Genes and Variation

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Genes and Variation
For evolution to be acceptable, there has to be a
way of explaining the variation in a species.
Darwin did not know of the work of Mendel.
Gregor Mendel
Charles Darwin
Laws of Heredity
Theory of Evolution
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Genes and Variation
Mendels Laws of Heredity showed how members of
species can inherit different forms of a trait.
B blue
b yellow
Bb
Bb
bb
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Genes and Variation
Darwin did not know of the work of Watson and
Crick.
Charles Darwin
James Watson Francis Crick
Theory of Evolution
Model of DNA
Sections of the DNA molecule called genes give us
our traits and our variations.
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Genetic Variation
Variation in a species is due to different
alleles for the genes.
most common
white
crimson
pink
4 color variations of the tentacles of the sea
anemone Urticina felina multiple alleles are
likely.
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How Common Is Genetic Variation?
Actually, its quite common.
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Some Variation is Invisible
two weeks later
two weeks later
Some corn is more resistant to disease than other
corn this is a biochemical variation.
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For vertebrates, 4 - 8 of their genes are
heterozygous. (Bb)
These are members of the same species of guppy
but with different alleles for body color.
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Gene Pools
The term gene pool refers to all of the alleles
for all of the genes in a population.
This picture shows the gene pool for just one
gene in a small population.
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Relative Frequency in a Gene Pool
The percent that an allele exists in a gene pool
is called its relative frequency in the pool.
For this gene pool
allele 12/20 60
allele 8/20 40
Future populations would be expected to have a
higher of whichever color increases fitness (if
either).
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Defining Evolution Using Gene Frequencies
change in
allele

population
frequency
appearance
therefore
allele
evolution
frequency
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Sources of Genetic Variation
Mutations
- any change in the DNA
Some do not change the phenotype.
parents
offspring
Some change the phenotype but not fitness.
parents
offspring
Some change the phenotype and fitness.
parents
offspring
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Sources of Genetic Variation
Gene Shuffling
Variation will occur whenever
1. homologous chromosomes separate independently
2. crossing over occurs
Independent Assortment
Crossing Over
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Sources of Genetic Variation
Single-Gene Traits
1 gene (2 alleles)
The allele s and dominant vs. recessive
determine phenotypes.
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Sources of Genetic Variation
Polygenic Traits
gt1 gene (gt2 alleles)
A range of phenotypes usually exists.
normal distribution