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Michael R. Cummings
Chapter 5
The Inheritance of Complex Traits
David Reisman • University of South Carolina
5.1 Polygenic Traits
 Discontinuous variation
• Phenotypes that fall into two or more distinct,
nonoverlapping classes or varieties
• Mendels’ peas
 Continuous variation
• A distribution of phenotypes from one extreme to
another in an overlapping fashion (like height in
tobacco plants and humans)
• The phenotypes together represent a bell-shaped
curve
Comparison of Discontinuous and
Continuous Phenotypes
Example of a Continuous phenotype
Fig. 5-2, p. 96
Polygenic Inheritance
 The distribution of polygenic traits through the
population follows a bell-shaped (normal) curve
Types of Traits
 Polygenic traits
• Traits controlled by two or more genes
 Multifactorial traits
• Polygenic traits resulting from interactions of two or
more genes and one or more environmental factors
Polygenic Inheritance
 Two or more genes contribute to the phenotype
 Phenotypic expression varies across a wide range so a large
population must be analyzed when studying a trait
 Interactions with the environment often participate in creating
the phenotype.
 Height, weight, skin color, eye color, and intelligence
5.3 The Additive Model of Polygenic
Inheritance
 The number of phenotypic classes increases as the
number of genes controlling a trait increases
The Additive Model of Polygenic Inheritance
Regression to the Mean
 Averaging out the phenotype is called regression to
the mean
• In a polygenic system, parents who have extreme
differences in phenotype tend to have offspring that
exhibit a phenotype that is the average of the two
parental phenotypes
A Polygenic Trait: Eye Color
 Five basic eye colors fit a model with two genes,
each with two alleles
Fig. 5-6, p. 99
The Threshold Model
 Explains the discontinuous distribution of some multifactorial
traits (clubfoot, cleft lip, congenital hip dislocation in females,
pyloric stenosis in males)
5.5 Heritability Measures the Genetic
Contribution to Phenotypic Variation
 Phenotypic variation is derived from two sources:
 Genetic variance
• The phenotypic variance of a trait in a population that
is attributed to genotypic differences
 Environmental variance
• The phenotypic variance of a trait in a population that
is attributed to differences in the environment
Heritability of a Trait
Heritability
The proportion of a phenotype that is dependent upon
genotype.
Measuring heritability involves study of twins and
adopted children.
Heritability Estimates
 Heritability is estimated by observing the amount of
variation among relatives who have a known fraction
of genes in common (known as genetic relatedness)
 Heritability can be estimated only for the population
under study and the environmental condition in
effect at the time of the study
Correlation
 Correlation coefficient
• The fraction of genes shared by two relatives
 Identical twins have 100% of their genes in common
(correlation coefficient = 1.0)
• When raised in separate environments identical twins
provide an estimate of the degree of environmental
influence on gene expression
5.6 Twin Studies
and Multifactorial Traits
 Monozygotic (MZ)
• Genetically identical twins derived from a single
fertilization involving one egg and one sperm
 Dizygotic (DZ)
• Twins derived from two separate and nearly
simultaneous fertilizations, each involving one egg
and one sperm
• DZ twins share about 50% of their genes
Fig. 5-11, p. 105
Concordance
• Agreement between traits exhibited by both twins
 In twin studies, the degree of concordance for a trait is
compared in MZ and DZ twins reared together or apart
• The greater the difference, the greater the heritability
Concordance, Heritability, and Obesity
 Concordance can be converted to heritability by
statistical methods
 Twin studies of obesity show a strong heritability
component (about 70%)
Table 5-3, p. 106
Genetic Clues to Obesity: The ob Gene
 The ob gene encodes the weight-controlling hormone leptin
in mice; receptors in the hypothalamus are controlled by the
db gene
 The ob gene encodes the hormone Leptin
• produced by fat cells that signals the brain and ovary
• As fat levels become depleted, secretion of leptin slows
and eventually stops
Fig. 5-13, p. 108
Fig. 5-13, p. 108
Human Obesity Genes
 In humans, mutations in the gene for Leptin (LP) of
the Leptin receptor (LEPR) account for about 5% of
all cases of obesity; other factors cause the recent
explosive increase in obesity
Scanning the Human Genome for Additional
Obesity Genes
5.7 More on the Genetics of Height
 The development of new technologies allows
researchers to survey the genome to detect
associations with phenotypes such as height,
weight, etc.
 The use of single nucleotide polymorphisms (SNPs)
allows the association between haplotypes and
phenotypes.
• Haplotype: specific combinations of SNPs located
close to gather on a chromosome that are very likely
inherited as a group.
DNA
source
SNP
SNP
SNP
Reference
standard
SNP
Original
haplotype
10,000 nucleotides
Person 1
Haplotype 1
Person 2
Haplotype 2
Person 3
Haplotype 3
Person 4
Haplotype 4
Fig. 5-16, p. 110
5.8 Skin Color and IQ are Complex Traits
 Skin color is a polygenic trait
 It is controlled by 3 or 4 genes, plus environmental
factors (most obvious—sun exposure)
 Can intelligence be measured quantitatively?
• Psychological measurements and the ability to
perform specific tasks at a specific age led to the
development of the intelligent quotient (IQ) test.
• There is no evidence that intelligence can be
measured objectively (like height or weight)
• Interestingly, IQ measurements do have a significant
heritable components
Are Intelligence and IQ Related?
 Can intelligence be measured quantitatively?
• Early studies believed that physical dimensions of
regions of the brain were a measure of intelligence.
Fig. 5-19, p. 112
More meaningful measures of intelligence and the search
for genes that control intelligence
 IQ test scores can’t be equated with intelligence
• Relative contributions of genetics, environment, social and
cultural influences can’t be measured
 General cognitive ability
• Characteristics include verbal and spatial abilities,
memory and speed of perception, and reasoning
• Genes associated with reading disability (dyslexia)
and cognitive ability have been discovered by
comparing haplotypes
• Both genetic and environmental factors make
important contributions to intelligence
Correlation coefficients of IQ measurements
Expected
value
Pairs studied
Nonbiological sibling pairs (adopted/natural pairings)
(5)
0.0
Nonbiological sibling pairs (adopted/adopted pairings) (6)
0.0
Foster-parent child
(12)
0.0
Single-parent offspring reared together
(32)
0.5
Single-parent offspring reared apart
(4)
Siblings reared apart
(2)
0.5
Siblings reared together
(69)
0.5
Dizygotic twins, opposite sex
(18)
0.5
Dizygotic twins, same sex
(29)
0.5
(3)
1.0
(34)
1.0
Monozygotic twins reared apart
Monozygotic twins reared together
0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Correlation coefficient
Fig. 5-20, p. 113