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Chapter 5 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Chapter 5Complex Patterns of Inheritance


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


Continuousand

DiscontinuousVariation

Fig. 5.2


Continuous Variation in Humans

Fig. 5.1


Genotype + Environment

Produce the Phenotype

P = G+E


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


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


• 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


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


Characteristics of MultifactorialTraits

• Polygenic• Genes controlling trait act additively• Environmental factors interact with

the genotype to produce thephenotype

• Assessing interactions can be difficult


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


Threshold Model

Fig. 5.7


Familial Risk


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


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


Heritability ofFingerprints

Fig. 5.8


Twin Studies

• Monozygotic twins– Single fertilization– Genetically identical

• Dizygotic twins– Independentfertilizations– Share approximatelyhalf their genes

Fig. 5.10

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