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BIO120 Lecture 4: Variation (Where does genetic/heritable variation come…
BIO120 Lecture 4: Variation
Where does genetic/heritable variation come from?
Mutation: stable change in DNA sequence; can be neutral, deleterious, lethal, or beneficial
Characteristics
Unstoppable despite effective cellular mechanisms to correct DNA replication errors
Not directed by organism or environment (random)
Low rate depends on type of mutation or environment (e.g. high temperatures, mutagens, etc.)
72 new mutations per individual
DNA structure
Point mutation (one base sequence is changed)
Ex. mutation causing G6PD deficiency (causes anemia but protects against malaria)
Insertion/deletion
Changes in repeat number of codon
Useful for DNA fingerprinting
Chromosomal rearrangement
Independent assortment allows different combinations of parental chromosomes (i.e. 2^n possible gamete combinations, where n = set of chromosomes)
Recombination during meiosis further contributes to diversity where chiasma are created
How is it inherited?
1700s - only one parent contributed
1800s - theory of blending inheritance (parents' genes mixed together irreversibly)
PROBLEM: advantageous gene will eventually become diluted --> phenotype can't spread through population
Mendel
Experiment: hybrid cross of pure-breeding lines, selfed them (hermaphrodites) giving 100% of one gene, selfed them again giving 3:1 ratio
Inheritance determined by discrete particles (i.e. genes)
Each organism carries two copies of gene (alleles)
Organisms form gametes containing only one allele (sperm/pollen, eggs/ovules)
Offspring inherit one allele from each parent randomly
How does it influence trait variation?
Continuous (quantitative)
Number of genes (polymorphism)
Important environmental effects
Selection response
Discrete/discontinuous (Mendelian)
Genes of major effect
Dominance vs. recessiveness
Spread of alleles