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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”
Turn to your table-mates:What are some of the genetic
variations in these babies?
Visible?- Hair texture
- Size- Skin color
Invisible?- Ability to resist certain diseases
- Blood type
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.
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).
What is the relative frequency of the brown allele? The black allele?
(50 total/20 black/30 brown)
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.
If the relative frequency of the black allele decreased, what would happen to the relative frequency of the other allele?
In genetic terms, evolution is any change in the relative frequency
of alleles in a population.
Over many generations
Sources of Genetic Variation
1. Mutations
2. Gene Shuffling that results from sexual reproduction.
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
Gene Shuffling
•Most heritable differences are due to gene shuffling–occurs during the production of gametes during
meiosis.
• Crossing-over further increases the number of different genotypes that can appear in offspring.
– 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.
Q: What determines how many phenotypes there are for a given
trait?
Single Gene TraitsSingle-gene traits are controlled by a single gene that
has two alleles. Variation only leads to two possible phenotypes.
Polygenic Traits
Polygenic traits are controlled by two or more genes.
Polygenic traits have many possible genotypes and phenotypes.
They exhibit a continuum of
expression in a bell shaped or
“normal distribution”
curve.
A: The number of phenotypes produced for a given trait depends
on how many genes control the trait.
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.
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