Download Population Genetics

Population Genetics
History
• Monk at the Brno
monastery
• Mathmatician –
statistics – probability
– physics
• Assigned to tend the
gardens
Plant Molecular Genetics Lecture 1 courtesy Dr. Michael Neff
http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html
X
X
http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html
(self fertilized)
X
Mendel’s work went
unnoticed for over 30 years
until other scientists started
to observe the same things.
All were European except
one.
William Spillman – In 1894
came to Washington
Agricultural College. It was
while Spillman was here
that he experimented with wheat, attempting to hybridize the
crop for the growing conditions here in the Palouse. He also
independently rediscovered Mendel’s Law of Heredity and is
credited with being a major role in the acceptance of Mendel’s
Law by scientists and agriculturists.
Plant Molecular Genetics Lecture 1 courtesy Dr. Michael Neff
http://www.wsulibs.wsu.edu/Holland/masc/finders/cg250.htm
Vocabulary
• Alleles – different forms of the same gene
• Genotype – forms of genes an individual possesses
• Meiosis – happens during sexual reproduction when only one
copy of the genes are passed to the gamete
• Gamete – sexual reproductive cell
• Homozygous – when both copies (alleles) of the gene are the
same
• Heterozygous – when the alleles of the same gene are
different
• Phenotype – the expressed type of the gene(s)
• Fixation – when a population becomes limited to just one
allele of a gene – loss of genetic diversity
• Directional selection – shift in a trait that confers fitness
(higher survival) or greater reproduction in individuals
• Stabilizing selection – maintaining a range of traits
Reginald Punnett – 1875 - 1967
1st Generation
♀
0.50 B
maternal
gametes 0.50 b
• Alleles – B & b
• Meiosis produces the
haploid gametes that
merge to form the
diploid zygote
♂ paternal gametes
0.50 B
0.50 b
0.25 BB
0.25 Bb
0.25 Bb
0.25 bb
1:2:1 ratio of genotypes
3:1 ratio of phenotypes
25% homozygous dominant
25% homozygous recessive
50% heterozygous dominant
http://thehandiestone.typepad.com/blog/2010/03/sixyearold-chinese-boy-with-31-fingers-and-toes.html
There was debate about how if Mendel’s Laws apply to
people, and disorders such as brachydactyly is a
dominant trait, why doesn’t this phenotype affect 3 out
of 4 people?
It is said that Punnett played cricket with G.H. Hardy.
Both G.H. Hardy and Wilhelm Weinberg, independent of
each other, pointed out that it was the percentage of the
alleles in the population that had to be taken into
consideration. Due to the research efforts of both we
now have a theorem that we know of as :
The Hardy – Weinberg equilibrium
This equilibrium is only valid if certain conditions are met.
1. The population is very large – small populations have a
tendency to cause genetic drift
2. There is random mating
3. No mutation – from B to b or b to B
4. No migration – movement of individuals out of
population or in to the population
5. No selection – opposite is what we think of as natural
selection
http://www.geocities.com/haplogroupb/mtdnamigrations.jpg
http://aquarium.nefsc.noaa.gov/FAQs/GeneralFAQ/im
ages/bluelob2.jpg
http://www.angelo.edu/faculty/kboudrea/lagnia
ppe/pictures_san_antonio_zoo_4/3_Albino_Allig
ator_01.JPG
The Hardy – Weinberg equilibrium
(remember in this instance 1 equals a population)
(B + b) or (p + q) = 1 (100%)
Where p = the percentage (frequency) of the
dominant allele and q = the percentage (frequency) of
the recessive allele
&
Diploids have a 2 alleles that make up their genotype
(BB, Bb, bb) so:
p2 + 2pq + q2 = 1 *Not technically squared
BB = p2
where
Bb = 2pq bb = q2
(p2 + 2pq + q2) or [(B)(B) + 2(B)(b) + (b)(b)] = 1
The Hardy – Weinberg equilibrium
BB = 25% (0.25)
Bb = 50% (0.50)
bb = 25% (0.25)
B+b=1
Allelic Frequencies
Genotypic Frequencies
B = (25% + ½ 50%) = 50%
b = (25% + ½ 50%) = 50%
1st Generation
♂ paternal gametes
♀
0.50 B
maternal
gametes 0.50 b
0.50 B
0.50 b
0.25 BB
0.25 Bb
0.25 Bb
0.25 bb
(p2 + 2pq + q2) or [(B)(B) + 2(B)(b) + (b)(b)] = 1
The Hardy – Weinberg equilibrium
How to figure out the allelic frequency based on phenotype observation
9 black cats (TT)
23 black & white cats (Tt)
18 white cats (tt)
1st figure out where the phenotype fits - ( TT is homozygous dominant or p2 )
- ( Tt is heterozygous dominant or 2pq )
- ( tt is homozygous recessive or q2 )
2nd figure out what the percentage of the total number of alleles the selected
phenotype represents –18 white cats = 36 t alleles, 9 black = 18 T alleles, 23
B&W = 23 T alleles and 23 t alleles
3rd add up all the t’s and T’s
36 + 23 = 59 t’s, 18 + 23 = 41 T’s
4th is to do the math - There are 100
total alleles so the allelic frequency is
– t’s = 59/100 or 59%
- T’s = 41/100 or 41%
p + q = 1 check .41 + .59 = 1
(.41)(.41) + 2(.41)(.59) + (.59)(.59) = 1
0.1681 + 0.4838 + 0.3481 = 1
And you can see from the calculation that
the homozygous populations are not the
square root of the frequencies
Lab Exercises
Exercise 1 – Testing the Hardy - Weinberg Equilibrium :
• using a bead model
• calculation to estimate heterozygote frequency
Phenylketonuria (PKU) is a rare condition in which a baby is born
without the ability to properly break down an amino acid called
phenylalanine due to an enzyme called phenylalanine hydroxylase is
missing. This can be represented in our genotype as aa.
Exercise 2 – Effect of Small Population Size: Simulation of
Genetic Drift
• Bottleneck effect
• Founder effect
Exercise 3 – Simulation of Natural Selection
Summary at the end of this exercise is what your group will
present at the end of lab to the rest of the groups
Next Week
Quiz
Microscope skills/Protists & Fungi