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Fundamental Principles of Variation
Chapter 9
Chris Gaffron
According to Futuyama, “Genetic variation is the foundation of evolution,
for the great changes in organisms that have transpired over time and the
differences that have developed among species as they diverged from theif
common ancestors all originated as genetic variants within species.”
Review of vocabulary:
_Penotype__-refers to a characteristic in an individual organism, or in a group of
individuals that are alike in respect.
_Genotype__-the genetic constitution of an individual organism, or of a group of
organisms that are alike in that respect..
_Locus__-is a site on a chromosome, or more usually, the gene that occupies that
site.
_Allele__-is a particular form of a gene, usually distinguished from other allels by
its effects on the phenotype.
_Haplotype___-is one of the sequences of a gene or DNA segment that can be
distinguished from homologous sequences by methods like DNA sequencing.
_Gene Copy__-the term we will use when counting the number of
representatives of a gene.
Fundamental Principles of Genetic Variation in Populations
___Allele Frequency__________-the relative commonness or rarity of an allele,
its proportion of all gene copies in population.
___Genotype Frequency__________-the proportion of a population that has a
certain genotype.
Hardy-Weinberg principle (p² + 2pq + q² = 1)
Developed by G.H Hardy and W. Weinberg
_____HW principle______________-is the foundation on which almost all of the
theory of population genetics of sexually producing organisms
Two important implication of the principle:
1) __Genotype frequencies______-attain their H-W values after a single
generation of random mating.
2) According to the H-W principle not only ___Genotype frequencies_, but
also__allele frequencies___, remain unchanged from generation to
generation.
The H-W principle only holds true if you take into consideration certain
assumptions.
1) ___Mating is random_____
2) ___The population is infinitely large______
3) Genes are not added from an outside population.
4) ___Genes do not mutate from one allelic state to another (no
mutation)_________
5) All individuals have equal probabilities of survival and reproduction
Example of Hardy-Weinberg equilibrium (223)
If we use the human MN locus for red blood cells from a sample of 320 people
from the village of Deluso.
Please Write the problems as I do on the back of this page.
Number of people carrying each genotype
MM=187
MN=114
NN=19
We can now estimate the frequency of each genotype as a proportion of total
sample
Frequency of MM=187/320=0.584
Frequency of MN= 114/320=0.356
Frequency of NN= 19/320= 0.59
Once we have these we can calculate the frequency of M and N, Each person
carries 2 genes so total sample is 320x2=640. MM is a homozygote it has two M
alleles and MN is heterozygote so it has 1 allele.
So
Number of M alleles in the sample is (187 x 2)+ (114)=488
Number of N alleles is (19 x 2) + (114)= 152
So the to find the frequency of M and N
F of M =p=488/640=0.763
F of N=q= 152/640=0.237
As with genotype frequencies p and q must add to 1.
These are estimates because it is a sample and not the whole population.
MM (p^2)
MN (2pq)
NN (q^2)
Expected HW freq
0.582
0.362
0.056
Exp #(HW x
sample size)
186
116
18
Observed number
187
114
19
Bibliography
Futuyma, Douglas J. Evolution 2nd ed. Sunderland, MA: Sinauer Associates Inc.,
2009. Print
Hillis, David M. et al. Molecular Systematics 2nd ed. Sunderland, MA: Sinauer
Associates Inc., 1996. Print.
Lewontin, R.C. The Genetic Basis of Evolutionary Change. New York and London:
Columbia University Press. 1974. Print.