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Active Lecture PowerPoint® Presentation for
Essentials of Genetics
Seventh Edition
Klug, Cummings, Spencer, Palladino
Chapter 22
Quantitative Genetics
Copyright © 2010 Pearson Education, Inc.
Mendelian Traits
• Distinct and separate categories
• Discontinuous variation
• Qualitative inheritance
• Single gene inheritance
• Simple inheritance
Quantitative Traits
• No distinct and separate categories
• Continuous variation
• Quantitative inheritance
• Polygenic inheritance – multiple genes can
influence the trait
• Complex or multifactorial inheritance –
environment can influence the trait
Quantitative Traits
Plants:
• Size of fruits, size of seed; weight of fruits, yield
• Protein content, oil content, etc
Animals:
• Milk production, meat production (size)
Humans:
• Height, weight, IQ, proneness to some diseases
Continuous Variation:
has large number of phenotype classes
Not all polygenic traits show continuous
variation
• Threshold traits are polygenic traits, but they
have a small number of phenotypic classes
• Environmental factors affect the phenotypes
• Eg: Type II diabetes, multiple sclerosis,
psoriasis, and schizophrenia.
• See Fig. 22-1.
Figure 22-1
Copyright © 2010 Pearson Education, Inc.
Quantitative Traits Can Be Explained
in Mendelian Terms
• Multiple genes individually behave in
Mendelian fashion in a cumulative or
quantitative way
• Multiple-factor hypothesis of Bateson &
Yule
Quantitative
Inheritance
Corolla size of Nicotiana
Grain Color
in Wheat
Additive alleles
vs.
Nonadditive alleles
P generation
A model for polygenic inheritance
of skin color
aabbcc
AABBCC
(very light) (very dark)
F1 generation
AaBbCc
1
–
8
F2 generation
1
–
8
1
–
8
AaBbCc
Sperm
1
–
8
1–
8
1–
8
1–
8
1–
8
1
–
8
1
–
8
1–
8
1
–
8
Fraction of population
Eggs
20
––
64
1–
8
1
–
8
1
–
8
1–
8
1
––
64
6
––
64
15
––
64
20
––
64
15
––
64
6
––
64
1
––
64
15
––
64
6
––
64
1
––
64
Skin color
Additive Alleles: The basis of
continuous variation
1. Phenotypic traits can be quantified by measuring,
weighing, counting, etc.
2. Two or more gene loci scattered throughout the
genome control the trait in an additive way
3. Non additive alleles do not contribute
quantitatively to the phenotype
4. Contribution of each additive allele is small but
equal
5. Together, additive alleles produce substantial
phenotypic variation
Polygenic Inheritance
Polygenic Inheritance
Polygenic Inheritance
Polygenic Inheritance
Polygenic Inheritance
Mean & Variance
Mean is the average.
Variance is the average squared distance of all measurements
from mean
Heritability (H2)
Phenotype = Genotype + Environment + Interaction
between G X E
VP = VG + VE + VG x E
• Proportion of total phenotypic variation in a
population due to genetic factors
• Broad-sense heritability
H2 = VG/VP
Heritability
• Heritability explains how much of the observed
phenotypic variation in a population is due to
genotypic differences
• Heritability of 1.0 indicates that environmental
conditions have little impact on the phenotypic
variance.
• Low H2 values, close to 0.0 indicates that
environmental factors, not the genotype, are
largely responsible for phenotypic differences.
Interaction
Between
Genotype
& Environment
Twin Studies on Heritability
• Monozygotic (MZ) or identical twins
– Arise from splitting of a single fertilized egg
– Variation between these twins is due to
environment alone (VP = VE)
– Epigenetic differences are included in the envt
• Dizygotic (DZ) or fraternal twins
– Arise from two separate fertilization events
– Variation is due to (VP = VG + VE + VG x E)
• Comparison of the same trait in both kinds of twins
is used to estimate broad sense heritability
– H2 = VG/VP
http://www.guardian.co.uk/lifeandstyle/2011/sep/24/twi
ns-black-white?fb=optOut
QTL could be mapped, isolated,
and transferred
Read 22.8 in the textbook