Introduction to Genetics and Evolution-week4
05 Oct 2019
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Single Gene Variation
Origin of Genetic Variation - Mutations as the origin of Genetic Variation
- what is a mutation?
- Error in replication/ meiosis leading to a change in a base
- Mutations do not happen preferentially “when we needthem”, they happen randomly.
- Many mutations are “bad”, but rarely(though it happens), a mutation is “good”.
- Some mutations “do not matter”, like “GCT” and “GCC” are tanslated to Alanine.
- Is itmore important that this mutation is passed on by meiosis or mitosis?
- Meiosis, because it will get pass down to offspring
Shortcomings of the Single Gene Model - Sources of Genetic Variation: Why the simple single gene/2 allele model is insufficient
- More than one gene controls trait
- True for almost every phrnotype
- Variable “penetrance”
- Mutant froms may “sometimes” affect phenotype but now always do so.
- BRCA1 breast cancer susceptility, buts BRCA1 breast cancer phenotype is not fullt penetrant- you CAN be “bb” and still have all non-cancerous cells in breast.
- BB or Bb is 12% risk of breast cancer
- bb is 60% risk
- Interaction among genes: “epistasis”
- Effect of genotype at one gene modifies effects of genotype at another gene.
- Pea flower color: C gene’s protein, C protein, is upstream of the P gene’s protein - P protein. If there is pp the flower is white, on the other hand, Px is the pink flower.
- THere can be >2 alleles at a locus!
- ABO blood types
- Blood genetypes: AA, AO, BO, AB, OO
- Blood phenotypes: A, B, AB, and O
- ABO blood types
- Environment, and interactions with it.
- We will begin “heritability” soon.
- Sun tanning.
- In absence of sun, do not tan(Environmental effect).
- Some people naturally taneasier than others(Interaction of environment with genetics)
Mutations and Complex Traits
Mutations Rates - Mutation “Rates”: How often do mutations happen?
- Mutations “rate” per generation
- Depends on type of mutation
- Specific base to specific base (A->G)
- Specific base to “any different” base (A-> nonA)
- Other types(inversions, translocations, deletions)
- Anythins in a gene that tields a phenotype
- There is the show, if the worm mutation rate to human, how many base mutations do human have? and the ans is 65 new mutations
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- Mutations introduce a lot of variation into populations
Mapping Complex Traits - Principles of mapping complex traits
- Most variable traits people study are conteolled by many genes
- Fruit size
- Human height
- Bristle numbers in Drosophila
- Many diseases
- Cancers
- Diabetes
- Schizophrenia
- QTLs
- Association of marker genotype with phenotype
- Markers near (“linked to”) gene(s) causing different phenotypes will show an association with that phenotype
- If there are many genes, and/ or if the effect is “complicated”, the assocation may not be very strong.
Complex Traits in Crosses
- Association depicted using “LOD” plot
- You try it ! and ans is between B and CPPT