GEN-3000 Lecture Notes - Lecture 20: Soil Fertility, Coronary Artery Disease, Locule
2 views19 pages
Chapter 20 - Quantitative Genetics and Multifactorial Traits
• Many traits not distinct and clear-cut show a continuous range of
– Height in humans, milk and meat production in cattle, and yield in
• Continuous variation:
– Measured and described in quantitative terms known as
• Quantitative inheritance—genetic phenomenon:
– Continuous variation can be measured in quantitative terms.
• Polygenic (many genes):
– Varying phenotypes result from input of genes at more than one
– Genes involved referred to as polygenes
• Multifactorial—complex traits:
– Quantitative (polygenic) traits
– Phenotypes results from both gene action and environmental
• Quantitative patterns of inheritance:
– Mendelian factors could not account for range of phenotypes
seen in quantitative patterns of inheritance.
• Multiple-gene hypothesis (multiple-factor):
– Many genes, individually behaving in Mendelian fashion,
contribute to phenotype in a cumulative/quantitative way.
– Example: grain color in wheat—Figure 20-1
How the multiple-factor hypothesis accounts for the 1:4:6:4:1 phenotypic ratio of grain color when all
alleles designated by an uppercase letter are additive and contribute an equal amount of pigment to
Additive and Non-additive Alleles
• Multiple-gene hypothesis—additive alleles and nonadditive alleles:
– Various grain color phenotypes due to additive and nonadditive
– Additive allele: contributes equally to red grain color
– Nonadditive allele: fails to produce red pigment
• Greater number of additive alleles in genotype = more intense red color
expressed in phenotype.
• Quantitative loci:
– Numerous loci can function in similar fashion.
– Greater and greater numbers of classes appear in F2 generation
in more complex ratios.
– Figure 20-2
The genetic ratios (on the x-axis) resulting from crossing two heterozygotes when polygenic
inheritance is in operation with 1 to 5 gene pairs. The histogram bars indicate the distinct F2
phenotypic classes, ranging from one extreme (left end) to the other extreme (right end). Each
phenotype results from a different number of additive alleles.