Download Gene Pools

1.2: Most Genes Do Not Function Alone
• Essential Question:
– How do allele frequencies vary between
populations
Gene Pools:
• All members of a population can interbreed, they
share a common group of genes, called a gene
pool.
– A gene pool is the combined genetic information
of all the members of a particular population.
• Typically contains two or more alleles—or forms of a
certain gene—for each inheritable trait.
– 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
occur.
Relative Frequencies of Alleles
Section 16-1
Sample Population
Frequency of Alleles
allele for brown fur
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for black fur
Sources of Genetic Variation
• The two main sources of genetic variation
are mutations and the genetic shuffling
that results from sexual reproduction.
– Sexual reproduction can thus produce many
different phenotypes, but this does not change
the relative frequency of alleles in a
population. (Card deck analogy)
Evolution as Genetic Change
• Natural selection on single-gene traits
can lead to changes in allele
frequencies and, thus, to evolution.
– Ex: Color Mutations
Most Genes Do Not Function Alone:
• Multifactorial Traits- Traits that are determined
by one or more genes and the environment.
– Some illnesses occur in different forms
• Inherited or not
• Caused by one gene or more than one
– Alzheimer Disease, Breast Cancer, Parkinson
Disease, Familial Hypercholesterolemia
– Knowing whether a trait or illness is single-gene or
multifactorial (polygenic) is important for predicting
the risk of recurrence.
Single-Gene and Polygenic Traits
• Inheritable variation can be expressed in a
variety of ways.
• The number of phenotypes produced
for a given trait depends on how many
genes control the trait
Single-gene trait
• Trait controlled by a
single gene
• Variation in this gene
leads to only two distinct
phenotypes
• The number of
phenotypes a given trait
has is determined by how
many genes control the
trait.
In humans, having a widow’s
peak or not having a widow’s
peak is controlled by a single
gene with two alleles. As a result,
only two phenotypes are
possible.
Polygenic Traits:
• Most traits are controlled by
two or more genes and are,
therefore, called polygenic
traits.
• Each gene of a polygenic
trait often has two or more
alleles.
• As a result, one polygenic
trait can have many possible
genotypes and even more
possible phenotypes.
EX: height (A bell-shaped curve is
also called a normal distribution)
Checkpoint Questions:
1. What two processes can lead to inherited
variation in populations?
2. How does the range of phenotypes differ
between single-gene traits and polygenic
traits?
3. What is a gene pool? How are allele
frequencies related to gene pools?
4. How could you distinguish between a species
in which there is a lot of variation and two
separate species?
Genes & Disease Risk:
– Environmental factors & population groups
influence the different incidence of disease
associated with inheriting the same abnormal
gene.
Warm-up
• . . . All the Help I Can Get
• Natural selection operates on traits in
different ways.
• You might be able
to predict which traits natural selection
would favor if you think about the
demands of an organism’s environment.
Evolution as Genetic Change
• Natural selection
– does not act
directly on genes,
but on phenotypes.
– affects which
individuals having
different
phenotypes survive
and reproduce and
which do not.
– determines which
alleles are passed
from one
generation to the
next.
– can change the
relative
frequencies of
alleles in a
population over
time.
Exactly what factors change the relative
frequencies of alleles in a population?
• In genetic terms, any factor that causes alleles
to be added to or removed from a population
will change the relative frequencies of alleles.
• Evolution is any change in the relative
frequencies of alleles in a population’s gene
pool.
• Evolution acts on populations, not on
individuals.
Natural Selection on Single-Gene Traits
• Natural selection on single-gene traits
can lead to changes in allele
frequencies and, thus, to evolution.
– EX: Color Mutations (organisms of one color
may produce fewer offspring than organisms
of another color.
Genetic Determinism:
– The fact that the environment modifies gene
actions counters the idea of genetic
determinism, which is that an inherited trait is
inevitable.
• Harmful or Helpful
– Public Policy (Harmful)
– Identifying the genetic component to a trait (Helpful)