Download Genes and Variation

16-1
Objectives

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?
Darwin and Today

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.
How Common is
Genetic Variation?

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
Variation and Gene Pools

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.
Relative Frequency

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

Main Sources of Genetic
Variation in a Population

1. Mutations
1. Gene Shuffling: results from
sexual reproduction
Mutations

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.
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.
Question

What determines how many
phenotypes there are for a
given trait?
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.