HMB265H1 Lecture Notes - Lecture 16: Quantitative Trait Locus, Frequency Distribution, Chromosome

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HMB265 Lecture 14 Genetic mapping and complex traits
Dissection of quantitative traits
- A number of genes can have a major effect on the phenotype
- A quantitative effect can be attributed to 2< important genes
- Quantitative trait loci (QTL)
o Genes that control variation in quantitative traits
- Identified using genetic mapping and association of genetic markers with the trait
o Allows to identify the genes associated by using markers
- Statistical methods allow the identification of more than one QTL at a time
1) Make an informative cross between individuals that are “inbred” relative to each
other, and which differ at the traits of interest
o First identify what the trait of interest is
o This step emphasized the important of model organisms
o E.g. dissecting the basis of tomato fruit size
Tomato fruits have huge phenotypic variation
Can cross inbred large tomato fruit with inbred small tomato fruit
Cross between these two inbred strains was done and the F1 generation
had no genetic variation
The variation due to genetics is 0 so if there is any variation it is
due to environment
The F1 generation was then crossed to obtain the F2 generation
Variation due to genetics is also 0 for the F1 generation since they
are all identical in terms of genetics
F2 generation had phenotypic and genetic variation
All the chromosomes looked different because of random
2) Determine the frequency distribution of F2
o Use frequency distribution to figure out which markers are correlated with which
o See if any genetic markers are associated with the phenotype
3) Use molecular markers to “genotype” the individuals, attempting to find markers
that co-segregate with the trait
o Run the genotypes on a gel
o See how many bands each individual display
2 bands heterozygous
1 band homozygous
o Most loci show no association between the genotype and the phenotype
meaning that they are randomly segregated
o If there is some consistency in the bands then it means that there is an
association between the genotype and the phenotype
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