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The Inheritance of Complex Traits. Chapter 5. 5.1 Polygenic Traits. Discontinuous variation Phenotypes that fall into two or more distinct, nonoverlapping classes or varieties Mendels ’ peas Continuous variation - PowerPoint PPT Presentation
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Michael R. Cummings
David Reisman • University of South Carolina
The Inheritance of Complex TraitsChapter 5
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
HeritabilityThe 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.
Fig. 5-16, p. 110
DNA source
SNP SNP SNP SNP
Reference standard
Original haplotype
10,000 nucleotides
Person 1 Haplotype 1
Person 2 Haplotype 2
Person 3 Haplotype 3
Person 4 Haplotype 4
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
Fig. 5-20, p. 113
Pairs studiedExpected
valueNonbiological sibling pairs (adopted/natural pairings) (5) 0.0Nonbiological sibling pairs (adopted/adopted pairings) (6) 0.0Foster-parent child (12) 0.0Single-parent offspring reared together (32) 0.5Single-parent offspring reared apart (4) 0.5Siblings reared apart (2) 0.5Siblings reared together (69) 0.5Dizygotic twins, opposite sex (18) 0.5Dizygotic twins, same sex (29) 0.5Monozygotic twins reared apart (3) 1.0Monozygotic twins reared together (34) 1.0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Correlation coefficient
Correlation coefficients of IQ measurements