Transcript Population Genetics - Bev Facey Community High

What do you think of when you hear
POPULATION GENETICS?
• Population = a group
of individuals of the
same species
occupying a given
area at a certain time
• Genetics = the study
of heredity
Population Genetics = the study of
heredity within a given population
• Species  a group of organisms that
actually (or potentially) interbreed in nature
and are reproductively isolated from all
other groups
• Gene pool  all the alleles of all the
genes of all the individuals in a population
• Evolution  cumulative changes in the
gene pool (and therefore changes in
characteristics of populations) of
organisms from one generation to the next
Natural Selection
• Darwin studied variation in phenotypes
• Natural selection  some traits are better
equipped for survival in a given
environment
– They are “naturally selected” for through
survival of the fittest
– E.g. short beaked finches vs. long beaked finches
eat seeds
– E.g. ??
eat grubs
The Importance of Variation
• Variation among
organisms is not only
restricted to physical
appearance
– What other variations can
be seen between
organisms?
• Metabolism, fertility, mode
of reproduction, behaviour
etc. etc.
• In order to have variation
amongst organisms, we
must have variation in
our genes
Albatross’ elaborate and unique courtship
behaviour
Variation as a Means for Survival
• Certain genotypes
are better equipped
than others for
survival
• Sexual reproduction
 variations passed
on to offspring
• Leads to natural
selection for
individuals in given
conditions.
Studying Human Genetic Traits
• Studying humans is problematic
– WHY??
• Few offspring
• Slow reproductive time
• How do we study human populations
then?
• POPULATION SAMPLING!!
Population sampling  finding the gene
frequencies for a particular genetic trait in
a small sample of the population and
applying them to the whole population
Trends are studied in a small population
sample and then applied to the whole
population
Trends  Gene Frequencies!
Gene pool  all of the genes that occur within a
specific population
Gene pool size increases as the population
size increases!
Sometimes, gene frequencies are associated with
certain populations
Swedish people mostly blond haired and
blue eyed
- the frequency of blond hair and blue eyes is
HIGH in this population
Any other examples?
When studying GENE FREQUENCIES…
Must use…
HARDY WEINBERG EQUILIBRIUM!!!
FINDING THE HARDY-WEINBERG
EQUILIBRIUM
Punnett square crossing two heterozygous
individuals for tongue rolling ability
R
r
R
RR
Rr
r
Rr
rr
R = Tongue rolling
r = non-tongue rolling
Let p  DOMINANT ALLELE frequency
Let q  RECESSIVE ALLELE frequency
R
(p)
r
(q)
R
(p)
RR
(p2)
Rr
(pq)
r
(q)
Rr
(pq)
rr
(q2)
R
(p)
0.6
r
(q)
0.4
R
(p)
0.6
RR
(p2)
0.36
Rr
(pq)
0.24
r
(q)
0.4
Rr
(pq)
0.24
rr
(q2)
0.16
What do you notice about the numbers?
0.36 + 0.24 + 0.24 + 0.16 = 1
If we express this in algebra form, we
get…
p2 + 2pq + q2 = 1
p = Frequency of
DOMINANT ALLELE
ALLELES
q = Frequency of
RECESSIVE ALLELE
p2 = Frequency of
HOMOZYGOUS DOMINANT
GENOTYPE (RR)
2pq = Frequency of
HETEROZYGOUS
GENOTYPE (Rr)
q2 = Frequency of
RECESSIVE GENOTYPE
(rr)
Freq. RR
0.36
Freq. Rr
0.48
Freq. rr
0.16
GENOTYPES
HARDY-WEINBERG
EQUILIBRIUM
2
p
+ 2pq +
2
q
p+q=1
=1
Assumptions of the Hardy-Weinberg
Equilibrium (Text p. 560)
1. No mutations
2. No migration  no immigration or emigration
(no gene flow)
3. No selective advantage for any genotype
4. Random mating
5. Very large population size
H-W assumes no evolutionary forces are acting on
a population to change the frequencies of different
alleles or genotypes.
The frequencies of alleles in populations are
always changing but H-W equations help us to
detect changes.
Hardy-Weinberg Examples
1.In a hypothetical population of birds, the
dominant beak color is brown. A recessive
allele for blue beak color occurs in a
frequency of 0.4.
a) What is the genotypic frequency of the blue
beaked trait?
b) What is the genotypic frequency of the
heterozygous condition?
c) What is the genotypic frequency of the
brown beaked trait?
Hardy-Weinberg Examples
2.
In a hypothetical population of squirrels, the dominant
fur color is black. A recessive allele for grey fur color
occurs with a frequency of 0.3. What is the frequency
of the heterozygous genotype?
3. Suppose that within a population of garter snakes, 408
of 1700 snakes exhibit a recessive stripe color. In the
next generation, 528 out of a population of 2200
exhibit the same trait. Show the steps involved in
determining whether evolution has taken place.
4. The frequency of individuals displaying the dominant
phenotype is 0.36. Find p and q.
Hardy-Weinberg Practice
• H - W Practice sheet – I’ll hand out.
• Blue workbook: pages 130 - 131 and 133 – 134
• If you don’t have blue booklet today – make sure
to bring it tomorrow – we need it for an activity
we’re doing on pages 123 – 125. Bring a pencil
and calculator too.
• H – W Quiz on Thursday.
Try this link for an online quiz for HardyWeinberg equilibrium