

3 ways of estimating VA :(1) regression of the offspring average on

the mid-parental average

: (2) selection

(3) variance components in a breeding design.

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A regression line expresses the general lineartrend ofywith increasingx (y = mx + b)

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(1) The first method for estimating narrow heritability, h2, using

Parent - offspring regressions. The regression of breeding value

(offspring average) on phenotypic value (parental mean)

h2represents the

reliability of the

phenotypic value of the

parents as an index of

offspring phenotypic

expression .

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Narrow sense heritability (VA/VP= h2

) is a measurement of theextent to which variation in phenotypesis determined by genes

with additive effects, transmitted by parents to offspring in a

population.

In other words, h2

represents the reliability of the parentalphenotypic value as a predictor of offspring value or expression.

2 ways of estimating VA:(1) regression of the offspring average on

the mid-parental average

(2) selection

Both estimates assume you have approximately normally-distributed

character measurements.

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S=Selection differential(differencebetween the meanof the original

parental population, and the meanof the parental population subset

selected to breed) .

R=Response to selection or the differencebetween the meanof theparentalpopulation overall and the averageof their offspring)

R= h2 * S

Method of estimating h2(2)

How the average character expression of selected individualsare related to the average size of their offspringis measured by

the heritability (h2) of the character being selected.

R=h2*S

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Parentals

Offspring

S= difference betweenthe mean of the

selected and the

whole parental

population

R= difference between

the mean of the

offspring and parental

generations

R= 0 ? R=1.0 ?

h2= R/S = 0.71 18


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h2represents the reliability

of the phenotypic value ofthe parents as a guide to the

phenotypic value of their

offspring, it tells you how

much of the phenotypic

variation is governed byinheritance but it does not

tell you anything about

specific genes or alleles

h2= R/S

If R= S, h2= 1.0

If R= 0, h2= 0

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h2= ?

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If h2=0.64, S=5,

What do you predict will be corolla length

in the next generation (i.e. R)?


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Long - term selection for increased oil content in corn (left)

altered bristle number inDrosophila (right)

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Sometimes you select on one character and affect another,

because they share the same genes, they are negatively

genetically correlated or are in a tradeoff.

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Alternatively they may be positively correlatedbecause of

non-random assortment of genesor pleiotropic effectsof

genes on more than one character


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Flight muscle (DLM) and the number of eggs in

cricket ovaries (ovary weight) have a negative

genetic correlation, wing muscle size (DLM) and

mobile energy reserves (acylglycerol)are

positively correlated.

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Narrow sense heritability (h2), an estimate of

response to selection in populations of non-inbred,sexually-reproducing families.

h2=

VA

VE + (VA +VGxE +VD )

VA= additive genetic variation

VGxE

= genotype by environment interaction

VD= dominance variance

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Genotype (or Family) by Environment Interaction, VGxE,

different families or cloneshave different responses to a

change in environment.

"P

2="

G

2+"

E

2+"

GxE

2

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Sometimes, different genotypes have the same (a) or (b) different

responses to a change in environment.

Figure 18-10

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Fig. 15.10.Genotype-by-

environment

interaction in maize.

[Data from W. A.Russell. 1974.

Annual Corn &

Sorghum Research

Conference29: 81]

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How do you recognize GXE ?

Relations between

environment andcharacter state of

different inbred fly lines

(colors). Each line

connecting the character

states expressed at each

temperature is called a

norm of reaction

.

Note that different strains

in this population havedifferent norms of

reaction,although in

general, the population

optimum is at 21 degrees. 28


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Our understanding of Genetics, summarized by the authors of your text book,pre-eminant geneticist of our generation, is.. Genes cannot generate or evendictate the structure of an organism by themselves. The environment

(internal or external) has a crucial ongoing effect and sometimes it is acontrolling component of expression.

Ch 1, Griffiths et al 9thedition.

Model I:genetic determination- a set of instructions for turningundifferentiated environmental materials into a specific organism.

Model II:environmental determinism- genes determine proteins , but aspecific environmentdetermines the actual course of development.

Model III:genotype - environment interaction - the trajectory ofdevelopment depends on genes an individual inherits from its parents, and the

sequence of specific environmentsan individual encounters.

Model IV:genetic noise- the random element of expression that cannotbe attributed to specific genotype or environmental differences.

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E1

E2

size

small

large

? VP=VG, VE, or VGxE

1

E1

E2

size

small

large2

E1 E2

s

ize

small

large

3

E1 E2

size

small

large

4

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L22Biol261W2013Q Genetics Copy