Quantitative Genetics
Quantitative Genetics
• Continuous phenotypic variation within populations- not discrete characters
• Phenotypic variation due to both genetic and environmental factors
Quantitative traits are described by a frequency distribution
Figure 18-3b
The distribution of a trait is composed of the distributions of thedifferent genotypes
Figure 18-16
A norm of reaction is the relation between environment and phenotype
Figure 18-6
Crosses are performed to test for heritability
Figure 18-11
Quantitative Trait Loci (QTL):the specific loci whose allelic differences are
responsible for the genetic variation in a quantitative trait (e.g. total sleep time)
Note: QTL does not refer to the sum total of all loci that influence a particular trait, only those
loci that are functionally polymorphic (with respect to the trait of interest in a given
environment) between the parental strains or within the population. In mice, Mutagenesis and engineered KOs can artificially alter any gene, however, “natural” polymorphisms can
represent more subtle variations.
Selection altered bristle number
Figure 18-14
Selection increased egg production
Figure 18-15
An experimental protocol for localizing genes
Figure 18-17
Only a small percentage of character difference is associated withany one DNA marker
Figure 18-18
QTL Mapping• QTL mapping: identification of chromosomal regions containing
gene(s) that correlate with measured phenotypes
• Different methods– Single-marker analysis: compares phenotypic means of different
marker genotypes – Interval mapping: estimates position of QTL between two markers
using maximum likelihood (compares null hypothesis of no QTL vs. a QTL between the markers).
– Composite Interval mapping: IM and multiple regression– Multiple QTL models
• QTL present when LOD score exceeds critical threshold– LOD = Log of the Odds = log10 (H1/H0) – often for single locus analysis, 3.0 is significant and 2.0 is
suggestive depending on sample size, number of markers, and other variables.
Crosses are performed to test for heritability
Figure 18-11
Generating the Backcross
Cast/EiJ x C57BL/6J
F1 x Cast/EiJ
BC1s
Backcross progeny have on average:
75% CE, 25% B6 alleles50% C/C, 50% C/B genotypes for all loci
C57BL/6J (B6)
Cast/EiJ (CE)
Some types of detectable variation
• RFLPs (Restriction fragment length polymorphisms)
• VNTRs (Variable nucleotide tandem repeats) = minisatellites
• Microsatellites
• SNPs (Single nucleotide polymorphisms)
LOD Scores
• Null hypothesis: assume no linkage.
• Alternative hypothesis: assume the disease (or phenotype) and the marker locus are linked.
Mice have 20 chromosomes
Generating the Backcross
Cast/EiJ x C57BL/6J
F1 x Cast/EiJ
BC1s
C57BL/6J (B6)
Cast/EiJ (CE)
# Scored:
(21/20)
(16)
(224)
CE
B6
CE/B6
F2 lines
CE
B6
Genetic Map of Markers used in Analysis
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