Lecture 16: Extensions of Mendelian Analysis
(version 4 September 2006)
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Extensions of Mendelian Analysis
Lots of example, don't worry about most details, just basic concepts
Levels of Dominance
Overdominance
Incomplete dominance
Codominance (additivity) = no dominance
Aa = the average of the AA and aa
Multiple alleles
- The ABO blood group is a nice example, three alleles
- IA, IB are codominant, IA and
IB are dominance over i
Lethals
Yellow allele in mice
An example of Pleiotropy ( Single gene influences many traits)
| Manx gene in cats Mm: tail-less
mm: normal
MM: dead
|
Self-incompatibility alleles in plants
Pollen tube will not grow if the S allele that it contains is present in the female parent.
Multiple Genes
Coat color genes in mammals
Episatic interactions --Two (or more) genes act in a nonadditive fashion
Coat color in Labrador retrievers
| Black Labrador B- E - |
| Brown (or Chocolate) Labrador bb E - |
| Golden Labrador -- ee
|
The dilute Gene
Suppressors
- Suppresses the effect of one gene
- An example: Malvidin production in plants
- K = Malvidin, D = suppressor of M
Duplicate genes
- Example: Seed shape in Shepherd' s purse
- Thin shape versus wide shape
Penetrance vs expressivity
- Penetrance: What fraction of individuals with a genotype show the expected phenotype
- Expressivity: What is the extent of expression
- Example: a human disease
- If only 75% of AA individuals have disease, penetrance = 75%
- Variation in severity of disease among AA individuals showing disease = expressivity
