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Lecture 8

BIOL 103 Lecture Notes - Lecture 8: Blending Inheritance, Mendelian Inheritance, Chromosome Segregation

Course Code
BIOL 103
Virginia K Walker

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Biology Week 8
Quantifying variation
o Heterozygosity is a common way to measure variation
o Allele richness
Sustaining Variation
o Mutations allows for more variation and creation of new alleles
o Varying Pressures environmental changes can lead to new forms of selection
o Sex reshuffling of genetic and variation
o Trade off certain traits that are beneficial come at a loss for something else
o Balancing when heterozygotes have an advantage
Leski didt stop there
o Number of lines became mutators, having much elevated rates of spontaneous mutation
o New adaptation occurred in occasional jumps
o Gnentic complecity of populations became apparent
o While expecting new high fitness variants to sweep to fixation (100% frequency) genetic diversity remained
o Just after generation 33,000 population A-3 exhibited a sudden increase in fitness: not 1 or 2% but several fold
o A key innovation a trait allowing colonization of a new resource or habitat
The inability to aerobically metabolize citrate is a diagnostic test for E. coli
Was it a single, extraordinary, mutation event or was it a series of ordinary mutations by chance turning out to create a good team
Clines provide some of the most compelling evidence for environmentally mediated maintenance of genetic variation
o Shown are the clines in Australia and NA for wing size and allele frequency of the cpo allele in Drosophila melanogaster
o This species has spread out of Africa as a human commensal and parallel latitudinal clines have been shown on multiple
o Clines are easier to identify than associations in complex patchy habitats
An early window into the genome
o Mutations are sometimes beneficial:
When double stranded breaks occur, genomic repair mechanisms occasionally cause a flip
The inverted sequence does not align with its standard homolog, stopping cross over
Can be visualized in the polytene chromosomes for fruit flies
Chromosome rearrangements can thus link together particular alleles at different loci
If super genes function well together, the inversion increases in frequency
Sex shuffling the deck
o Recombination mixes up gene combinations within (via crossing over) and between (via chromosome segregation)
o Gene mixing creates not only new alleles but an enormous assortment of gene combinations that may interact in different
o Sex is there fore a double edged sword:
Good if the eiroet is upreditale or ou hold a ad had
Bad if ou hold the roal flush ad are ell adapted to oditios
The problem of blending
o Darwin and contemporaries did not understand genetics and inheritance
o Quantitative traits blend in offspring generally as intermediates
o Few traits are actually discrete and medelian
o Rediscovery of mendelian Genetics was presented a challenge to the concept of
blending and resolving it was ultimately crucial to evolutionary theory
Early problems:
o Blending:
Observations of intermdiacy for many traits led some to believe in a blending of
maternal and paternal alleles
Blending inheritance should lead to a constant loss of genetic variation
o Dominance:
Dominance was misunderstood to reflect a kind of conversion of the other
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