Last day''' outlined Darwins theory of natural selection,

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Last day... outlined Darwins theory of natural
selection, some of the evidence he used to
support evolution natural selection
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Also - Mendel discovery of rules of heredity
(1866)
- traits do not blend in offspring - when
cross-bred, only dominant trait expressed in F1
generation, but dominant (3/4) recessive
traits (1/4) both show up in F2 generation
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Today, understand this occurs because
hereditary info is contained in genes
located on chromosomes
Most plants animals carry 2 sets of
chromosomes diploid
One set from mother (in egg), other set
from father (in sperm)
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One gene (e.g. for flower color) has different
versions called alleles (e.g. purple or white)
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Gene can be thought of as segment of DNA that
codes for a protein (or polypeptide)
Code is written using 4 bases Adenine Thymine Gua
nine Cytosine
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Sequence of 3 bases codes for a particular amino
acid in the protein
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How does this make a flower purple or white?
Purple allele codes for a protein that makes
flower purple
White allele may code for a defective protein
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One copy of dominant purple allele produces
sufficient protein to turn flower purple
2 copies of same allele homozygous
1 copy each of different alleles heterozygous
Phenotype (observable characteristics of an
organism) does not always reveal the
genotype (set of genes or alleles an
individual has)
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• Not all genetic characters
• work the same way
• some traits show
• incomplete dominance
• (heterozygotes show
• intermediate phenotype)

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Another complication - some characters are
affected by gt1 gene, each gene may
have gt2 alleles
Quantitative characters typically show
continuous variation
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Diploid organisms get one copy of gene from
mother and one copy from father
But parents have 2 copies of each gene - how does
it work so only one copy gets passed on?
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Normal cell divisions (mitosis) produce 2
daughter cells with same set of genes as
original cell - no good for producing
gametes in diploid organisms
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Gametes instead produced by meiosis -
reduction division that results in
haploid cells
- chromosomes replicate before meiosis -
diploid organisms now with 4 copies
of genes
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• THEN 2 rounds of cell division,
• 4 daughter cells, not 2
• each haploid (1 copy of genes)

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Meiosis does not just reduce chromosome number,
it also creates new gene combinations

• homologous chromosomes come together as pairs
• (tetrad), process called synapsis

- chiasmata form crossing-over of chromatids
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Other ways of creating new gene combinations

• Independent assortment of chromosomes
• when chromosomes align during meiosis,
• chromosome from same parent may move
• into different cells

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• Random fertilization
• combination of any sperm with any egg allows
  trillions of possible combinations

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New gene combinations detected by Mendel when he
crossed peas that differed in 2 characters
e.g. pea color (yellow or green) shape
(round or wrinkled
Original combinations were found, but also
new ones
How do new combos arise?
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Ultimately, genetic variation must come from
mutation - many ways for this to happen...
Chromosomes may break fragments may be lost,
or added elsewhere
Errors also during crossing-over
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deletion segment removed
- usually lethal
duplication segment repeated
- usually harmful
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inversion segment in reversed position
reciprocal translocation fragments switched
between non-homologous chromosomes

• inversions translocations can change phenotype
  
• because gene expression may depend on
  position

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Point mutations (change in one nucleotide of DNA)
can be caused by chemicals, UV light,
errors in replication, etc.
- most repaired by enzymes
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Most mutations are deleterious, some neutral (do
not change fitness), a very few are beneficial

• generally detrimental, but could not have
  evolution
• without them

Chapter 14 (esp. pp. 262-75)
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Evolution after Mendel After Darwin, evolution
generally accepted but skepticism about
natural selection - poor understanding of
heredity a problem
Mendels paper noticed in 1900, theory of
heredity rapidly developed
Initially thought Mendelism was in conflict with
natural selection - emphasis on major
mutations causing discrete traits, not small
variations
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Eventually, population genetics developed,
combined with findings from paleontology,
systematics, etc. ? The Modern Synthesis -
built on clarified Darwins theory
Sewall Wright
Ronald Fisher
J. B. S. Haldane