Download GENETIC VARIATION - anderson1.k12.sc.us

SC STANDARD B-5: Students will demonstrate an
understanding of biological evolution and the diversity of
life.
Inquiry Activity
 Page 392 in textbook
 Page 47 in notebook
 Work in table groups
 CN page 44
 Topic: Genetic Variation
 EQ: What factors contribute to genetic variation
in a population?
Genetic Variation :
How common is it?
Most mammals are
heterozygous for 4
to 8% of genes
Gene Pools
 Consists of all the
different alleles
present in population.
 Population: group of
individuals of same
species that live in
same area
Gene Pools
 The # of times an allele
occurs in a gene pool
compared with the # of
times other alleles
occur is called the
allele’s
relative frequency
 In genetic terms,
evolution is any
change in the relative
frequency of alleles in
a population
Sources of Genetic Variation
1. MUTATIONS
2. Gene Shuffling
 May have a change in DNA
 Cause of most heritable
sequence that does/does
not result in a change in
phenotype
 Some mutations alter the
individual’s fitness, some
do not
differences
 Due to meiosis
 Crossing over increases
genetic variation
 Homologous
chromosomes assort
independently in
anaphase I
 Random gamete fertilized
 Does not change gene
frequency
The # phenotypes produced for given trait
depends on # genes that control the trait.
Single-Gene Trait
Polygenic Traits
 Example: Widow’s Peak is
 >2 genes control trait &
dominant over no widow’s
peak
there are often >2 forms of
alleles
Natural Selection on Single-Gene
Traits
 can lead to changes in
allele frequencies and
so to evolution
 Example: whitish tree
bark fed on by moths
with variations in
color white-gray to
gray-black
Natural Selection on Polygenic Traits
 can affect distribution
of phenotypes in 3
ways
 1. Directional Selection
 occurs when individuals
at one end of curve have
higher fitness than
individuals at other end
or in middle of curve
How Natural Selection
Affects Phenotypes
2. Stabilizing Selection
3. Disruptive Selection
 Individuals near center of
 Individuals at both ends have
curve have higher fitness than
individuals at either end
higher fitness than those near
middle of curve
Genetic Drift
 is the random change in allele frequencies that occurs
in small population
 In small populations individuals that carry a particular
allele may leave more offspring than other individuals,
just by chance.
 Over time, a series of chance occurrences of this type
can cause an allele to become common in a
population.
Genetic Drift
 seen when a small population colonizes a new
habitat
 these individuals may carry different relative
frequencies than did the larger population they
came from
Founder Effect
 the change in allele
frequencies as a result of
migration of a small
subgroup of a population
 Example: in 1814 15 British
subjects colonized an
uninhabited island in the
Atlantic
 1 individual was
heterozygous for a
recessive allele for retinitis
pigmentosa
 by 1960 4 of 240 citizens
afflicted with R.P. a much
greater incidence than in
Britain
Hardy-Weinberg Principle
 states that allele frequencies in a population will
remain constant unless 1 or more factors cause
those frequencies to change
 genetic equilibrium: situation in which allele
frequencies remain constant
5 conditions to maintain genetic
equilibrium
 1. random mating
 ensures each individual has equal opportunity to pass
on its alleles
 2. large population
 genetic drift has less effect on larger #s
 3. no immigration or emigration
 allele frequencies remain same
 4. no new mutations
 allele frequencies stay the same
 5. no natural selection
 All genotypes have equal chance of surviving &
reproducing
p² + 2pq + q² = 1
 p = dominant allele
 q = recessive allele
 pq = heterozygotes