Download POPULATION GENETICS Terms 1.

POPULATION GENETICS
Terms
1.
Population - The total of all individuals in a breed or species or those
individuals that inhabit a particular area.
2.
Population Genetics - Dealing with the genetics of individuals that inhabit a
particular area.
3.
Gene Frequency - The relative abundance or relative rarity of a particular gene
in a population as compared with its own alleles in a population. Any gene
frequency takes a range from 0 to 1.
A.
Gene frequency is determined by the mode of inheritance.
The variance is due to the frequency of the genes.
B.
Factors that may modify the gene frequency.
1)
Selection - artificial selection.
2)
Natural selection - what goes on in nature. Those
with more viability continue to reproduce.
4.
Heterosis - Superiority of heterozygous genotypes in comparison with
corresponding homozygous genotypes in respect to one or more traits.
Heterosis = (Avg. of hybrid offspring - Avg. of crossed breeds) x 100
Average of crossed breeds
5.
Heritability (h 2) - Degree to which a given trait is controlled by inheritance.
h2 = ?G/Sd
6.
Hardy-Weinberg Law - Use when dominance is complete and when there are
two phenotypes and three genotypes. This law states that in large
populations, there are the following conditions:
A.
Frequency of one or two alleles is equal to p 2.
B.
Frequency of the other alleles is equal to q 2.
C.
Sum of frequency p + q = 1.
D.
Mating is random.
Offspring of three genotypes in a definite ratio will be in equilibrium in the next
generation in the frequency of:
p2 + 2pg + q 2 = 1
A.
p2 is the number of homozygous dominant individuals.
B.
2pg is the number of heterozygous individuals.
C.
q2 is the number of homozygous recessive individuals.
Population Genetics
(Uncontrolled environment)
Population - Total number of individuals of a breed or species that inhabit an
area.
Population genetics - Genetic variation of the population and how it interacts with the
environment.
AA AA
aa Aa aa
Aa Aa Aa
aa
Env 1
Genotype:
Env 2
Env 3
AA
AA
AA AA
AA
AA Aa
aa
Aa aa
AA
aa
aa
aa aa
aa
AA
AA
Aa
aa
aa
Partitioning
of genotypes
in
environments
With 1 locus:
* 2 alleles - A, a
* 3+ alleles - ?A, ?B, ?O (ABO blood group) multiple alleles
* In humans - > 1,000 loci
To study populations: Use 1 locus models.
AA
Aa
aa
* How many A's and a's?
(allele frequency)
* How many AA's, Aa's, and aa's?
(genotypic frequency)
Consider 1 locus with 2 alleles, A and a.
Allele Frequencies:
* Fr (A) = p = 2 (#AA) + (#Aa)
2N
* Fr (a) = q = 2 (#aa) + (#Aa)
2N
or
* Fr (a) = 1 - P (since p + q = 1)
Genotypic frequencies:
Fr (AA) = #AA
N
Fr (Aa) = #Aa
N
Fr (aa) = #aa
N
Are all genotypes present in proportion to allele frequencies?
* product rule:
Ex. .5 x .5 = .25
A=p a=q
A=p
p2
pq
a=q
qp
q2
Thus: p2 + pq + qp + q 2 = 1
Hardy - Weinberg eq. (law)
p2 + 2pq + q 2 = 1
(AA) (Aa (aa)
+ aA)
Assumptions: A. No selection
B. Random mating
C. No migration (in or out)
AA aa
Aa Aa
Aa
Aa aa
Aa
D. No mutation
E. No genetic drift
Population Genetics (Continued)
To test, use Chi-square - "Goodness of Fit"
x2 = Summation of (obs - Exp.)2
Exp.
*If x2 < x2 at 0.05 with 1 df, then accept Ho that genotypic and allelic frequencies are in
equilibrium; otherwise, reject Ho.
Ex. AA = 21, Aa = 53, aa = 26;
Genotypic Frequencies:
Fr (AA) = #AA = 21 = 0.21
N 100
Fr (Aa) = #Aa = 53 = 0.53
N 100
Fr (aa) = #aa = 26 = 0.26
N 100
N = 100
Allele Frequencies:
Fr (A) = p = 2 (AA) + Aa = 2(21) + 53
2N
200
Fr (A) = p = 0.48
Fr (a) = q = 2(aa) + Aa = 2(26) + 53
2N
200
Fr (a) = q = 0.52
Chi-Square Analysis:
Genotype
HWeFr
HWeFr (N)
Exp#
AA
p2 = .23
.23 (100)
= 23
Aa
2pq = .50
.50 (100)
= 50
aa
q2 = .27
.27 (100)
= 27
Genotype
Obs.
Exp.
21
53
26
23
50
27
AA
Aa
aa
(Obs - Exp)
-2
3
-1
(Obs - Exp)2
4
9
1
(Obs - Exp)2
Exp
____________________________
AA
4/23 = 0.17
Aa
9/50 = 0.18
aa
1/27 = 0.04
Genotype
x2 = 0.39
x2 = 0.39 < x2 (0.05) = 3.841
(with df = 1)
.39 < 3.841; therefore, accept Ho
No statistical difference
Naturalists and Breeders (agricultural) developed the science of population genetics.
Forces Affecting Allele Frequency:
*Genetic Drift - Differential contribution of genotypes (offspring) to the next
generation due to random events.
Populations usually small.
AA
AA
Aa
aa
AA
AA
*Selection (S)
Differential contribution of offspring to the next generation due to differences in
survivorship or fecundity (reproductive potential).
a)
b)
artificial - imposed by man
natural - interaction of environment and genotype.
For selection to occur, trait has to be:
a)
b)
c)
Heritable
Must have genetic variation
Reproductive potential is greater than the carrying
capacity - more offspring are produced than are able
to live.
Heritability (h 2) = the degree to which a particular trait is inherited.
R = ?G = response of a population (given by the new population mean after selection)
S = Sd = selection imposed on the trait (can be strong or weak, and is different
between selected and unselected population means)
R = h2S
The wider the curve, the more loci affecting the trait (polygenic).
R = h2S
As selection proceeds, curve gets narrower and narrower.