15 The Genetic Basis of Complex Inheritance

Multifactorial Traits Multifactorial traits are determined by multiple genetic and environmental factors acting together Multifactorial = complex traits = quantitative traits Most traits that vary in the population, including common human diseases with the genetic component, are complex traits Genetic architecture of a complex trait = specific effects and combined interactions of all genetic and environmental factors

Quantitative Inheritance Quantitative traits = phenotypes differ in quantity rather than type (such as height) In a genetically heterogeneous population, genotypes are formed by segregation and recombination Variation in genotype can be eliminated by studying inbred lines = homozygous for most genes, or F1 progeny of inbred lines = uniformly heterozygous Complete elimination of environmental variation is impossible

Quantitative Inheritance Continuous traits = continuous gradation from one phenotype to the next (height) Categorical traits = phenotype is determined by counting (hen’s eggs) Threshold traits = only two, or a few phenotypic classes, but their inheritance is determined by multiple genes and environment (adult-onset diabetes)

Distributions Distribution of a trait in a population = proportion of individuals that have each of the possible phenotypes Mean = peak of distribution x = ∑fixi /N Variance = spread of distribution estimated by squared deviation from the mean s2=∑fi(xi - x )/N-1 Standard deviation = square root of the variance s =√ s2

Normal Distribution Normal distribution = symmetrical curve produced by data in which half points are above and half points are below mean ~68% of a population have a phenotype within one standard deviation (s) of the mean ~95% - within 2 s ~99.7% - within 3 s • The distribution of a trait in a population implies nothing about its inheritance

Fig. 15.5

Phenotypic Variation Variation of a trait can be separated into genetic and environmental components Genotypic variance sg2 = variation in phenotype caused by differences in genotype Environmental variance se2 = variation in phenotype caused by environment Total variance sp2 = combined effects of genotypic and environmental variance sp2 = sg2 + se2

Fig. 15.9

Phenotypic Variation Genotype and environment can interact or they can be associated Genotype-environment (G-E) interaction = environmental effects on phenotype differ according to genotype Genotype-by-sex interaction: same genotype produces different phenotype in males and females (distribution of height among women and men)

Fig. 15.10

Genetic Variation D = difference between parental strains Genotype-environment (G-E) association = certain genotypes are preferentially associated with certain environments There is no genotypic variance in a genetically homogeneous population sg2 = 0 When the number of genes affecting a quantitative trait is not too large, the number, n, of genes contributing to the trait is n = D2/8sg2 D = difference between parental strains

Broad-Sense Heritability Broad-sense heritability (H2) includes all genetic effects combined H2 = sg2 / sp2 = sg2 / sg2 + se2 Knowledge of heritability is useful in plant and animal breeding because it can be used to predict the magnitude and speed of population improvement

Heritability: Twin Studies Twin studies are often used to assess genetic effects on variation in a trait Identical twins arise from the splitting of a single fertilized egg = genetically identical Fraternal twins arise from two fertilized eggs = only half of the genes are identical Theoretically, the variance between identical twins would be equivalent to se2 , and between fraternal twins - sg2/2 + se2

Heritability: Twin Studies Potential sources of error in twin studies of heritability: Genotype-environment interaction increases the variance in fraternal twins but not identical twins Frequent sharing of embryonic membranes by identical twins creates similar intrauterine environment Greater similarity in treatment of identical twins results in decreased environmental variance Different sexes can occur in fraternal but not identical twins

Narrow-Sense Heritability Narrow-sense heritability (h2) = proportion of the variance in phenotype that is transmissible from parents to offspring Narrow-sense heritability can be used to predict changes in the population mean in with individual selection h2 = (M’ - M)/(M* - M) In general, h2 < H2 . They are equal only when the alleles affecting the trait are additive in their effects = heterozygous phenotype is exactly intermediate between homozygous dominant and recessive

Artificial Selection Artificial selection =“managed evolution” = the practice of selecting a group of organisms from a population to become the parents of the next generation h2 is usually the most important in artificial selection Individual selection = each member of the population to be selected is evaluated according to its individual phenotype Truncation point = arbitrary level of phenotype that determines which individuals will be used for breeding purposes

Artificial Selection There are limits to the improvement that can be achieved by artificial selection: Selection limit at which successive generations show no further improvement can be reached because natural selection counteracts artificial selection due to indirect harmful effects of selected traits (weight at birth versus viability) Correlated response = effect of selection for one trait on a non-selected trait (number of eggs and their size)

Inbreeding Inbreeding can have harmful effects Inbreeding depression = decrease in fitness due to harmful recessive alleles which become homozygous Heterosis = hybrid vigor refers to superior fitness of heterozygote; often used in agricultural crop production Fig. 15.14

Correlation Between Relatives Genetic variation is revealed by correlations between relatives Covariance (Cov), the tendency for traits to vary together, is Cov(x,y)=∑fi(xi - x )(yi - y )/N-1 Correlation coefficient (r) = statistical evaluation of paired data (pairs of parents, twins, parent and offspring) r =Cov(x,y)/sxsy Covariance and correlation coefficient are important in heritability estimates

Correlation Between Relatives Correlation coefficient of a trait between relatives is related to the narrow- or broad-sense heritability

Threshold Traits: Heritability Liability = quantitative trait that presents a genetic risk for a threshold trait Individuals with a liability above threshold develop the trait The risk of manifesting a threshold trait has H2 and h2 that cannot be estimated directly, but can be inferred from the incidents of the trait among individuals and their relatives

Threshold Traits: Heritability Many congenital abnormalities are inherited as threshold traits Heritability analyses can be used to determine recurrence risks Theoretical curves show incidence, type of inheritance and risk among first-degree relatives of an affected individual

Multifactorial Disorders Most common disorders in human families are multifactorial Pedigree studies of genetic polymorphisms are used to map loci for quantitative traits Quantitative trait locus (QTL) = gene that affects a quantitative trait Simple tandem repeat polymorphisms (STRPs) are used to locate QTLs Candidate gene = gene for which there is some a priori basis for suspecting that it affects the trait