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Unit 13: Quantitative Genetics and Heritability

Traits controlled by many genes at once, how to split their variation into genetic and environmental parts, and how breeders exploit that split.

6 topics · 22 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

Continuous Traits and the Normal Distribution

  1. How it worksHow does the combined effect of many genes, each adding a small amount, produce a continuous, bell-shaped trait distribution?
  2. Worked exampleFind how many phenotype classes appear when three genes each contribute additively to a quantitative trait.
  3. Common mistakeWhy doesn't a continuous trait mean the underlying genes stop following normal Mendelian inheritance?
  4. Going furtherWhat happens to a quantitative trait's distribution as the number of genes contributing to it rises from two to eight?
Your turn1 of 2
A quantitative trait is controlled by 8 genes, each contributing additively (each gene has two alleles, one that adds to the trait and one that doesn't). How many distinct phenotype classes appear in the offspring of a cross between two individuals heterozygous at all 8 genes?

Polygenic Inheritance and Additive Alleles

  1. How it worksHow do additive alleles at several different genes each nudge a phenotype up or down by a small fixed amount?
  2. Worked exampleFind the range and number of genotype classes for kernel color in wheat controlled by two additive genes.
  3. Going furtherWhat happens to the range of a polygenic trait's distribution as more genes are found to contribute to it?
Your turn1 of 2
Kernel color in wheat is controlled by 5 additive genes, and each 'dark' (increaser) allele adds 4 units of pigment above the baseline. What is the total range in pigment score between the darkest possible genotype (all increaser alleles) and the lightest possible genotype (all decreaser alleles)?

Broad-Sense and Narrow-Sense Heritability

  1. How it worksHow does partitioning phenotypic variance into genetic and environmental components define heritability?
  2. Worked exampleFind the broad-sense heritability of a trait given a phenotypic variance of 40 and a genetic variance of 30.
  3. Common mistakeWhy doesn't a heritability of 0.8 for a trait tell you how much of one individual's trait value came from their genes?
  4. Going furtherWhat happens to a trait's heritability estimate when the environment becomes much more uniform across a population?
  5. Worked exampleFind the narrow-sense heritability given an additive genetic variance of 24 and a total phenotypic variance of 40.
Your turn1 of 2
A trait has a total phenotypic variance of 79 and a genetic variance of 20. What is the broad-sense heritability of this trait?

Twin Studies and Estimating Heritability

  1. How it worksHow does comparing trait concordance between monozygotic and dizygotic twins estimate a trait's heritability?
  2. Worked exampleFind the heritability estimate given identical twins show 90 percent concordance and fraternal twins show 40 percent concordance for a trait.
  3. Common mistakeWhy doesn't a twin study's heritability estimate apply equally well to a completely different population?
Your turn1 of 2
For a particular trait, identical (monozygotic) twins show 0.724 concordance and fraternal (dizygotic) twins show 0.48 concordance. Using Falconer's formula (heritability equals twice the difference between the two concordance rates), what is the estimated heritability?

QTL Mapping

  1. How it worksHow does tracking molecular markers alongside a quantitative trait across many offspring locate a QTL?
  2. Worked exampleTrace how a QTL peak on a linkage map identifies the chromosome region most strongly associated with plant height.
  3. Going furtherWhat happens to QTL mapping resolution when many more recombinant offspring are added to the analysis?
  4. Common mistakeWhy doesn't a QTL peak pinpoint the exact gene responsible for a trait without further fine-mapping?
Your turn1 of 2
A QTL mapping study using 100 recombinant offspring locates a broad chromosome region associated with plant height, spanning many genes. Researchers then repeat the cross with 2,000 recombinant offspring instead. What happens to the mapping resolution?

Selection Response and the Breeder's Equation

  1. How it worksHow does the breeder's equation combine heritability and selection differential to predict a trait's response to selection?
  2. Worked exampleFind the predicted response to selection given a heritability of 0.5 and a selection differential of 10 kilograms.
  3. Common mistakeWhy doesn't selecting for a trait with low heritability produce much change across generations?
Your turn1 of 2
A trait has a narrow-sense heritability of 0.657 and a selection differential of 20 kilograms (the difference between the selected breeders' average and the whole population's average). Using the breeder's equation (response equals heritability times selection differential), what response to selection is predicted in the next generation?