Download 27. Population Genetics II

Biology 212 General Genetics
Spring 2007
Lecture 27: Population Genetics II
Reading: Chap. 14 pp. 507-510 and 513
Lecture Outline:
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Identifying polymorphisms
DNA polymorphisms in humans
Inbreeding
Processes that contribute to evolutionary change
Lecture
1. Identifying polymorphisms
polymorphism: genetic variation among individuals of the same species.
Detecting genetic variation in a population:
1) Observe morphological differences
2) Identify differences in chromosome number or structure
3) Use antibodies to detect antigens
 Blood group antigens
 MHC (major histocompatibility) antigens: 121 distinct phenotypes in humans
4) Molecular approaches
 Electrophoresis: protein sizes, DNA fragment sizes
 Sequences: proteins, DNAs
2. DNA polymorphisms in humans
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1/1200 nucleotide sites differ from one individual to the next
each human genotype is unique except for identical twins
applications
 human population genetics
 forensic analysis
SSR = simple sequence repeat; also known as VNTR, variable number of tandem repeats;
variation in numbers of copies of a simple sequence repeat
Table 14.2 and Fig. 14.17
 Repeat numbers vary among individuals in human subpopulations
 Repeat numbers vary among different population groups
 Particular alleles may be more or less common in population
 Critical for DNA tests used in forensics
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Some applications:
 Fig. 14.18. Genetic typing of SSRs from individuals. Each paternal and maternal
allele in this test shows up as a separate band. In this test, each of the 9
individuals has a different test result.
 Fig. 14.19 Paternity testing. Also can apply DNA testing in criminal cases
(murder, rape, etc.)
These figure show RFLP-based tests. Here DNA is isolated, cut with restriction
enzymes, separated by electrophoresis, and the alleles are detected by hybridization to a
unique probe (Southern blot). The number of repeats in the SSR increases or decreases
the size of the DNA fragments, giving different profiles.
DNA polymorphism tests for fingerprinting or forensics can also involve
 PCR: to reproduce rare or limited DNA samples
 DNA sequence based tests (SNP or single nucleotide polymorphism analysis).
May use dideoxy sequencing or new technologies.
3. Inbreeding = mating between close relatives
Non-random mating leads to inbreeding
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frequency of heterozygotes is smaller than with random mating
increases frequency of homozygous recessive traits
inbred populations have been crucial to the study of human genetic diseases
examples:
Amish: choose mates within religious order
Icelanders: isolated island country; choose mates within region
Argentina: families with Huntington's disease localized to particular region
4. Processes that contribute to evolutionary change
a. mutation: change in base sequence of DNA induced by chemicals or radiation
b. migration: movement of new members into a population
c. natural selection: various genotypes have differing abilities to survive and
reproduce.
d. genetic drift: random fluctuations in allele frequencies
In the next lecture, we will illustrate how evolutionary forces influence allele frequencies,
emphasizing major trends.
Simulations at the following web site will be used to illustrate some of the evolutionary
trends: http://darwin.eeb.uconn.edu/simulations/selection.html
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