BIOL 2030 Lecture Notes - Lecture 9: Quantitative Trait Locus, Quantitative Genetics, Epistasis
Quantitative Genetics
January 25th, 2016
*need to know components of genetic variance equations and the heritability equations
Phenotypic Traits:
1. Discontinuous or Qualitative
-presence of a few distinct phenotypes; easy to assign individuals to the different
phenotypic classes
-for many discontinuous traits, simple relationships are between phenotypes and
genotypes
-even in presence of dominance and epistasis, relationship remains simple
-environment has no or little influence
-genotypes inferred from phenotypes
-basic principles of heredity deduced *not true for all traits
2. Continuous or Quantitative
- traits for which there is a continuum of possible phenotypes – continuous
distribution
-traits measured quantitatively *often should normal distribution
What is treated as a quantitative trait:
• Truly continuous – traits for which phenotype can take any values between two extremes
• Meristic traits – measured in whole numbers
• Threshold traits – traits that only have two phenotypes but same same underlying
polygenic model
Study of Inheritance of Quantitative Traits:
• Early studies did recognize that many quantitative traits of parents and offspring were
statistically associated Pattern of inheritance were unclear & different from that of
discontinuous traits (mendelian trait) Polygenic model was proposed to reconcile
Mendelian principles with the inheritance of quantitative traits
Polygenic Inheritance Model:
• Simultaneous segregation of many Mendelian genes affect the trait: polygenes or
Quantitative Trait Loci (QTL)
• Each polygene has a small effect The effects of the different polygenes “add” Polygenic
Inheritance Model together to give the phenotype (additive)
• Interaction between the alleles at one locus (dominance) and interaction between the
various loci (epistasis) also influence the phenotype
• Phenotypic variation also arises from environmental causes
Kernel Colour in Wheat:
• Analyzed by Nilson-Ehle – proposed that the intensity of red pigmentation (a quasi
quantitative trait) was controlled by 3 genes each with 2 alleles
• For each of the 3 genes, 1 allele produced red pigment (A+,B+,C+) while the other one
did not (A-,B-,C-)
find more resources at oneclass.com
find more resources at oneclass.com
• The effect of the 3 genes were additive, each gene seemed to contribute equally to the
colour
• Ex. White (A-A-B-B-) X Purple (A+A+B+B+)
o F1: red (A+A-B+B-)
▪ Purple 1/16
▪ Dark-red 4/16
▪ Red 6/16
▪ Light-red 4/16
▪ White 1/16
1:4:6:4:1
A good model system to deduce how to reconcile quantitative trait inheritance and Mendelian
principles:
• Limited number of genes and alleles
• Incomplete dominance
• Additive effects
• No effect of environment
• Non-linkage
A simple polygenic model: 2 loci
• Assumptions:
o Only two types of alleles
▪ Contributing (c)
▪ Non-contributing (nc)
o Incomplete dominance
o Each gene has small effects
o No environmental effect
o No linkage
(c + nc)4 = 1:4:6:4:1 ratio of all possible combinations of c and nc alleles
• In wheat kernel colour:
o 1 loci: 1 c and 1 nc per locus
o p(c) = 0.5 ; p(nc) = 0.5
o (c+nc)4 = 1c4 (purple) + 4c3nc (dark-red) + 6c2nc2 (red) + 4c1nc3 + 1nc4
• Hypothetical example: ear length in corn
o 2 loci: 1 c and 1 nc allele each
o p(c)=0.5; p(nc)=0.5
o base length = 8cm
o nc allele: +0cm (notation A0,B0..etc)
o c allele: +2cm (notation A2,B2..etc)
o P1: A2A2B2B2 (16cm) X A0A0B0B0
▪ F1: A2A0B2B0
• F2: 1/16 (16cm): 4/16 (14cm): 6/16 (12cm): 4/16 (10cm): 1/16
(8cm) 1:4:6:4:1
***with 4 loci
o Assume now that we have twice as many loci but with half as much effect
o 4 loci, each with 1 contributing (c=+1cm) and 1 noncontributing (nc=+0cm) allele
o P(c)=0.5; P(nc)=0.5
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
*need to know components of genetic variance equations and the heritability equations. Presence of a few distinct phenotypes; easy to assign individuals to the different. For many discontinuous traits, simple relationships are between phenotypes and phenotypic classes genotypes distribution. Even in presence of dominance and epistasis, relationship remains simple. Basic principles of heredity deduced *not true for all traits: continuous or quantitative. Traits for which there is a continuum of possible phenotypes continuous. Mendelian principles with the inheritance of quantitative traits. Polygenic inheritance model: simultaneous segregation of many mendelian genes affect the trait: polygenes or. Quantitative trait loci (qtl: each polygene has a small effect the effects of the different polygenes add polygenic. Inheritance model together to give the phenotype (additive) Interaction between the alleles at one locus (dominance) and interaction between the various loci (epistasis) also influence the phenotype: phenotypic variation also arises from environmental causes.
