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Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Chapter 5Complex Patterns of Inheritance
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Phenotypes Can BeDiscontinuous or Continuous
• Discontinuous variation shows distinctphenotypes–Short and tall peas phenotypes
• Continuous variation shows a series ofoverlapping phenotypic classes–Height in humans
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Continuousand
DiscontinuousVariation
Fig. 5.2
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Continuous Variation in Humans
Fig. 5.1
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Genotype + Environment
Produce the Phenotype
P = G+E
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Terms
• Polygenic traits are determined by twoor more genes
• Multifactorial traits are controlled bytwo or more genes and show significantinteraction with the environment
• Complex traits are ones where relativecontribution of genes and environment arenot yet established
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Polygenic Inheritance
• Traits are usually quantified bymeasurement
• Two or more genes contribute to thephenotype
• Phenotypic variation varies across a widerange
• Better analyzed in populations than inindividuals
• Example: human eye color
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
• As the number ofloci increases, thenumber of classesincreases
• As classes increase,phenotypicdifference betweenclasses decreases
• Averaging out ofthe phenotype iscalled regressionto the mean
Fig. 5.5
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Multifactorial Traits
• Genotype does not change afterfertilization (except by mutation)
• Phenotype is the sum of the observablecharacteristics and may changethroughout life
• Environment includes all genetic andnongenetic factors
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Characteristics of MultifactorialTraits
• Polygenic• Genes controlling trait act additively• Environmental factors interact with
the genotype to produce thephenotype
• Assessing interactions can be difficult
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Methods Used to StudyMultifactorial Traits
• Threshold modelFrequency of disorder among relatives iscompared with the frequency of thedisorder in the general population
• Recurrence riskEstimates the risk that the disease willrecur
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Threshold Model
Fig. 5.7
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Familial Risk
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Phenotypic Variation
Sources of phenotypic variation• Genotypes in the population• Variation in the environment
Heritability – how much of the observedphenotypic variation is due to differencesin genotype
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Factors that Contribute toPhenotypic Variance
• Genetic varianceVariance attributed to the genotypicdifferences
• Environmental variance Variance attributed to differences in the
environment• Correlation coefficients
Measure the degree to which variablesvary together
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Heritability ofFingerprints
Fig. 5.8
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Twin Studies
• Monozygotic twins– Single fertilization– Genetically identical
• Dizygotic twins– Independentfertilizations– Share approximatelyhalf their genes
Fig. 5.10