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Quantitative Traits and Heritability

Guest Lecture for Biology 230, Principles of Genetics
by

Geoff Morris

on 16 February 2016

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Transcript of Quantitative Traits and Heritability

Quantitative Traits and Heritability
What is the difference ?
Geoff Morris
morrisg@uvic.ca
can be explained in
Mendelian terms by many genes, each
behaving individually in Mendelian fashion, contributing to the phenotype in a
cumulative or quantitative way

Quantitative traits
Figure 23.2
Controlled by two genes

Each gene has an additive allele (A) and a nonadditive allele (a)
A/B
Red
=
Figure 23.3
n = number of genes involved

1/4 = number of F2 progeny
expressing either extreme phenotype
n
Example: A cross is made between a red flowering and a white flowering plant. The F1 is a uniform pink, but in the F2, 1/64 of the plants have white flowers, 1/64 have red, and the rest have varying shades of pink to red. How many genes are estimated to be affecting the trait?
a) 3
b) 4
c) 8
Many polygenic traits are
difficult or impossible to categorize
into several discrete genotypic categories.

This is especially true when:
The number of genes
controlling the trait increases
The influence of the environment increases
1
2
Heritability
All traits of biological organisms may be influenced by genetics and the environment.
Determining the relative amount that genetics and environment control a quantitative trait is very difficult.
Instead, focus on how variation in genetics and environment affect the phenotype.
= part due to genetics
If all phenotypic variation in a group is due to genetics,
Heritability = 1

If all phenotypic variation is due
to environment,
Heritability = 0
Rank these traits from
most heritable, to least:
Height

Heart Disease

Eye Colour

IQ

Type-2 Diabetes
describes the amount of phenotypic due to genetic variation for a raised in a
Heritability
variation
particular population
particular environment
Twin Studies Allow an Estimation of
Heritability in Humans
Identical
Fraternal
V = V + V
P
G
E
Concordant: both twins express the trait, or neither does.
Discordant: only one twin expresses the trait.
Most meaningful to compare concordance between twin types
Blood Type
Eye Colour
Measles
Handedness
Schizophrenia
Cleft Lip
Mammary Cancer
100
98
95
89
69
42
6
Trait
MZ
Concordance Values Between Twin Types
Quantitative traits are often at an intermediate
value in unselected populations
Selective Breeding of Species Can Alter Quantitative Traits Dramatically
Selective breeding (artificial selection) is the modification of phenotypes in plants and animal species of economic importance
Results 80 generations later
Differs from natural selection as it does not result from reproductive success
Greyhound
German Shephard
Bulldog
Charlie
Wild Mustard Plant
Strain
Modified Trait
Kohlrabi
Stem
Kale
Leaves
Broccoli
Flower Buds
and Stems
Brussels Sprouts
Lateral
Leaf Buds
Cabbage
Terminal
Leaf Bud
Cauliflower
Flower Buds
Variation Underlies Selective Breeding
Study began in 1869
163 ears of corn with oil content of 4-6%
19%
< 1%
Selective Breeding
Twin Studies
Quantitative Traits
Heritability
However, there is a limit
81%
Limitations?
By the time they are born, MZ twins do not necessarily have identical genomes

Gene-expression patterns in MZ twins change with age, leading to phenotypic differences
66
28
87
77
10
5
3
DZ
+ V
G x E
1/64 is one of the extreme phenotype

1/4 = 1/64
n=3
n
1/64 is one of the extreme phenotype

1/4 = 1/64
n=3
n
34-53%

98%

60-70%

26%
2.

4.

1.

3.

5.
V = V
P
E
V = 1/2V + V
P
G
E
At the start of the experiment the heritability
for the trait is fairly high. At the end, it is near zero
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