Genes and Variation

16-1

What are the main sources of

heritable variation in a population?

How is evolution defined in genetics terms?

What determines the numbers of phenotypes for a given trait?

Objectives

Darwin was unaware of Mendel’s work

with genes and heredity. What did Mendel study again?

Today’s understanding of genes, DNA, variation, and mutations is central to our understanding of how evolution works.

Darwin and Today

Many genes have more than one form or alleles Examples? There is also additional

variation that is “invisible” because it

involves small variations in

biochemical processes

How Common is Genetic Variation?

It is important to study entire populations for genetic variation

Population: group of the same species that interbreed.

Interbreeding populations share common groups of genes, including all the different alleles, in their gene pool.

Variation and Gene Pools

The relative frequency of an allele is the number of times that the allele

occurs in a gene pool, compared with the number of times other alleles for

the same gene occur.

Has nothing to do with whether the allele is dominant or recessive.

EVOLUTION in genetics terms: change in the relative frequency of alleles in a population.

Relative Frequency

Relative Frequency

1.Mutations

2.Gene Shuffling: results from sexual reproduction

Main Sources of Genetic Variation in a Population

A mutation is any change in

a sequence of DNA.

Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment.

Some mutations do not always affect an organisms phenotype, some do.

Some mutations do not affect an organism’s fitness, some do.

Mutations

Gene Shuffling

Most heritable differences are due to gene shuffling that occurs during the production of gametes during meiosis.

Crossing-over further increases the number of different genotypes that can appear in offspring.

Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

i.e. no matter how many times you shuffle a deck of cards, the probability of drawing a particular card will remain the same.

What determines how many phenotypes there

are for a given trait?

Question

Single Gene Traits

Single-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.

Represented with a bell shaped or “normal distribution” curve.

Answer

The number of phenotypes produced

for a given trait depends on how many genes

control the trait.