genetic variation there are two main sources of genetic variation: – mutation – gene shuffling...
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Genetic Variation• There are two main sources of genetic
variation:– Mutation– Gene shuffling that results from sexual
reproduction• Some variation is “invisible”
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Turn to your table-mates:What are some of the genetic
variations in these babies?
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Visible?- Hair texture
- Size- Skin color
Invisible?- Ability to resist certain diseases
- Blood type
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Genetic variation is studied in populations.
• Recall: What is a population?
• A gene pool consists of all genes, including all the different alleles, that are present to a population.
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Relative Frequency of Alleles
• The relative frequency of an allele is the number of times that allele occurs in a gene pool (compared with the number of times other alleles for the same gene occur).
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What is the relative frequency of the brown allele? The black allele?
(50 total/20 black/30 brown)
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Relative frequency of…- Dominant (black) = 40%- Recessive (brown) = 60%
Note: The relative frequency of an
allele has nothing to do with whether
it is dominant or recessive.
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If the relative frequency of the black allele decreased, what would happen to the relative frequency of the other allele?
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In genetic terms, evolution is any change in the relative frequency
of alleles in a population.
Over many generations
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Sources of Genetic Variation
1. Mutations
2. Gene Shuffling that results from sexual reproduction.
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Mutations
• A mutation is any change in a sequence of DNA
• Mutations occur because of– mistakes in DNA replication– radiation or chemicals in the
environment• Some mutations do not affect
an organism’s phenotype, some do– Fitness
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Gene Shuffling
•Most heritable differences are due to gene shuffling–occurs during the production of gametes during
meiosis.
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• Crossing-over further increases the number of different genotypes that can appear in offspring.
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– i.e. no matter how many times you shuffle a deck of cards, the probability of drawing a particular card will remain the same.
Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.
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Q: What determines how many phenotypes there are for a given
trait?
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Single Gene TraitsSingle-gene traits are controlled by a single gene that
has two alleles. Variation only leads to two possible phenotypes.
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Polygenic Traits
Polygenic traits are controlled by two or more genes.
Polygenic traits have many possible genotypes and phenotypes.
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They exhibit a continuum of
expression in a bell shaped or
“normal distribution”
curve.
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A: The number of phenotypes produced for a given trait depends
on how many genes control the trait.
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Exit slip questions:
How do you think the relative frequency of alleles varies between the salamander subspecies in CA?
Give an example of a polygenic trait for salamanders.