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Biology

BIO207H5

Karen Williams

Winter

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Ch. 21 Population Genetics
-transmission genetics: concerned primarily with genetic processes that occur within the individuals and
how genes are passed from one ind. to another. Thus, the unit of study for transmission genetics is the
individuals
-Molecular genetics: we are interested largely in the molecular nature of heredity: how genetic
information is encoded within the DNA and how biochemical processes of the cell translate the genetic
information into influencing the phenotype. Focus is on the cell
-Population genetics: applies the principles of transmission genetics to large groups of individuals,
focusing on the transmission process at one or a few loci
-Quantititave genetics: considers the transmission of traits simultaneously determined by many genes
-population geneticists investigate the patters of genetic variation found among individuals within
groups (genetic structure of population) and how these patterns vary geographically and change over
time.
-A Mendelian population is a group of interbreeding individuals who share a common set of genes.
-the genes shared by the individuals of a Mendelian population are called the gene pool
-the principle aim of population genetics is to understand genetics of evolution, a change in a
population or species over time.
Genetic Structure of Populations
Genotype frequencies
-a frequency is a proportion, and it always ranges between 0 and 1.
-if 43% of the people in a group have red hair, the frequency of red hair in the group is 0.43. the
frequency that don’t is
1 -0.43 = .57 or 57%
-to calculate the genotype frequencies at a specific locus, we count the number of individuals with one
particular genotype and divide this number by the total number of individuals in the population. We do
this for each of the genotypes at the locus. The sum of the genotype frequencies should be 1.
Example: typically 3 genotypes: BB, Bb, and bb
f(BB) = 452/497 = 0.909
f(Bb) = 43/497 = 0.087
f(bb) – 2/497 = 0.004
-genotype frequencies at a single locus are useful for examining the effects of certain evolutionary
processes on a population
Allele Frequency
-frequencies of alleles to describe how the gene pool changes over time
-alleles are passed from one generation to the next. Consequently only alleles have continuity over
time, and the gene pool evolves when allele frequencies change
-Allele Frequencies: can be calculated in two ways: from the observed number of different genotypes at
a particular locus or from the genotype frequencies
Allele frequency = number of copies of a given allele / sum of counts of all alleles in the
population
Example of this first method -the second method of calculating allele frequencies goes through the step of first calculating genotype
frequencies as demonstrated previously
p = f(A) = (frequency of the AA homozygote) + (1/2 x frequency of the Aa heterozygote)
q = f(a) = (frequency of the aa homozygote) + (1/2 x frequency of the Aa heterozygote)
p+q=1
Allele Frequencies with multiple alleles
1 2 3
-suppose we have three alleles A , A , and A
First method
(DON’T FORGET TO DIVIDE BY (2 x Total number of individuals))
Second method
Allele frequencies at X-Linked Locus
-females have 2 X linked alleles while males only have 1 thus A a
For when searching for the frequenciesof two alleles at an X-linked locus (X and X ) are determined
with the following equation:
First method (Just use the first one its easier) The hardy-Weinberg Law
-serves as a foundation for population genetics because It offers a simple explanation for how the
Mendelian principle of segregation influences allele and genotype frequencies in a population
-The Hardy-Weinberg law is divided into 3 parts: a set of assumptions and 2 major results
1. Assumptions: in an infinitely large (1), randomly mating population (2), free from mutation
(3), migration (4) and natural selection (5)
2. Result 1: the frequencies of the alleles do not change over time (p = A and q=a)
3. Result 2: the genotypic frequencies remain in the proportions p (frequency of AA), 2pq
2 2 2
(frequency of Aa), and q (frequency of aa). The sum of the genotype frequencies equals 1 (p + 2pq + q
= 1)
-summary: the Hardy-Weinberg law explains what happens to the allele and genotype frequencies of a
population as the alleles are passed from generation to generation in the absence of evolutionary
forces.
Assumptions of the Hardy-Weinberg Law
-the population must be infinitely large. If a population is limited in size, change deviations from
expected ratios can result in changes in allele frequency, a phenomenon called genetic drift. (unlikely
but large populations can look like infinitely large populations
-proportional deviations from the assumption results in proportional difference in results from the
Hardy-Weinberg law.
-random mating: mating between genotypes occurring in proportion to the frequencies of the
genotypes in the population. More specifically, the probability of a mating between two genotypes is
equal to the product of the genotypic frequencies.
-must not be open to the effects of mutation, migration, and natural selection (all things that change the
allele frequency)
Predictions of the Hardy-Weinberg Law
-the frequencies of the alleles will not change from one generation to the next
2 2
-second the genotypic frequencies will be in proportions of p , 2pq, q after one generation of random
mating and they will remain in those proportions in every generation that mating follows as all the
conditions are met.
Extension of the Hardy-Weinberg Law to Loci with more than two alleles
-2for alleles2A, B, and C 2
p + 2pq + q + 2pr +2qr +r
p (AA), pq = (AB), pr= (AC), qr= (BC), r 2 =(CC)
example: ^^^frequency of the allele are calculated by the formulas earlier (first thing on page 2)
Then use the formula to see the expected frequency if it were to fit uner the Hardy-Weingberg Law
Extensions of the hardy-Weinberg Law to X-linked alleles
-for X-linked alleles in females, the Hardy_weingberg frequencies are the same as those for autosomal
loci: p (X X ), 2pq (X X ), and q (X X ).
-In males, the genotype frequencies are p(X Y) and q (X Y) B
-for this reason recessive X linked traits are more frequent in males
-if the alleles are X-linked and the sexes differ in allele frequency, the eqbm frequencies are approached
over several generations. (RECALL: the previous extension and normal conditions result in eqbm after 1
mating)
Testing for Hardy-weinberg Proprotions
1. calculate allele frequency
2. calculate genotype frequency using the allele frequency (p , 2pq, q ) 2
3. use chi-sq test to compare these to observed (note: here df = n-2)
Genetic Variation in Space and Time
-when allele frequencies change in a systematic way across geographic transect, we call this an allele
frequency cline
-often clines are associated with changes in a phsycial attribute in the environment (ex. Temp or
water availability)
-can also change with time
Genetic Variation in natural populations
-protein electrophoresis reveleaed abundant genetic variation in natural populations
-The amount of genetic variation within a population was commonly measured with two parameters,
the proportion of polymorphic and heterozygosity.
-A polymorphic locus is nay locus that has more than one allele present within a population
-The proportion of polymorphic loci (P) is calculated by dividing the number of polymorphic loci
by the total number of loci examined
-for example: suppose we dounf that of 33 loci in a population of green frogs, 18
were polymorphic. The proportion of polymorphic loc

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