Chapter 5 Power Point Slides

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Transcript Chapter 5 Power Point Slides

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
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
 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
The proportion of a phenotype that is dependent upon
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 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
• 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
• Haplotype: specific combinations of SNPs located
close to gather on a chromosome that are very likely
inherited as a group.
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
Pairs studied
Nonbiological sibling pairs (adopted/natural pairings)
Nonbiological sibling pairs (adopted/adopted pairings) (6)
Foster-parent child
Single-parent offspring reared together
Single-parent offspring reared apart
Siblings reared apart
Siblings reared together
Dizygotic twins, opposite sex
Dizygotic twins, same sex
Monozygotic twins reared apart
Monozygotic twins reared together
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