Independent Assortment of Homologous Chromosomes

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Independent Assortment of Homologous Chromosomes
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pyramidines
purines
C
G
T
A
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Point mutation change a single base in
the DNA sequence substitution Transitions
change a purine for a purine (A for G) or
pyramidine for pyramidine (C for T)
Transversions change a purine for a pyramidine
(A for T C for G) Effects of a point mutation
will vary depending on whether or how the protein
they code for is changed. If amino acid is
changed replacement substitution If amino
acid is unchanged silent-site substitution
(neutral mutation)
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Missense substitution replacement substitution
that disrupts function EXAMPLE sickle-cell
disease The replacement of A by T at the 17th
nucleotide of the gene for the beta chain of
hemoglobin changes the codon GAG (for glutamic
acid) to GTG (which encodes valine). Thus the 6th
amino acid in the chain becomes valine instead of
glutamic acid.
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Normal adult hemoglobin gene (Hb) Sickle Cell
hemoglobin gene (HbS) HbHb HbHbS HbSHbS Homoz
ygotes for HbS, develop severe anemia and do not
typically live past puberty without aggressive
medical intervention How does the HbS gene remain
at relatively high frequencies in the
population? The fitness of a gene depends on its
selective value in its natural environment West
Africa Normal homozygotes (HbHb) in West Africa
are highly susceptible to malaria high death
rates! Heterozygotes (HbHbS) are often only
mildly anemic, but they are also less affected by
malaria heterosis!
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Missense point mutations are an example of
loss-of-function mutations These are most
commonly caused by another kind of point mutation
frame-shift mutation Frame shifts are caused by
insertions or deletions
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Mutation rates

• Mutations occur at low but steady rates
• Mutation rates may differ among genes, among
  individuals and among species

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Variability among individuals

• DNA polymerase (necessary for replication) varies
  among individuals,
• i.e., some isomers of DNA polymerase are more
  error-prone, producing more mutations during
  replication
• Why dont they disappear due to selection?
• Compromise between speed and accuracy

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Variability among species

• Mutation rates should be highest in species with
  the fastest generation times, i.e., more
  replications/time (bacteriagthumans)
• But per generation, longer lived species actually
  have higher mutation rates more cell divisions
  in meiosis older gametes

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Variation among genes

• Mutation rates are lowest in genes or regions of
  genes whose structure is highly correlated with
  its function, i.e., those controlling the active
  site of an enzyme as opposed to a non-active site
• Mutation rates are lowest in genes whose
  conservative function is essential for life,
  i.e., cytochrome c Hb

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Macromutation

• Gene duplication an extra copy of a gene is
  produced in the genome
• How?
• 1) retrotransposition mRNAgtDNA
• 2) incomplete crossing-over (more common)
  
• What are the effects of the extra copy?
• Its a freebie! It can mutate w/o functional
  consequences (since you still have 2 normal
  copies) gt increases liklihood of positive
  mutations gt new alleles with new functions

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Inversion
Original sequence
Inverted sequence
Inverted sequence cant be broken up by crossing
over gt linkage (supergene)
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• Aneuploidy duplication of chromosomes, usually
  1 or a few. Bad (Downs syndrome)! Usually
  sterile.
• Polyploidy duplication of entire genome (N)
• due to unreduced (2N) gametes
• 2N x N gt triploid usually sterile
• 2N x 2N gt tetraploid
• Autotetraploids (within a species) usually
  sterile due to 4 homologs trying to synapse
  during meiosis gt tearing unequal division
• Allotetraploids (hybrids between species) often
  fertile double the number of homologous pairs,
  but still in pairs
• Other multiples usually in even numbers (6N, 8N,
  etc.)

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• Successful polyploidy can lead to instantaneous
  formation of new species gt if they try to cross
  with parent (2N x N), leads to sterile
  triploidsgtgenetic isolation
• Just as with gene duplication, genome duplication
  provides new DNA that is free to accumulate
  mutations. Though most mutations are neutral or
  deleterious, since polyploids have a whole extra
  complement of DNA they can continue to function
  normally so the chances of incorporating a new
  functional mutation increase