Fundamental Principles of Variation

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.