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.